Basic Electrical - Short Question and Answer

TOPIC : Basic Electrical Short Question and Answer 

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

Ans. It is the tendency of every body to have minimum electric potential i.e., to obtain zero potential. A positive charge means deficit of electrons while the negative charge indicates 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. The thermal speeds of free electrons are very large. Inspite of these high speeds, why they fail to escape from the surface of a conductor ?

Ans. The free electrons in a conductor are free only to the extent that they may transfer from one atom to another within the conductor. It is because the free electrons that start at the surface of a conductor find behind them the positive ions pulling them back and none pulling forward .Thus at the surface of a conductor, a free electron encounters forces that prevent it to leave the conductor surface. External energy is required for the free electron to escape the conductor surface.

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 we device such as a battery.
(ii)E.M.F maintains p.d while p.d . cause 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 an 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 give up an energy of joules in moving from A to B.

Q.5. Why are alloys constantan and manganin used to make standard resistances ?

Ans. It is because these alloys have high resistivity and negligibly small temperature co-efficient  of resistance.

Q.6. The drift velocity of free electrons is very small. Why does room light turn on at once as the switch is closed? 

Ans. The electric field is transmitted with the speed of light. As soon as the switch is closed, the electric field is at once established in the whole circuit. As a result, free electrons everywhere in the wire begin drifiting at once.

Q.7. We know that a large number of free electrons are present in metals. Why is no current established in the absence of electric field?

Ans. In the absence of electric field the free electrons in the metal have random motions, i.e., free electrons move in all directions haphazardly. During motion, they collide with positive ions of the metal again and again and 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.8. Is ohm's law true for all conductors?

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

 

Q.9. Does the formula V= IR define ohm's law?

Ans. No. This formula defines resistance and can be applied to any conducting device whether or not it obey's  ohm's law. The essence of ohm's law is that the graph between V and I is linear i.e. the value of R is independent of V.

 

Q.10. Can we verify Ohms law by 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 law is that relation between V and I is linear if R is independent of the magnitude of V .

Q.11. Why do conductors have positive temperature co-efficient of resistance ?

Ans. The rise of temperature of a substance causes two effects:

(i)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 positive temperature co-efficient of resistance i.e., their resistance increases with the rise in temperature.

Q.12. Why do insulators have negative temperature co-efficient of resistance ?

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

Q.13. What happens if power rating of a machine is exceeded? 

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

Q.14. 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 release of 60 joules of energy per second. Clearly, a 100 watt bulb will give more light than a 60 watt bulb.

Q.15. 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.16. 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 short circuit should be suspected.

Q.17. 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.18. Is the formula V = IR true for non-ohmic conductors ?

Ans.Yes. This formula defines resistance and not ohm's law.

Q.19. Is matter electrical in nature ?

Ans. Yes, 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.20. Why do some high resistance alloys (Eureka, manganin etc) have almost zero temperature co-efficient of resistance ?

Ans.In case of some high resistance alloys (e.g.. Eureka, Manganin, Constantan etc.), the rise of temperature causes negligible increase in resistance i.e., they have almost zero temperature co-efficient of resistance. It is because the two effects of temperature (mentioned in answer to Q. 11) almost cancel each other...