# NCERT Physics Class 12 Exemplar Chapter 3 Current Electricity Part 3 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Q . 9 Temperature dependence of resistivity of semiconductors, insulators and metals is significantly based on the following factors:

(a) Number of charge carriers can change with temperature T.

(b) Time interval between two successive collisions can depend on T.

(c) Length of material can be a function of T.

(d) Mass of carriers is a function of T.

Ans: (a, b)

- Resistivity is the intrinsic property of the substance. For a metallic conductor, resistivity is given by

- where
*n*is the number of charge carriers per unit volume (number density) which can change with temperature*T*and r is relaxation time (time interval between two successive collisions) which decreases with the increase of temperature

Q. 10 The measurement of an unknown resistance R is to be carried out using Wheatstone bridge (see Fig. of NCERT Book) . Two students perform an experiment in two ways. The first students takes and The other student takes and . In the standard arm, both take . Both find within errors.

(a) The errors of measurement of the two students are the same.

(b) Errors of measurement do depend on the accuracy with which and can be measured.

(c) If the student uses large values of and , the currents through the arms will be feeble. This will make determination of null point accurately more difficult.

(d) Wheatstone bridge is a very accurate instrument and has no errors of measurement.

Ans: (b, c) ′

Wheatstone bridge:

- Wheatstone bridge is an arrangement of four resistances which can be used to measure one of them in terms of rest.
- Her arms AB and BC are called ratio arm and arms AC and BD are called conjugate arms.

*Balanced bridge*: The Bridge is said to be balanced when deflection in galvanometer is zero, i.e.. no currant flows through the galvanometer or in other words*V1, V11*. In the balanced condition on mutually changing the position of cell and galvanometer this condition will not change.*Unbalanced bridge*: lf the bridge is not balanced current will flow from D to B if . i.e.. . .*Which*gives .- According to the problem for first student,
- For second student,
- Let us take
- Now, according to Wheatstone bridge rule.

- Now putting all the values in above equation, we get for both students. Thus, we can analyze that the Wheatstone bridge is most sensitive and accurate if resistances are of same value.
- Thus, the errors of measurement of the two students depend on the accuracy and sensitivity of the bridge, which in turn depends on the accuracy with which and can be measured.
- The currents through the arms of bridge are very weak, when and are larger.
- This can make the determination of null point accurately more difficult.

Q. 11 In a meter bridge the point D is a neutral point.

(a) The meter bridge can have no other neutral point for this set of resistances.

(b) When the jockey contacts a point on meter wire left of D, current flows to B from the wire.

(c) When the jockey contacts a point on the meter wire to the right of D, current flows from B to the wire through galvanometer.

(d) When R is increased, the neutral point shifts to left.

Ans: (a) , (c)

- The meter bridge can have no other neutral point for this set of resistances.
- When the jockey contacts a point on the meter wire to the right of D, current flows from B to the wire through galvanometer.

## Very Short Answer

Q. 12 Is the motion of a charge across junction momentum conserving? Why or why not?

Ans:

- When an electron approaches a junction, in addition to the uniform E that it normally faces (which keep the drift velocity fixed) , there are accumulation of charges on the surface of wires at the junction.
- These produce electric field. These fields alter direction of momentum.