NCERT Physics Class 12 Exemplar Chapter 3 Current Electricity Part 1
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Multiple Choice Questions
Q 1.Consider a current carrying wire (current I) in the shape of a circle. Note that as the current progresses along the wire, the direction of j (current density) changes in an exact manner, while the current I remain unaffected. The agent that is essentially responsible for is
(a) Source of emf.
(b) Electric field produced by charges accumulated on the surface of wire.
(c) The charges just behind a given segment of wire which push them just the right way by repulsion.
(d) The charges ahead.
Ans:

Key concept Current per unit area (taken normal to the current), I/A, is called current density and is denoted by

The SI unit of the current density is. The current density is also directed along E and which is also a vector quantity and the relationship is given by

Where conductivity and p resistivity or specific resistance of the substance.

The changes due o the electric field produced by charges accumulated on the surface of wire.
Q.2 Two batteries of emf and and internal resistances and respectively are connected in parallel as shown in Fig.
(a) The equivalent emf of the two cells is between and, i.e.
(b) The equivalent emf is smaller than.
(c) The is given by always.
(d) is independent of internal resistances and .
Ans: (a)
The equivalent emf of the two cells is between and, i.e.
Q.3 A resistance R is to be measured using a meter bridge. Student chooses the standard resistance S to be 100Ω. He finds the null point at l 1 = 2.9 cm. He is told to attempt to improve the accuracy. Which of the following is a useful way?
(a) He should measure more accurately.
(b) He should change S to and repeat the experiment.
(c) He should change S to and repeat the experiment.
(d) He should give up

In this problem, the concept of balanced Wheatstone bridge is to be used.

Condition of balanced wheat stone bridge: The Bridge is said to be balanced if the ratio of the resistances in same branch is equal

Wheatstone bridge is an arrangement of four resistances which can be used to measure one unknown resistance of them in terms of rest.

The percentage error in R can be minimized by adjusting the balance point near the middle of the bridge, i.e., when I, is close to 50 cm. This requires a suitable choice of S.

Since,

Since here,

Then the value of S is nearly 33 times to that of R. In order to make this ratio 1:1, it is necessary to reduce the value of S nearly 1/33 times, i.e., nearly.
Q 4. Two cells of emf’s approximately and are to be accurately compared using a potentiometer of length 400cm.
(a) The battery that runs the potentiometer should have voltage of 8V.
(b) The battery of potentiometer can have a voltage of and R adjusted so that the potential drop across the wire slightly exceeds 10V.
(c) The first portion of 50 cm of wire itself should have a potential drop of.
(d) Potentiometer is usually used for comparing resistances and not voltages.
Ans: (b)

The potential drop along the wires of potentiometer should be greater than emf’s of cells.

In a potentiometer experiment, the emf of a cell can be measured if the potential drop along the potentiometer wire is more than the emf of the cell to be determined.

Here, values of emf’s of two cells are given as 5 V and 10 V, therefore, the potential drop along the potentiometer wire must be more than 10 V.