Short Note OF SSC JE

Q. 1. Why does a positive charge attract a negative charge?

1Ans. It is the tendency of everybody to have minimum electric potential i.e., to obtain zero potential. A positive charge means a deficit of electrons while a negative charge indicates an excess of electrons. Consequently, positive and negative charges attract each other to have minimum electric potential.


Q. 2. A wire is carrying current. Is it charged?

Ans. No. The current in a wire is due to the drifting of free electrons in a definite direction. But the number of electrons in the wire at any instant is equal to the number of protons. Hence, the net charge on the wire is zero.


Q. 3. What is a resistor? What is its function?

Ans. A component whose function in a circuit is to provide a specified value of resistance is called a resistor. Its function is to limit current, divide voltage, and in certain cases generate heat.


Q. 4. What are the fundamental differences between e.m.f and p.d.?

Ans. The following are the differences between e.m.f. and p.d. :

(I) The name e.m.f. at first, sight implies that it is a force that causes current to flow. But this is not correct because it is not a force but energy supplied to charge by some active device such as a battery.
(ii) E.M.F. maintains p.d. while p.d. causes current to flow.

(iii) When we say that e.m.f. of a device (e.g., a cell) is 2V, it means that the device supplies the energy of 2 joules to each coulomb Of charge. when we say that a p.d. between points A and B of a circuit (suppose point A is at higher potential ) is 2V, it means that each coulomb of charge Will gives up the energy of 2 joules in moving from A to B.

Q•5. Why is a carbon resistor given a color code?

Ans. The physical size of a carbon resistor is very small. Therefore. it is more convenient to use color code indicating, the resistance value than to imprint the numerical value on the body of the resistor.


Q• 6. We know that a large number of free electrons are present in metals. Why is no current established in the absence of an electric field?

Ans. In the absence of an electric field. the free electrons in the metal have e random motions, i.e., free electrons move in all directions haphazardly. During motion, they collide with positive ions of the metal again and again after each collision. their direction changes. The result is that the net motion in any particular direction is zero. however, when an electric field is applied, the free electrons experience a force and start drifting towards the positive terminal of the source with a small velocity (drift velocity).


Q.7. Is ohm s lass true for all conductors?

Ans. No, it is true only for metallic conductors provided physical conditions do not change.


Q. 8. Can Ohm’s law verify using a filament lamp?

Ans. No. As the voltage across the lamp is varied. the temperature of the filament also changes. This in turn changes the resistance of the filament. The essence of Ohm’s lass is that relation between V and I is linear if R is independent of the magnitude of V.

Q. 9. Why do conductors have positive temperature co-efficient of resistance?

Ans. The rise in temperature of a substance causes two effects :
(1) It causes the molecules to vibrate more rapidly, thus impeding the movement of free electrons through the substance.
(ii) It may break some bonds, releasing free electrons.
With the rise in temperature of a conductor (e.g., metals), there is no increase in the number of free electrons and the sole effect of temperature rise is to increase resistance due to the increased molecular vibration. Hence conductors have a positive temperature coefficient of resistance i.e., their resistance increases with the rise in temperature.


Q. 10. Why do insulators have a negative temperature coefficient of resistance?

Ans. A temperature rise in insulators creates many more free electrons than existed in the cooler state. Often this increase in the number of free electrons more than offsets the interference to the drift movement caused by the increased molecular activity. Hence the resistance of insulators (and for the same reason that of semiconductors) decreases with the increase in temperature i.e., their insulators have a negative temperature coefficient of resistance.


Q. 11. Why do some high resistance alloys (Eureka, manganin, etc), have almost zero temperature co-efficient of resistance?

Ans. In the case of some high resistance alloys (e.g., Eureka, Manganin, Constantan, etc.), the rise of temperature causes a negligible increase in resistance i.e., they have almost zero temperature coefficient of resistance. It is because the two effects of temperature almost ‘cancel each other. The rise in temperature of a substance causes two effects :
(1) It causes the molecules to vibrate more rapidly, thus impeding the movement of free electrons through the substance.
(ii) It may break some bonds, releasing free electrons.
With the rise in temperature of a conductor (e.g., metals), there is no increase in the number of free electrons and the sole effect of temperature rise is to increase resistance due to the increased molecular vibration. Hence conductors have a positive temperature coefficient of resistance i.e., their resistance increases with the rise in temperature.

Q. 12. What happens if the power rating of a machine is exceeded?

Ans. Every electrical equipment has a power rating marked on its body. While the equipment is in operation. care should be taken that the power rating is not exceeded otherwise the equipment may be damaged or burnt due to excessive heat.


Q. 13. Why does a 100-watt bulb give more light than a 60-watt bulb?

Ans. A 100-watt bulb means that it is giving out 100 joules of energy per second while a 60-watt bulb indicates the release of 60 joules of energy per second. Clearly. a 100-watt bulb will give more light than a 60-watt bulb.


Q. 14. How is work done in an electric circuit?

Ans. When voltage is applied in a circuit. it causes current (i.e., movement of free electrons) to flow through it. Clearly, work is being done in moving the electrons in the circuit.


Q. 15. What is the difference between an overload and a short circuit?

Ans. An overload means that the circuit is carrying more than the rated current. A short circuit is said to occur if the circuit carries an exceedingly high current. In general, if the circuit carries more than.10 times the rated current, then a short circuit should be suspected.


Q. 16. What does an electric circuit do?

Ans. The function of an electric circuit is to convert electrical energy into some other form of energy. For example, electrical energy is converted into heat energy by devising a suitable heater circuit. In fact, the innumerable uses of electricity have been possible only due to the proper use and applications of electric circuits.


Q. 17. Is the formula V = IR true for non-ohmic conductors?

Ans. Yes. This formula defines resistance and not Ohm’s law.


Q. 18. Is matter electrical in nature?

Ans. Yes, the matter is electrical in nature. It is because every matter is made up of atoms and every atom contains charged particles viz protons and electrons.


Q. 19. Name the charge carriers in (i) platinum (ii) a primary cell (iii) germanium (iv) superconductor
.

Ans. (i) Electrons (ii) Positive and negative ions (iii) Holes and electrons (iv) Electrons.

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