CHANGES PROPOSED FOR AP SSC EXAM BY APSCERT

APSCERT Proposed Changes in SSC AP 10th Class Exam System from 2014-15

APSCERT The Andhra Pradesh State Counsel for Research and Training has proposed reforms(Changes) in SSC 10th Class Examination System and Evaluation System from 2014-15 academic year, hence students and Teachers should get ready for New Exam System and Pattern for 9th, 10th Classes from 2014-15. We already knew that New Text books for 10th Class has been released basing on CCE. Basing on the APSCF-2011, The APSCERT has designed New pattern of Examination for 10th Class (SSC AP) and going to be implemented from 2014-15. These are the proposals made by APSCERT. There will be Formative and Summative Assessments instead of Unit and Terminal Exams in 10th Class.

Why reforms are needed in SSC Examinations System from 2014-15: APSCF-2011 was framed in accordance with RTE-2009 and NCF-2005. The textbooks from classes I to X are revised (modernized) on the basis of APSCF-2011. The new textbooks aim at achieving the class wise, subject wise, targeted competencies and developing multifaceted competencies such as thinking, expressing and analyzing. CCE for classes I to VIII is being implemented in our State since April, 2010. When the above aspects are taken in to consideration it is necessary to bring in reforms in class X examination.
Need for Changes (Reforms) in SSC 10th Class Exam System 1. The aspect that is to be worried about is mainly the role played by ‘school’. The role of school is reduced to prepare the children mechanically for their exams limiting their knowledge to textbooks. Learning by rote memory and mechanical completion of syllabus do not prove to be beneficial for achieving the objectives of education. A school must enable the children to explore, search, discuss, read the reference books, discriminate and analyze. When these are kept in view, the present system of examination has to be changed.2. School should function for the all-round development of children. For their all- round development, children should participate in various activities and programmes to develop physically, mentally, socially, emotionally and morally. Presently, teaching is limited to languages, Science, Mathematics, Social Studies. Only these are being evaluated. Teaching must take place considering Health & Physical Education, Arts & Cultural Education, Work & Computer Education and Value Education & Life Skills as curricular areas. They should not be treated as co-curricular activities. Even these areas also be evaluated periodically.
Proposed Reforms (Changes) in SSC 10th Class Exam System Presently there are two papers in class X for all subjects except second language (Telugu/ Hindi) which are of 2 ½ hours duration. Instead, there should be only one paper in each subject which will be of 3.00 hours duration

Public Exam and Exam Marks
Subject Marks
First Language (Telugu/ Hindi/ Urdu etc.)	100 marks
Second Language (Telugu/ Hindi)	100 marks
Third Language (English)	100 marks
Mathematics	100 marks
Physical Science	50 marks
Biological Science	50 marks
Social Studies	100 marks
Total	600 marks

Marks and Weightage (Formative, Summative, Public Exams) 1. Public exams in each subject will be conducted for 80% of marks. The remaining 20% of marks are through Formative Assessment [FA].
2. The average of four Formative Assessments conducted in an academic year will be accounted for 20% of marks.
3. First and second Summative Assessments shall be conducted, preparing question papers for 80% of marks. This
ensures that the children are trained to write public exams. The remaining 20% of marks shall be awarded on the
basis of the Formative Assessment.
4. First Summative Assessment – 20% (FA)+80% question papers. Second Summative Assessment – 20% (FA)+80%
question papers.

In each Formative Assessment marks are allotted as follows:
1. Children’s participation and responses	5 marks
2. Children’s written work in their notebooks	5 marks
3. Project works	5 marks
4. Slip test	5 marks
Total marks	20 marks

Children must attend all the four formative assessments. In case any child is absent for any formative assessment,
teachers should conduct the test immediately (when they come back) and award marks.
After completion of the 4th Formative Assessment in February, the headmaster should submit these details to the
board in a fixed format supplied to them.
HM is responsible for proper conducting of Formative Assessment. District Deputy Educational Officers shall form in
to a two member HM Committee to monitor the proper recording of the results/ formative assessment. Experienced
HMs of aided/ recognized schools can also be included in this committee along with HMs working under government management.
Mandal shall be considered as a unit for monitoring. The monitoring committee should observe/ monitor the details
of at least 10 to 15 schools.
PASS MARKS AND GRADIING SYSTEMEach subject will have only one paper. So the exam must be conducted on alternate days. Each exam is of 3 hours duration. Similarly, 15 minutes will be allotted in each exam to read and understand the
paper.
Even though 20% of marks are allotted to formative assessment, children must attend the public exam to children
will be considered as passed only when they score 30 marks out of 80 marks in each subject in the public exam.
Physical Science and Biological Science exams will be conducted for 50 marks. So the children should score a
minimum of 15 marks in each subject to pass.
Children must score a minimum of 40% in each subject when the marks of formative assessment and public exam
are added i.e. children are passed only when they score 40 marks. They will be failed even if they score full marks in
formative assessment and score below 30 in public exams.
Grading System Though the present be better if the grading method that is being implemented is good, it would following range of marks and concerned grades are implemented.

91 to 100 marks	A1
81 to 90 marks	A
71 to 80 marks	B1
61 to 70 marks	B
51 to 60 marks	C1
40 to 50 marks	C
0 to 39 marks	D

Other curricular subjects and Evaluation In our State, evaluation is being conducted for classes I to VIII in Physical & Health Education, Arts & Cultural Education, Work & Computer Education, Value Education & Life Skills considering them as a part of curriculum. Hence, the same is to be followed for classes IX and X. Each subject has 50 marks.
Grade details of these subjects also must be recorded in the memorandum of marks of class X.
No public exam shall be conducted in these subjects. Teachers shall award marks based on the results of first and
second summative assessment. Their average must taken into account and the details of grade must be submitted
on-line by HM.
HM should allot the responsibility of conducting these exams to the teachers.
Example:* Value Education & Life Skills to language teachers
* Art & Cultural Education to Social Studies teacher
* Work & Computer Education to Physical Science or Biological Science teacher
* Physical & Health Education will be conducted by Physical Director/ Physical Education Teacher.
Qualitative Aspects and Exam Question Papers Pattern1) Nature of the question papers Questions in public exam question papers should not facilitate memorizing and merging up. The questions should make the children think and write. The questions should be analytical, application oriented and open ended. The questions once appeared in public exams should not be repeated under any circumstances. The questions given in the exercises of the textbook must not be given as they are. They should prepare additional
questions of same nature.
2) Academic standards/ competencies to be achieved – nature of questions The questions in the public exam should be in relevance to the fixed academic standards/ competencies to be achieved in the subjects concerned. Weightage for the competencies of the subjects must be fixed and the questions must be prepared accordingly.
3) Number of questionsIt should better if the questions in public exam in Non Languages are given as follows:

5 marks questions – 4	(total 20 marks)
3 marks questions – 10	(total 30 marks)
2 marks questions – 10	(total 20 marks)
1 mark questions – 10	(total 10 marks)
Total	80 Marks

Marks can be allotted to language with slight changes as per their competencies.
* 5 marks questions mean essay answer questions. Answers to these must be written in 3 to 4 paragraphs or 10 to 12
sentences.
* 3 marks questions mean short answer questions. Answers to these must be written in a paragraph or 5 to 6
sentences.
* 2 marks questions means very short answer questions. Answers to these must be written in two or three sentences.
* 1 mark questions mean objective type questions.
4) Questions – Choice Each question paper may contain internal choice for essay type questions. (If a question is asked from a chapter, then the second question or additional question must be given from the same chapter) or questions of internal choice must given from the same chapter. Children must be given a choice to choose any one.
5) Questions – Weightage Lesson wise weightage should not be given as in past. Questions shall be based on academic standards and can be given from any lesson. The following weightage tables must be kept in view while preparing question papers. Type of questions (essay, short answer, very short answer and objective questions) and academic standard wise questions (how many marks and questions to each academic standards).
6) Correcting the answer scripts A teacher must correct 20 papers in the morning and 20 papers in the afternoon i.e. only 40 papers must be
corrected per day. As the questions are thought provoking and open ended, correction must be done carefully.
Opportunity must be provided for re-correction of the answer scripts if necessary. It is only recounting in vogue now. Guidelines for correcting the papers must be framed. Indicators must be framed to evaluate the methods of writing answers to different questions. A ‘key paper’ was made previously and the papers were corrected based on the key. But now the need has arisen to incorporate those indicators in the guidelines which will properly evaluate the writing of children on their own, their power of expression, power of analysis, power of self writing, power of application and power of explanation.

OHMMETER

You remember that the current through a circuit depends on the resistance. This prin- ciple can be used to manufacture a voltmeter using an ammeter and a resistor. The larger the value of the resistance in series with the meter, the more voltage is needed to produce a reading of full scale. This has a converse, or a “flip side”: Given a constant voltage, the current through the meter will vary if the resistance varies. This provides a means for measuring resistances. An ohmmeter is almost always constructed by means of a milliammeter or microammeter in series with a set of fixed, switchable resistances and a battery that provides a known, constant voltage (Fig. 3-8). By selecting the resistances appropri- ately, the meter will give indications in ohms over any desired range. Usually, zero on the meter is assigned the value of infinity ohms, meaning a perfect insulator. The full-scale value is set at a certain minimum, such as 1 Ω, 100 Ω, or 10 KΩ (10,000 Ω). Ohmmeters must be precalibrated at the factory where they are made. A slight er- ror in the values of the series resistors can cause gigantic errors in measured resistance. Therefore, precise tolerances are needed for these resistors. It is also necessary that the battery be exactly the right kind, and that it be reasonably fresh so that it will provide the appropriate voltage. The smallest deviation from the required voltage can cause a big error in the meter indication. The scale of an ohmmeter is nonlinear. That is, the graduations are not the same everywhere. Values tend to be squashed together towards the “infinity” end of the scale.
It can be difficult to interpolate for high values of resistance, unless the right scale is se- lected. Engineers and technicians usually connect an ohmmeter in a circuit with the meter set for the highest resistance range first; then they switch the range until the me- ter is in a part of the scale that is easy to read. Finally, the reading is taken, and is mul- tiplied (or divided) by the appropriate amount as indicated on the range switch. Figure 3-9 shows an ohmmeter reading. The meter itself says 4.7, but the range switch says 1 KΩ. This indicates a resistance of 4.7 KΩ, or 4700 Ω.
Ohmmeters will give inaccurate readings if there is a voltage between the points where the meter is connected. This is because such a voltage either adds to, or sub- tracts from, the ohmmeter battery voltage. This in effect changes the battery voltage, and the meter reading is thrown way off. Sometimes the meter might even read “more than infinity” ohms; the needle will hit the pin at the left end of the scale. Therefore, when using an ohmmeter to measure resistance, you need to be sure that there is no voltage between the points under test. The best way to do this is to switch off the equip- ment in question.

VOLTMETER

Current is a flow of charge carriers. Voltage, or electromotive force (EMF), or potential difference, is the “pressure” that makes a current possible. Given a circuit whose resis- tance is constant, the current that will flow in the circuit is directly proportional to the voltage placed across it. Early electrical experimenters recognized that an ammeter could be used to measure voltage, since an ammeter is a form of constant-resistance circuit. If you connect an ammeter directly across a source of voltage—a battery, say—the meter needle will deflect. In fact, a milliammeter needle will probably be “pinned” if you do this with it, and a microammeter might well be wrecked by the force of the needle striking the pin at the top of the scale. For this reason, you should never connect mil- liammeters or microammeters directly across voltage sources. An ammeter, perhaps with a range of 0-10 A, might not deflect to full scale if it is placed across a battery, but it’s still a bad idea to do this, because it will rapidly drain the battery. Some batteries, such as automotive lead-acid cells, can explode under these conditions. This is because all ammeters have low internal resistance. They are designed that way deliberately. They are meant to be connected in series with other parts of a circuit, not right across the power supply. But if you place a large resistor in series with an ammeter, and then connect the ammeter across a battery or other type of power supply, you no longer have a short cir- cuit. The ammeter will give an indication that is directly proportional to the voltage of the supply. The smaller the full-scale reading of the ammeter, the larger the resistance to get a meaningful indication on the meter. Using a microammeter and a very large value of resistor in series, a voltmeter can be devised that will draw only a little current from the source. A voltmeter can be made to have different ranges for the full-scale reading, by switching different values of resistance in series with the microammeter (Fig. 3-6). The internal resistance of the meter is large because the values of the resistors are large. The greater the supply voltage, the larger the internal resistance of the meter, because the necessary series resistance increases as the voltage increases. It’s always good when a voltmeter has a high internal resistance. The reason for this is that you don’t want the meter to draw much current from the power source. This cur- rent should go, as much as possible, towards working whatever circuit is hooked up to the supply, and not into just getting a reading of the voltage. Also, you might not want, or need, to have the voltmeter constantly connected in the circuit; you might need the voltmeter for testing many different circuits. You don’t want the behavior of the circuit to be affected the instant you connect the voltmeter to the supply. The less current a voltmeter draws, the less it will affect the behavior of anything that is working from the power supply.

AMMETER

Getting back to electromagnetic deflection, and the workings of the galvanometer, you might have thought by now that a magnetic compass doesn’t make a very convenient type of meter. It has to be lying flat, and the coil has to be aligned with the compass nee- dle when there is no current. But of course, electrical and electronic devices aren’t all turned in just the right way, so as to be aligned with the north geomagnetic pole. That would not only be a great bother, but it would be ridiculous. Imagine a bunch of scien- tists running around, turning radios and other apparatus so the meters are all lying flat and are all lined up with the earth’s magnetic field! In the early days of electricity and electronics, when the phenomena were confined to scientific labs, this was indeed pretty much how things were. Then someone thought that the magnetic field could be provided by a permanent magnet right inside the meter, instead of by the earth. This would supply a stronger magnetic force, and would therefore make it possible to detect much weaker currents. It would let the meter be turned in any direction and the operation would not be af- fected. The coil could be attached right to the meter pointer, and suspended by means of a spring in the field of the magnet. This kind of meter, called a D’Arsonval move- ment, is still extensively used today. . This is the ba- sic principle of the ammeter.
A variation of this is the attachment of the meter needle to a permanent magnet, and the winding of the coil in a fixed form around the magnet. Current in the coil pro- duces a magnetic field, and this in turn generates a force if the coil and magnet are aligned correctly with respect to each other. This meter movement is also sometimes called a D’Arsonval movement. This method will work, but the inertial mass of the per- manent magnet causes a slower needle response. This kind of meter is also more prone to overshoot than the true D’Arsonval movement; the inertia of the magnet’s mass, once
overcome by the magnetic force, causes the needle to fly past the actual current level before finally coming to rest at the correct reading. It is possible to use an electromagnet in place of the permanent magnet in the me- ter assembly. This electromagnet can be operated by the same current that flows in the coil attached to the meter needle. This gets rid of the need for a massive, permanent magnet inside the meter. It also eliminates the possibility that the meter sensitivity will change in case the strength of the permanent magnet deteriorates (such as might be caused by heat, or by severe mechanical vibration). The electromagnet can be either in series with, or in parallel with, the meter movement coil. The sensitivity of the D'Arsonval meter, and of its cousins, depends on several fac- tors. First is the strength of the permanent magnet, if the meter uses a permanent mag- net. Second is the number of turns in the coil. The stronger the magnet, and the larger the number of turns in the coil, the less current is needed in order to produce a given magnetic force. If the meter is of the electromagnet type, the combined number of coil turns affects the sensitivity. Remember that the strength of a magnetomotive force is given in terms of ampere turns. For a given current (number of amperes), the force in- creases in direct proportion to the number of coil turns. The more force in a meter, the greater the needle deflection, and the smaller the amount of current that is needed to cause a certain amount of needle movement. The most sensitive ammeters can detect currents of just a microampere or two. The amount of current for full scale deflection (the needle goes all the way up without banging against the stop pin) can be as little as about 50 uA in commonly available me- ters. Thus you might see a microammeter, or a milliammeter, quite often in electronic work. Meters that measure large currents are not a problem to make; it’s easy to make an insensitive device. Sometimes, it is desirable to have an ammeter that will allow for a wide range of current measurements. The full-scale deflection of a meter assembly cannot easily be changed, since this would mean changing the number of coil turns and/or the strength of the magnet. But all ammeters have a certain amount of internal resistance. If a re- sistor, having the same internal resistance as the meter, is connected in parallel with the meter, the resistor will take half the current. Then it will take twice the current through the assembly to deflect the meter to full scale, as compared with the meter alone. By choosing a resistor of just the right value, the full-scale deflection of an ammeter can be increased by a factor of 10, or 100, or even 1000. This resistor must be capable of car- rying the current without burning up. It might have to take practically all of the current flowing through the assembly, leaving the meter to carry only 1/10, or 1/100, or 1/1000 of the current. This is called a shunt resistance or meter shunt . Meter shunts are frequently used when it is necessary to measure very large cur- rents, such as hundreds of amperes. They allow microammeters or milliammeters to be used in a versatile multimeter, with many current ranges.

THERMAL HEATING

Another phenomenon, sometimes useful in the measurement of electric currents, is the fact that whenever current flows through a conductor having any resistance, that con- ductor is heated. All conductors have some resistance; none are perfect. The extent of this heating is proportional to the amount of current being carried by the wire. By choosing just the right metal or alloy, and by making the wire a certain length and diameter, and by employing a sensitive thermometer, and by putting the entire as- sembly inside a thermally insulating package, a hot-wire meter can be made. The hot-wire meter can measure ac as well as dc, because the current-heating phenomenon does not depend on the direction of current flow. A variation of the hot-wire principle can be used by placing two different metals into contact with each other. If the right metals are chosen, the junction will heat up when a current flows through it. This is called the thermocouple principle. As with the hot-wire meter, a thermometer can be used to measure the extent of the heating. But there is also another effect. A thermocouple, when it gets warm, generates a di- rect current. This current can be measured by a more conventional, dc type meter. This method is useful when it is necessary to have a faster meter response time. The hot-wire and thermocouple effects are used occasionally to measure current at radio frequencies, in the range of hundreds of kilohertz up to tens of gigahertz.

ELECTROMAGNETIC DEFLECTION

Early experimenters with electricity and magnetism noticed that an electric current produces a magnetic field. This discovery was probably an accident, but it was an ac- cident that, given the curiosity of the scientist, was bound to happen. When a mag- netic compass is placed near a wire carrying a direct electric current, the compass doesn’t point toward magnetic north. The needle is displaced. The extent of the er- ror depends on how close the compass is brought to the wire, and also on how much current the wire is carrying. Scientific experimenters are like children. They like to play around with things. Most likely, when this effect was first observed, the scientist tried different arrangements to see how much the compass needle could be displaced, and how small a current could be detected. An attempt was made to obtain the greatest possible current-detecting sensitivity. Wrapping the wire in a coil around the compass resulted in a device that would indicate a tiny electric current (Fig. 3-1). This effect is known as galvanism, and the meter so devised was called a galvanometer.

Once this device was made, the scientist saw that the extent of the needle dis- placement increased with increasing current. Aha—a device for measuring current! Then, the only challenge was to calibrate the galvanometer somehow, and to set up some kind of standard so that a universal meter could be engineered. You can easily make your own galvanometer. Just buy a cheap compass, about two feet of insulated bell wire, and a six-volt lantern battery. Set it up as shown in Fig. 3-1. Wrap the wire around the compass four or five times, and align the compass so that the needle points right along the wire turns while the wire is disconnected from the battery. Connect one end of the wire to the minus (–) terminal of the battery. Touch the other end to the plus (+) terminal, intermittently, and watch the compass needle. Don’t leave the wire connected to the battery for any length of time unless you want to drain the battery in a hurry. You can buy a resistor and a potentiometer at a place like Radio Shack, and set up an experiment that shows how galvanometers measure current. For a 6-V lantern bat- tery, the fixed resistor should have a value of at least 330 Ω at 1/4 watt, and the poten- tiometer should have a value of 10 KΩ(10,000 Ω) maximum. Connect the resistor and potentiometer in series between one end of the bell wire and one terminal of the bat- tery, as shown in Fig. 3-2. The center contact of the potentiometer should be short-cir- cuited to one of the end contacts, and the resulting two terminals used in the circuit. When you adjust the potentiometer, the compass needle should deflect more or less, depending on the current through the wire. Early experimenters calibrated their me- ters by referring to the degree scale around the perimeter of the compass.

POWER AND WATT

Whenever current flows through a resistance, heat results. This is inevitable. The heat can be measured in watts, abbreviated W, and represents electrical power. Power can be manifested in many other ways, such as in the form of mechanical motion, or radio waves, or visible light, or noise. In fact, there are dozens of different ways that power can be dissipated. But heat is always present, in addition to any other form of power in an electrical or electronic device. This is because no equipment is 100-percent efficient. Some power always goes to waste, and this waste is almost all in the form of heat.

CONDUCTANCE AND SIEMENS

The better a substance conducts, the less its resistance; the worse it conducts, the higher its resistance. Electricians and electrical engineers sometimes prefer to speak
about the conductance of a material, rather than about its resistance. The standard unit of conductance is the siemens, abbreviated S. When a component has a conduc- tance of 1 S, its resistance is 1 ohm. If the resistance is doubled, the conductance is cut in half, and vice-versa. Therefore, conductance is the reciprocal of resistance. If you know the resistance in ohms, you can get the conductance in siemens by tak- ing the quotient of 1 over the resistance. Also, if you know the conductance in siemens, you can get the resistance in ohms by taking 1 over the conductance. The relation can be written as:
siemens 1/ohms, or ohms 1/siemens
Smaller units of conductance are often necessary. A resistance of one kilohm is equal to one millisiemens. If the resistance is a megohm, the conductance is one mi- crosiemens. You’ll also hear about kilosiemens or megasiemens, representing resis- tances of 0.001 ohm and 0.000001 ohm (a thousandth of an ohm and a millionth of an ohm) respectively. Short lengths of heavy wire have conductance values in the range of kilosiemens. Heavy metal rods might sometimes have conductances in the megasiemens range. As an example, suppose a component has a resistance of 50 ohms. Then its con- ductance, in siemens, is 1⁄50, or 0.02 S. You might say that this is 20 mS. Or imagine a piece of wire with a conductance of 20 S. Its resistance is 1/20, or 0.05, ohm. Not often will you hear the term “milliohm”; engineers do not, for some reason, speak of subohmic units very much. But you could say that this wire has a resistance of 50 milliohms, and you would be technically right. Conductivity is a little trickier. If wire has a resistivity of, say, 10 ohms per kilome- ter, you can’t just say that it has a conductivity of 1/10, or 0.1, siemens per kilometer. It is true that a kilometer of such wire will have a conductance of 0.1 S; but 2 km of the wire will have a resistance of 20 ohms (because there is twice as much wire), and this is not twice the conductance, but half. If you say that the conductivity of the wire is 0.1 S/km, then you might be tempted to say that 2 km of the wire has 0.2 S of conductance. Wrong! Conductance decreases, rather than increasing, with wire length. When dealing with wire conductivity for various lengths of wire, it’s best to convert to resistivity values, and then convert back to the final conductance when you’re all done calculating. Then there won’t be any problems with mathematical semantics. Figure 2-5 illustrates the resistance and conductance values for various lengths of wire having a resistivity of 10 ohms per kilometer.

RESISTANCE AND OHM

Resistance is a measure of the opposition that a circuit offers to the flow of electric current. You might compare it to the diameter of a hose. In fact, for metal wire, this is an excellent analogy: small-diameter wire has high resistance (a lot of opposition to current flow), and large-diameter wire has low resistance (not much opposition to electric currents). Of course, the type of metal makes a difference too. Iron wire has higher resistance for a given diameter than copper wire. Nichrome wire has still more resistance. The standard unit of resistance is the ohm. This is sometimes abbreviated by the upper-case Greek letter omega, resembling an upside–down capital U (Ω).  In this book, we’ll just write it out as “ohm” or “ohms.” You’ll sometimes hear about kilohms where 1 kilohm 1,000 ohms, or about megohms, where 1 megohm 1,000 kilohms 1,000,000 ohms. Electric wire is sometimes rated for resistivity. The standard unit for this purpose is the ohm per foot (ohm/ft) or the ohm per meter (ohm/m). You might also come across the unit ohm per kilometer (ohm/km). Table 2-1 shows the resistivity for vari- ous common sizes of wire.

AMPERE

Current is a measure of the rate at which charge carriers flow. The standard unit is the ampere. This represents one coulomb (6,240,000,000,000,000,000) of charge carriers per second past a given point. An ampere is a comparatively large amount of current. The abbreviation is A. Often, current is specified in terms of milliamperes, abbreviated mA, where 1 mA 0.001 A or a thousandth of an ampere. You will also sometimes hear of microamperes (µA), where 1 µA 0.000001 A 0. 001 mA, a millionth of an ampere. And it is increasingly common to hear about nanoamperes (nA), where 1 nA 0. 001 µA 0.000000001 A (a billionth of an ampere). Rarely will you hear of kiloamperes (kA), where 1 kA 1000 A. A current of a few milliamperes will give you a startling shock. About 50 mA will jolt you severely, and 100 mA can cause death if it flows through your chest cavity. An ordinary 100-watt light bulb draws about 1 A of current. An electric iron draws approximately 10 A; an entire household normally uses between 10 A and 50 A, depending on the size of the house and the kinds of appliances it has, and also on the time of day, week or year. The amount of current that will flow in an electrical circuit depends on the voltage, and also on the resistance. There are some circuits in which extremely large currents, say 1000 A, flow; this might happen through a metal bar placed directly at the output of a massive electric generator. The resistance is extremely low in this case, and the gen- erator is capable of driving huge amounts of charge. In some semiconductor electronic devices, such as microcomputers, a few nanoamperes will suffice for many complicated processes. Some electronic clocks draw so little current that their batteries last as long as they would if left on the shelf without being put to any use at all.

CURRENT FLOW

If a conducting or semiconducting path is provided between two poles having a poten- tial difference, charge carriers will flow in an attempt to equalize the charge between the poles. This flow of electric current will continue as long as the path is provided, and as long as there is a charge difference between the poles. Sometimes the charge difference is equalized after a short while. This is the case, for example, when you touch a radiator after shuffling around on the carpet in your hard-soled shoes. It is also true in a lightning stroke. In these instances, the charge is equalized in a fraction of a second. The charge might take longer to be used up. This will happen if you short-circuit a dry cell. Within a few minutes, or maybe up to an hour, the cell will “run out of juice” if you put a wire between the positive and negative terminals. If you put a bulb across the cell, say with a flashlight, it takes an hour or two for the charge difference to drop to zero.
24 Electrical units
2-1 Electric lines of flux always exist near poles of electric charge.
In household electric circuits, the charge difference will essentially never equalize, unless there’s a power failure. Of course, if you short-circuit an outlet (don’t!), the fuse or breaker will blow or trip, and the charge difference will immediately drop to zero. But if you put a 100-watt bulb at the outlet, the charge difference will be maintained as the current flows. The power plant can keep a potential difference across a lot of light bulbs indefinitely. You might have heard that “It’s the current, not the voltage, that kills,” concerning the danger in an electric circuit. This is a literal truth, but it plays on semantics. It’s like saying “It’s the heat, not the fire, that burns you.” Naturally! But there can only be a deadly current if there is enough voltage to drive it through your body. You don’t have to worry when handling flashlight cells, but you’d better be extremely careful around household utility circuits. A voltage of 1.2 to 1.7 V can’t normally pump a dangerous cur- rent through you, but a voltage of 117 V almost always can. Through an electric circuit with constant conductivity, the current is directly propor- tional to the applied voltage. That is, if you double the voltage, you double the current; if the voltage is cut in half, the current is cut in half too. Figure 2-2 shows this relationship as a graph in general terms. But it assumes that the power supply can provide the neces- sary number of charge carriers. This rule holds only within reasonable limits.

VOLT

An accumulation of static electric charge, such as an excess or shortage of elec- trons, is always, associated with a voltage. There are other situations in which voltages exist. Voltage is generated at a power plant, and produced in an electrochemical reac- tion, and caused by light falling on a special semiconductor chip. It can be produced when an object is moved in a magnetic field, or is placed in a fluctuating magnetic field. A potential difference between two points produces an electric field, represented by electric lines of flux . There is always a pole that is relatively positive, with fewer electrons, and one that is relatively negative, with more electrons. The positive pole does not necessarily have a deficiency of electrons compared with neutral objects, and the negative pole might not actually have a surplus of electrons with respect to neu- tral things. But there’s always a difference in charge between the two poles. The nega- tive pole always has more electrons than the positive pole. The abbreviation for volt is V. Sometimes, smaller units are used. The millivolt (mV) is equal to a thousandth (0.001) of a volt. The microvolt (µV) is equal to a mil- lionth (0.000001) of a volt. And it is sometimes necessary to use units much larger than one volt. One kilovolt (kV) is equal to one thousand volts (1,000). One megavolt (MV) is equal to one million volts (1,000,000) or one thousand kilovolts. In a dry cell, the EMF is usually between 1.2 and 1.7 V; in a car battery, it is most often 12 V to 14 V. In household utility wiring, it is a low-frequency alternating current of about 117 V for electric lights and most appliances, and 234 V for a washing machine, dryer, oven, or stove. In television sets, transformers convert 117 V to around 450 V for the operation of the picture tube. In some broadcast transmitters, kilovolts are used. The largest voltages on Earth occur between clouds, or between clouds and the ground, in thundershowers; this potential difference is on the order of tens of megavolts. In every case, voltage, EMF, or potential difference represents the fact that charge carriers will flow between two points if a conductive path is provided. The number of charge carriers might be small even if the voltage is huge, or very large even if the volt- age is tiny. Voltage represents the pressure or driving force that impels the charge car- riers to move. In general, for a given number of charge carriers, higher voltages will produce a faster flow, and therefore a larger current. It’s something like water pressure. The amount of water that will flow through a hose is proportional to the water pressure, all other things being equal.

NON ELECTRICAL ENERGY

In electricity and electronics, there are many kinds of phenomena that involve other forms of energy besides electrical energy. Visible light is an example. A light bulb converts electricity into radiant energy that you can see. This was one of the major motivations for people like Thomas Edison to work with electricity. Visible light can also be converted into electric current or voltage. A photovoltaic cell does this. Light bulbs always give off some heat, as well as visible light. Incandescent lamps actually give off more energy as heat than as light. And you are certainly acquainted with electric heaters, designed for the purpose of changing electricity into heat energy. This “heat” is actually a form of radiant energy called infrared. It is similar to visible light, except that the waves are longer and you can’t see them. Electricity can be converted into other radiant-energy forms, such as radio waves, ultraviolet, and X rays. This is done by things like radio transmitters, sunlamps, and X-ray tubes. Fast-moving protons, neutrons, electrons, and atomic nuclei are an important form of energy, especially in deep space where they are known as cosmic radiation. The en- ergy from these particles is sometimes sufficient to split atoms apart. This effect makes it possible to build an atomic reactor whose energy can be used to generate electricity. Unfortunately, this form of energy, called nuclear energy, creates dangerous by- products that are hard to dispose of. When a conductor is moved in a magnetic field, electric current flows in that conductor. In this way, mechanical energy is converted into electricity. This is how a
18 Basic physical concepts
generator works. Generators can also work backwards. Then you have a motor that changes electricity into useful mechanical energy. A magnetic field contains energy of a unique kind. The science of magnetism is closely related to electricity. Magnetic phenomena are of great significance in electron- ics. The oldest and most universal source of magnetism is the flux field surrounding the earth, caused by alignment of iron atoms in the core of the planet. A changing magnetic field creates a fluctuating electric field, and a fluctuating electric field produces a changing magnetic field. This phenomenon, called electro- magnetism, makes it possible to send radio signals over long distances. The electric and magnetic fields keep producing one another over and over again through space. Chemical energy is converted into electricity in all dry cells, wet cells, and bat- teries. Your car battery is an excellent example. The acid reacts with the metal elec- trodes to generate an electromotive force. When the two poles of the batteries are connected, current results. The chemical reaction continues, keeping the current going for awhile. But the battery can only store a certain amount of chemical energy. Then it “runs out of juice,” and the supply of chemical energy must be restored by charging. Some cells and batteries, such as lead-acid car batteries, can be recharged by driving current through them, and others, such as most flashlight and transistor-radio batteries, cannot.

STATIC ELECTRICITY

Charge carriers, particularly electrons, can build up, or become deficient, on things without flowing anywhere. You’ve probably experienced this when walking on a car- peted floor during the winter, or in a place where the humidity was very low. An excess or shortage of electrons is created on and in your body. You acquire a charge of static
Static electricity 15
electricity. It’s called “static” because it doesn’t go anywhere. You don’t feel this until you touch some metallic object that is connected to earth ground or to some large fixture; but then there is a discharge, accompanied by a spark that might well startle you. It is the current, during this discharge, that causes the sensation that might make you jump. If you were to become much more charged, your hair would stand on end, because every hair would repel every other. Like charges are caused either by an excess or a de- ficiency of electrons; they repel. The spark might jump an inch, two inches, or even six inches. Then it would more than startle you; you could get hurt. This doesn’t happen with ordinary carpet and shoes, fortunately. But a device called a Van de Graaff gen- erator, found in some high school physics labs, can cause a spark this large

CURRENT

Whenever there is movement of charge carriers in a substance, there is an electric current. Current is measured in terms of the number of electrons or holes passing a single point in one second. Usually, a great many charge carriers go past any given point in one second, even if the current is small. In a household electric circuit, a 100-watt light bulb draws a cur- rent of about six quintillion (6 followed by 18 zeroes) charge carriers per second. Even the smallest mini-bulb carries quadrillions (numbers followed by 15 zeroes) of charge carriers every second. It is ridiculous to speak of a current in terms of charge carriers per second, so usually it is measured in coulombs per second instead. A coulomb is equal to approximately 6,240,000,000,000,000,000 electrons or holes. A cur- rent of one coulomb per second is called an ampere, and this is the standard unit of electric current. A 100-watt bulb in your desk lamp draws about one ampere of current. When a current flows through a resistance—and this is always the case because even the best conductors have resistance—heat is generated. Sometimes light and other forms of energy are emitted as well. A light bulb is deliberately designed so that the resistance causes visible light to be generated. Even the best incandescent lamp is inefficient, creating more heat than light energy. Fluorescent lamps are better. They produce more light for a given amount of current. Or, to put it another way, they need less current to give off a certain amount of light. Electric current flows very fast through any conductor, resistor, or semiconductor. In fact, for most practical purposes you can consider the speed of current to be the same as the speed of light: 186,000 miles per second. Actually, it is a little less.

SEMICONDUCTORS

In a semiconductor, electrons flow, but not as well as they do in a conductor. You might imagine the people in the line being lazy and not too eager to pass the balls along. Some semiconductors carry electrons almost as well as good electrical conductors like copper or aluminum; others are almost as bad as insulating materials. The people might be just a little sluggish, or they might be almost asleep. Semiconductors are not exactly the same as resistors. In a semiconductor, the ma- terial is treated so that it has very special properties. The semiconductors include certain substances, such as silicon, selenium, or gal- lium, that have been “doped” by the addition of impurities like indium or antimony. Perhaps you have heard of such things as gallium arsenide, metal oxides, or silicon rectifiers. Electrical conduction in these materials is always a result of the motion of electrons. However, this can be a quite peculiar movement, and sometimes engi- neers speak of the movement of holes rather than electrons. A hole is a shortage of an electron—you might think of it as a positive ion—and it moves along in a direction opposite to the flow of electrons.

When most of the charge carriers are electrons, the semiconductor is called N-type, because electrons are negatively charged. When most of the charge carriers are holes, the semiconducting material is known as P-type because holes have a positive electric charge. But P-type material does pass some electrons, and N-type material car- ries some holes. In a semiconductor, the more abundant type of charge carrier is called the majority carrier. The less abundant kind is known as the minority carrier. Semiconductors are used in diodes, transistors, and integrated circuits in almost limitless variety. These substances are what make it possible for you to have a computer in a briefcase. That notebook computer, if it used vacuum tubes, would occupy a sky- scraper, because it has billions of electronic components. It would also need its own power plant, and would cost thousands of dollars in electric bills every day. But the cir- cuits are etched microscopically onto semiconducting wafers, greatly reducing the size and power requirements.

RESISTORS

Some substances, such as carbon, conduct electricity fairly well but not really well. The conductivity can be changed by adding impurities like clay to a carbon paste, or by wind- ing a thin wire into a coil. Electrical components made in this way are called resistors. They are important in electronic circuits because they allow for the control of current flow. Resistors can be manufactured to have exact characteristics. Imagine telling each person in the line that they must pass a certain number of balls per minute. This is anal- ogous to creating a resistor with a certain value of electrical resistance. The better a resistor conducts, the lower its resistance; the worse it conducts, the higher the resistance.
Resistors 13
1-5 In a conductor, electrons are passed from atom to atom.
Electrical resistance is measured in units called ohms. The higher the value in ohms, the greater the resistance, and the more difficult it becomes for current to flow. For wires, the resistance is sometimes specified in terms of ohms per foot or ohms per kilometer. In an electrical system, it is usually desirable to have as low a resistance, or ohmic value, as possible. This is because resistance converts electrical energy into heat. Thick wires and high voltages reduce this resistance loss in long-distance electrical lines. This is why such gigantic towers, with dangerous voltages, are necessary in large utility systems.

INSULATORS

If the people refuse to pass balls along the line in the previous example, the line repre- sents an electrical insulator. Such substances prevent electrical currents from flowing, except possibly in very small amounts. Most gases are good electrical insulators. Glass, dry wood, paper, and plastics are other examples. Pure water is a good electrical insulator, although it conducts some current with even the slightest impurity. Metal oxides can be good insulators, even though the metal in pure form is a good conductor.

INSULATORS

Electrical insulators can be forced to carry current. Ionization can take place; when electrons are stripped away from their atoms, they have no choice but to move along. Sometimes an insulating material gets charred, or melts down, or gets perforated by a spark. Then its insulating properties are lost, and some electrons flow. An insulating material is sometimes called a dielectric. This term arises from the fact that it keeps electrical charges apart, preventing the flow of electrons that would equalize a charge difference between two places. Excellent insulating materials can be used to advantage in certain electrical components such as capacitors, where it is im- portant that electrons not flow. Porcelain or glass can be used in electrical systems to keep short circuits from oc- curring. These devices, called insulators, come in various shapes and sizes for different applications. You can see them on high-voltage utility poles and towers. They hold the wire up without running the risk of a short circuit with the tower or a slow discharge through a wet wooden pole.

CONDUCTORS

In some materials, electrons move easily from atom to atom. In others, the electrons move with difficulty. And in some materials, it is almost impossible to get them to move. An electrical conductor is a substance in which the electrons are mobile. The best conductor at room temperature is pure elemental silver. Copper and alu- minum are also excellent electrical conductors. Iron, steel, and various other metals are fair to good conductors of electricity. In most electrical circuits and systems, copper or aluminum wire is used. Silver is impractical because of its high cost. Some liquids are good electrical conductors. Mercury is one example. Salt water is a fair conductor. Gases are, in general, poor conductors of electricity. This is because the atoms or molecules are usually too far apart to allow a free exchange of electrons. But if a gas be- comes ionized, it is a fair conductor of electricity. Electrons in a conductor do not move in a steady stream, like molecules of water through a garden hose. Instead, they are passed from one atom to another right next to it (Fig. 1-5). This happens to countless atoms all the time. As a result, literally trillions of electrons pass a given point each second in a typical electrical circuit. You might imagine a long line of people, each one constantly passing a ball to the neighbor on the right. If there are plenty of balls all along the line, and if everyone keeps passing balls along as they come, the result will be a steady stream of balls moving along the line. This represents a good conductor. If the people become tired or lazy, and do not feel much like passing the balls along, the rate of flow will decrease. The conductor is no longer very good.

MOLECULES

When atoms of elements join together to form a compound, the resulting particles are molecules. Figure 1-3 is an example of a molecule of water, consisting of three atoms put together. The natural form of an element is also known as its molecule. Oxygen tends to occur in pairs most of the time in the earth’s atmosphere. Thus, an oxygen molecule is some- times denoted by the symbol O2. The “O” represents oxygen, and the subscript 2 indi- cates that there are two atoms per molecule. The water molecule is symbolized H2O, because there are two atoms of hydrogen and one atom of oxygen in each molecule.
Sometimes oxygen atoms are by themselves; then we denote the molecule simply as O. Sometimes there are three atoms of oxygen grouped together. This is the gas called ozone, that has received much attention lately in environmental news. It is written O3. All matter, whether it is solid, liquid, or gas, is made of molecules. These particles are always moving. The speed with which they move depends on the temperature. The hotter the temperature, the more rapidly the molecules move around. In a solid, the molecules are interlocked in a sort of rigid pattern, although they vibrate continuously  In a liquid, they slither and slide around . In a gas, they are lit- erally whizzing all over the place, bumping into each other and into solids and liquids adjacent to the gas .

COMPOUNDS

Different elements can join together to share electrons. When this happens, the result is a chemical compound. One of the most common compounds is water, the result of two hydrogen atoms joining with an atom of oxygen. There are literally thousands of dif- ferent chemical compounds that occur in nature.
A compound is different than a simple mixture of elements. If hydrogen and oxy- gen are mixed, the result is a colorless, odorless gas, just like either element is a gas separately. A spark, however, will cause the molecules to join together; this will liber- ate energy in the form of light and heat. Under the right conditions, there will be a vi- olent explosion, because the two elements join eagerly. Water is chemically illustrated in Compounds often, but not always, appear greatly different from any of the ele- ments that make them up. At room temperature and pressure, both hydrogen and oxy- gen are gases. But water under the same conditions is a liquid. If it gets a few tens of degrees colder, water turns solid at standard pressure. If it gets hot enough, water be- comes a gas, odorless and colorless, just like hydrogen or oxygen. Another common example of a compound is rust. This forms when iron joins with oxygen. While iron is a dull gray solid and oxygen is a gas, rust is a maroon-red or brownish powder, completely unlike either of the elements from which it is formed.

 
 
 
 
 
A good example of an ionized substance is the atmosphere of the earth at high altitudes. The ultraviolet radiation from the sun, as well as high-speed subatomic par- ticles from space, result in the gases’ atoms being stripped of electrons. The ionized gases tend to be found in layers at certain altitudes. These layers are responsible for long-distance radio communications at some frequencies. Ionized materials generally conduct electricity quite well, even if the substance is normally not a good conductor. Ionized air makes it possible for a lightning stroke to take place, for example. The ionization, caused by a powerful electric field, occurs along a jagged, narrow channel, as you have surely seen. After the lightning flash, the nuclei of the atoms quickly attract stray electrons back, and the air becomes electrically neu- tral again. An element might be both an ion and an isotope different from the usual isotope. For example, an atom of carbon might have eight neutrons rather than the usual six, thus being the isotope C14, and it might have been stripped of an electron, giving it a positive unit electric charge and making it an ion.

ATOMIC WEIGTS ATOMIC NUMBERS

 Atomic numbers and weights.

Element name     Abbreviation     Atomic number       Atomic weight*

Actinium                                     Ac                     89                            227

Aluminum                                   Al                     13                              27

Americium**                              Am                   95                             243

Antimony                                     Sb                    51                             121

Argon                                           Ar                    18                               40

Arsenic                                         As                    33                               75

Astatine                                        At                     85                             210

Barium                                         Ba                     56                             138

Berkelium**                                Bk                     97                             247

Beryllium                                     Be                       4                                 9

Bismuth                                        Bi                      83                             209

Boron                                            B                        5                               11

Bromine                                        Br                      35                               79

Cadmium                                     Cd                      48                             114

Calcium                                        Ca                      20                               40

Californium**                              Cf                       98                             251

Carbon                                           C                        6                               12

Cerium                                          Ce                      58                              140

Cesium                                          Cs                      55                               133

Chlorine                                        Cl                       17                                 35

Chromium                                    Cr                       24                                52

Cobalt                                           Co                      27                                59

Copper                                         Cu                      29                                63

Curium**                                    Cm                     96                              247

 Dysprosium                                Dy                      66                             164

 Einsteinium**                            Es                     99                              254

 Erbium                                       Er                      68                              166

 Europium                                   Eu                     63                               153

 Fermium                                    Fm                   100                              257

 Fluorine                                      F                         9                                 19

Francium                                    Fr                      87                                223

Gadolinium                                Gd                     64                                158

 Gallium                                      Ga                     31                                  69

Germanium                                Ge                     32                                 74

 Gold                                            Au                     79                                 197

 Hafnium                                      Hf                      72                                 180

 Helium                                         He                        2                                   4

Holmium                                      Ho                     67                                  165

Hydrogen                                       H                        1                                    1

Indium                                           In                       49                                 115

Iodine                                              I                        53                                127

Iridium                                          Ir                        77                                193

Iron                                                Fe                       26                                 56

Krypton                                         Kr                       36                                 84

Lanthanum                                   La                      57                                139

Lawrencium**                              Lr or Lw            103                              257

IONS

If an atom has more or less electrons than neutrons, that atom acquires an electrical charge. A shortage of electrons results in positive charge; an excess of electrons gives a negative charge. The element’s identity remains the same, no matter how great the ex- cess or shortage of electrons. In the extreme case, all the electrons might be removed

ELECTRONS

Surrounding the nucleus of an atom are particles having opposite electric charge from the protons. These are the electrons. Physicists arbitrarily call the electrons’ charge negative, and the protons’ charge positive. An electron has exactly the same charge quantity as a proton, but with opposite polarity. The charge on a single elec- tron or proton is the smallest possible electric charge. All charges, no matter how great, are multiples of this unit charge. One of the earliest ideas about the atom pictured the electrons embedded in the nu- cleus, like raisins in a cake. Later, the electrons were seen as orbiting the nucleus, mak- ing the atom like a miniature solar system with the electrons as the planets (Fig. 1-1). Still later, this view was modified further. Today, the electrons are seen as so fast- moving, with patterns so complex, that it is not even possible to pinpoint them at any given instant of time. All that can be done is to say that an electron will just as likely be inside a certain sphere as outside. These spheres are known as electron shells. Their centers correspond to the position of the atomic nucleus. The farther away from the nucleus the shell, the more energy the electron . Electrons can move rather easily from one atom to another in some materials. In other substances, it is difficult to get electrons to move. But in any case, it is far easier to move electrons than it is to move protons. Electricity almost always results, in some way, from the motion of electrons in a material. Electrons are much lighter than protons or neutrons. In fact, compared to the nu- cleus of an atom, the electrons weigh practically nothing. Generally, the number of electrons in an atom is the same as the number of protons. The negative charges therefore exactly cancel out the positive ones, and the atom is electrically neutral. But under some conditions, there can be an excess or shortage of electrons. High levels of radiant energy, extreme heat, or the presence of an electric field (discussed later) can “knock” or “throw” electrons loose from atoms, upsetting the balance.

ISOTOPES AND ATOMIC WEIGHTS

For a given element, such as oxygen, the number of neutrons can vary. But no matter what the number of neutrons, the element keeps its identity, based on the atomic num- ber. Differing numbers of neutrons result in various isotopes for a given element. Each element has one particular isotope that is most often found in nature. But all elements have numerous isotopes. Changing the number of neutrons in an element’s nucleus results in a difference in the weight, and also a difference in the density, of the element. Thus, hydrogen containing a neutron or two in the nucleus, along with the pro- ton, is called heavy hydrogen. The atomic weight of an element is approximately equal to the sum of the num- ber of protons and the number of neutrons in the nucleus. Common carbon has an atomic weight of about 12, and is called carbon 12 or C12. But sometimes it has an atomic weight of about 14, and is known as carbon 14 or C14. Table 1-1 lists all the known elements in alphabetical order, with atomic numbers in one column, and atomic weights of the most common isotopes in another column. The standard abbreviations are also shown.

PROTON NEUTRON AND ATOMIC NUMBER

The part of an atom that gives an element its identity is the nucleus. It is made up of two kinds of particles, the proton and the neutron. These are extremely dense. A tea- spoonful of either of these particles, packed tightly together, would weigh tons. Protons and neutrons have just about the same mass, but the proton has an electric charge while the neutron does not. The simplest element, hydrogen, has a nucleus made up of only one proton; there are usually no neutrons. This is the most common element in the universe. Sometimes a nucleus of hydrogen has a neutron or two along with the proton, but this does not occur very often. These “mutant” forms of hydrogen do, nonetheless, play significant roles in atomic physics. The second most abundant element is helium. Usually, this atom has a nucleus with two protons and two neutrons. Hydrogen is changed into helium inside the sun, and in the process, energy is given off. This makes the sun shine. The process, called fusion, is also responsible for the terrific explosive force of a hydrogen bomb. Every proton in the universe is just like every other. Neutrons are all alike, too. The number of protons in an element’s nucleus, the atomic number, gives that element its identity. The element with three protons is lithium, a light metal that reacts easily with gases such as oxygen or chlorine. The element with four protons is beryllium, also a metal. In general, as the number of protons in an element’s nucleus increases, the num- ber of neutrons also increases. Elements with high atomic numbers, like lead, are there- fore much denser than elements with low atomic numbers, like carbon. Perhaps you’ve compared a lead sinker with a piece of coal of similar size, and noticed this difference.