Current Electricity Short answer questions
Table of Contents
How does the relaxation time of electron in the conductor change when temperature of the conductor decreases.
Ans: When temperature of the conductor decreases, ionic vibration in the conductor decreases so relaxation time increases.
Sketch a graph showing variation of resistivity with temperature of (i) Copper (ii) Carbon.
The emf of the driver cell (Auxiliary battery) in the potentiometer experiment should be greater than emf of the cell to be determined. Why ?
Ans: If emf of a driver cell is less, then null point will not be obtained on the potentiometer wire.
Two wire one of copper and other of manganin have same resistance and equal length. Which wire is thicker ?
Resistance of a conductor is given by, 
here, R and L is same for copper and manganin
Since, 
So, 
i.e. Manganin is thicker
V-I graph for a metallic wire at two different temperatures T1 and T2 is as shown in the figure. Which of the two temperatures is higher and why ?
Why is the potentiometer preferred to a voltmeter for measuring emf of a cell ?
Ans: Emf measured by the potentiometer is more accurate because
- (i) the cell is in open circuit giving no current.
- (ii) It is null method so no loss of energy.
- (iii) current supplied to drive potentiometer circuit is supplied by different battery so actual measurement of cell emf is done.
Why is copper not used for making potentiometer wires ? OR Why is Manganin and Constantan wire used for making potentiometer wires ?
Answer: Copper has a high temperature coefficient of resistance and is therefore not preferred for making potentiometer wires. Materials with a high temperature coefficient of resistance show greater variation in resistance with a slight variation in temperature, according to the following relationship:
Variation in resistance of a wire due to change in temperature, 
(temperature coefficient of resistance)
For higher value of 
there will be large variation in 
for small change in temperature 
.
During the experiment, there may be a time delay that causes the wire to heat up (an increase in wire temperature). which can cause an experimental error by changing the resistance of the potentiometer wire.
Potentiometer wire is made from manganin and constantan wire because they have a lower value of (the temperature coefficient of resistance).
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