# NCERT Physics Class 12 Exemplar Chapter 3 Current Electricity Part 4

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Q.13 The relaxation time τ is nearly independent of applied E field whereas it changes significantly with temperature T. First fact is (in part) responsible for Ohm’s law whereas the second fact leads to variation of ρ with temperature. Elaborate why?

Ans:

• Relaxation time is bound to depend on velocities of electrons and ions. Applied electric field affects the velocities of electrons by speeds at the order of 1mm/s, an insignificant effect.

• Change in T, on the other hand, affects velocities at the order of. This can affect significantly.

Q.14 What are the advantages of the null-point method in a Wheatstone bridge? What additional measurements would be required to calculate by any other method?

• The advantage of null point method in a Wheatstone bridge is that the resistance of galvanometer does not affect the balance point and there is no need to determine current in resistances and galvanometer and the internal resistance of a galvanometer.

• can be calculated applying Kirchhoff ‘s rules to the circuit.

• We would need additional accurate measurement of all the currents in resistances and galvanometer and internal resistance of the galvanometer.

Q.15 What is the advantage of using thick metallic strips to join wires in a potentiometer?

• The metal strips have low resistance and need not be counted in the potentiometer length of the null point.

• One measures only their lengths along the straight segments (of lengths 1 meter each).

• This is easily done with the help of centimeter rulings or meter ruler and leads to accurate measurements.

Q.16 For wiring in the home, one uses Cu wires or Al wires. What considerations are involved in this?

Ans:

• Two considerations are required:

• Cost of metal, and

• Good conductivity of metal.

• Cost factor inhibits silver. Cu and Al are the next best conductors.

Q.17 Why are alloys used for making standard resistance coils?

Ans:

• Alloys have low value of temperature co-efficient (less temperature sensitivity) of resistance and high resistivity.

Q.18 Power P is to be delivered to a device via transmission cables having resistance RC. If V is the voltage across R and I the current through it, find the power wasted and how can it be reduced.

Ans:

• Power wasted

• Where is the resistance of the connecting wires?

• In order to reduce, power should be transmitted at high voltage.

Q.19 AB is a potentiometer wire if the value of R is increased, in which direction will the balance point J shift?

Ans:

• If R is increased, the current through the wire will decrease and hence the potential gradient will also decrease, which will result in increase in balance length. So J will shift towards B.

Q.20 While doing an experiment with potentiometer it was found that the deflection is one sided and (i) the deflection decreased while moving from one end A of the wire to the end B; (ii) the deflection increased. While the jockey was moved towards the end B. A B (i) which terminal +or –ve of the cell, is connected at X in case (i) and how is related to E? (ii) Which terminal of the cell is connected at X in case (ii)?