NCERT Physics Class 12 Exemplar Chapter 3 Current Electricity Part 6

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Q.27 Suppose there is a circuit consisting of only resistances and batteries. Suppose one is to double (or increase it to n-times) all voltages and all resistances. Show that currents are unaltered. Do this for circuit of Example 3.7 in the NCERT Text Book for Class XII.


  • We can think of reducing entire network to a simple one for any branch R as shown in Fig.

  • Dimensionally has a dimension of voltage and has a dimension of resistance.

  • Therefore if all are increased n-fold and.

  • Current thus remains the same.

Long Answer

Q.28 Two cells of voltage and and internal resistances and respectively, are connected in parallel with the positive end of battery connected to negative pole of battery Find the effective voltage and effective resistance of the combination.

Image of two cells of voltage respectivelu

Image of Two Cells of Voltage Respectivelu

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Applying Kirchhoff’s junction rule:

Kirchhoff’s loop rule gives:



Image of Kirchhoffis loop rule

Image of Kirchhoffis Loop Rule

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Q.29 A room has AC run for 5 hours a day at a voltage of. The wiring of the room consists of Cu of 1 mm radius and a length of 10 m. Power consumption per day is 10 commercial units. What fraction of it goes in the joule heating in wires? What would happen if the wiring is made of aluminium of the same dimensions?


Power consumption

Power loss in wire

Power loss in Al wire

Q.30 In an experiment with a potentiometer,. R is adjusted to be (Fig. 3.9). A student wanting to measure voltage of a battery (approx. 8V) finds no null point possible. He then diminishes R to and is able to locate the null point on the last (4th) segment of the potentiometer. Find the resistance of the potentiometer wire and potential drop per unit length across the wire in the second case.

Image of experiment potentiometer

Image of Experiment Potentiometer

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Let be the resistance of the potentiometer wire.

  • Any between and will achieve.

  • Potential drop across cm of wire

  • Potential drop across cm of wire.


Q.31 (a) Consider circuit in figure How much energy is absorbed by electrons from the initial state of no current (ignore thermal motion) to the state of drift velocity?

Consider circuit in absorbed by electrons

Consider Circuit in Absorbed by Electrons

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(b) Electrons give up energy at the rate of per second to the thermal energy. What time scale would one associate with energy in problem (a)? , length of circuit, cross-section

  1. Ohmic loss

All of KE of electrons would be lost in