NCERT Physics Class 12 Exemplar Chapter 3 Current Electricity Part 2
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Q 5. A metal rod of length 10 cm and a rectangular crosssection of is connected to a battery across opposite faces. The resistance will be
(a) Maximum when the battery is connected across faces.
(b) Maximum when the battery is connected across faces.
(c) Maximum when the battery is connected across faces.
(d) Same irrespective of the three faces.
Ans: (a)

The resistance of a wire depends on various parameter, its area, material (resistivity) and length (length of the rod). Here, the metallic rod behaves as a wire.

Relationship between resistance and various parameters is given by.
The resistance of a wire is given by
For greater value of R. I must be higher and A should be lower and it is possible only when the battery is connected across cm (area of crosssection A).
Q 6 Which of the following characteristics of electrons determines the current in a conductor?
(a) Drift velocity alone.
(b) Thermal velocity alone.
(c) Both drift velocity and thermal velocity
(d) Neither drift nor thermal velocity.
Ans: A

The characteristic of an electron that determines the current in a conductor is drift velocity alone and not thermal velocity.

Drift velocity is the average velocity with which the free electrons get drifted towards the positive end of the conductor under the influence of an external electric field.

On the other hand thermal velocity of electron depends upon the temperature.

Drift velocity is related to current as:

Where A is area of crosssection of the conductor wire, e the charge on each electron and n the number density of electrons.

On the other hand Thermal velocity is given by:

Where me is the electron mass, the Boltzmann constant and T temperature Thus thermal velocity is depended on temperature not the current
One or More Than One Correct Answer Type
Q 7. Kirchhoff’s junction rule is a reflection of
(a) Conservation of current density vector.
(b) Conservation of charge.
(c) The fact that the momentum with which a charged particle approaches a junction is unchanged (as a vector) as the charged particle leaves the junction.
(d) The fact that there is no accumulation of charges at a junction.
Ans: (b), (d)

Conservation of charge.

The fact that there is no accumulation of charges at a junction.
Q .8 Consider a simple circuit shown in Fig.
(a) Potential drop across AB is nearly constant as is varied.
(b) Current through is nearly a constant as R′ is varied.
(c) Current I depend sensitively on
(d) always.
Parallel grouping

Samc potential difference appeared across each resistance but currant distributes in the reverse ratio of their resistance, i.e. In this problem, the potential drop is taking place across AB and r Since the equivalent resistance of parallel combination of R and R’ is always less than

R, therefore current will be greater than or equal to) always.

Important point: In parallel combination of resistances, the equivalent resistance is smallest than smallest resistance present in combination.