Solved Examples | Current Electricity | Class 12

Q1. What is a thermistor ? Give its two applications.

Ans: A thermistor is semiconducting metal oxide whose resistance varies appreciably with rise of tempe-rature.

Applications of thermistors :

(i) Thermistors with negative temperature coefficients are used to stabilise transistor receiver circuit against undesirable temperature rise.

(ii) Thermistors with positive temperature coefficients are used as temperature controlled switches.

Q2. Why is the use of a potentiometer preferred over that of a voltmeter for measurement of emf of a cell or in comparing the emfs of two cell ?

Ans: Potentiometer is a null method device. No current is drawn from the cell at null point, thus there is no potential drop due to the internal resistance of the cell. So it measures the p.d. in an open circuit. But voltmeter requires a small current for its operation. So voltmeter measures the p.d. in a closed circuit which is less than the actual emf of the cell.

Q3. Explain, giving reasons, how the internal resistance of a cell changes in the following cases :

(a) When concentration of the electrolyte is increased

 (b) When area of the anode is decreased.

(c) When temperature of the electrolyte is increased.


(a) When concentration of the electrolyte is increased, internal resistance of the cell increases.

Reason. In more concentrated electrolyte, inter ionic attractions increase and the movements of the ions become difficult.

(b) When area of the anode is decreased, internal resistance of the cell increases.

Reason. Lesser area of the anode decreases its tendency to attract oppositely charged ions.

(c) When temperature of the electrolyte is increased, internal resistance decreases.

Reason. Both interionic attractions and viscous forces decrease at higher temperature.

Q4. Answer the following :

(a) Why are the connections between the resist, in a metre bridge made of thick copper strips,

(b) Why is it generally preferred to obtain the balance point in the middle of the metre bridge wire ?

(c) Which material is used for the metre bridge wire and why ?


(a) Thick copper strips offer minimum resistance and hence avoid the error due to end resistance which have not be taken into account in the bridge formula.

(b) The metre bridge is most sensitive when the four resistances forming the bridge are equal. For this the balance point must be somewhere near the middle of the wire.

(c) Alloy such as manganin or nichrome which II & low temperature coefficient of resistance and higt resistivity.

Q5. Answer the following questions:

(a) What is the order of magnitude of the resistance of a (dry) human body ?

(b) If the resistance of our body is so large, why does one experience a strong shock (some-times even fatal) one accidentally touches the live wire of, say 240 V supply ?

(c) There is an impression among in that a person touching a high power line gets 'stuck' with the line. Is that true ? Explain.

(d) Currents of the order of 0.1 A through the human body are fatal. What causes the death : heating of the body due to electric current or something else ?

(e) A nerve fiber contains a membrane separating two conducting 'fluids' maintained at a potential difference. What is the order of this potential difference ?


(a) About 10 k , it is mainly due to skin through which current enters and leaves our body.

(b) It is because our body is sensitive to minute currents even as low as a few mA.

(c) This impression is misleading. There is no special attractive force that keeps a person 'stuck' with a high power line. Actually, current of the order of 0.05 A or even much less is enough to disorganise our nervous system. The result is that the affected person may temporarily lose his ability to exercise his nervous control to get himself 'free' from the high voltage point.

(d) The cause of death is not heating, though a person may receive burns if the currents are too large. The cause of death is the interference caused by our highly sensitive nervous system which is basically electrical in nature. External currents cause convulsive actions and especially interfere with the nerve processes related to our heartbeating. Beyond a certain point, this interference is fatal.

(e) About 0.1 V.