Physics Test 61

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Physics Test 61

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Question 1
4 / -1

In a transistor, the value of the base current depends on

A
Base thickness only

B
Bias voltages only

C
Doping levels of emitter, base and collector only

D
All of the above

Solution

In a transistor, the value of the base current depends on base thickness bias voltages and oping levels of the emitter, base, and collector.

Hence, the correct option is (D).
Question 2
4 / -1

A conducting square loop of side L and resistance R moves in its plane with a uniform velocity v perpendicular to one of its sides. A magnetic field B, constant in space and time, pointing perpendicular and into the plane of the loop, exists everywhere as shown in the figure given below. The current induced in the loop is

A
BLv / R clockwise

B
BLv/ R anti-clockwise

C
2BLv / R anti-clockwise

D
Zero

Solution

Since the magnetic field is uniform the flux through the square loop at any time t is constant.

Flux ϕ= BA = costant

Thus induced em f = −dϕ / dt=0

Thus, there is no current induced in the loop.

Hence, the correct option is (D).
Question 3
4 / -1

Two identical spheres A and B lie on a smooth horizontal circular groove at opposite ends of a diameter. A is projected along the groove and at the end of time t, it impinges on B. If e is the coefficient of restitution, the second impact will occur after a time

A
2t / e

B
t / e

C
(πt) / e

D
(2πt) / e

Solution

Before first collision the velocity of approach is v. Separation is πR

Therefore, v×t=πR

t=πR/ v

After first collision:

Velocity of separation: e=v′ / v ⇒ v′ = ev

For second collision:

Velocity of approach is v′ and distance between them is 2πR

v′×t′=2πR

t′=2πR / v′ = 2πR / ev = 2te

Hence, the correct option is (C).
Question 4
4 / -1

Two masses m and 2m are joined to each other by means of a frictionless pulley as shown in the figure below. When the mass 2m is released, the mass m will ascend with an acceleration of

A
g/3

B
g/2

C
g

D
2 g

Solution

The acceleration would be

a=g(2m−m) / m+2m

a=g(m) / 3m

a=g/3

Hence, the correct option is (A).
Question 5
4 / -1

Using mass (M), length (L), time (T) and current (A) as fundamental quantities, the dimensions of permeability are

A
[M^-1LT^-2A]

B
[ML^-2T^-2A^-1]

C
[MLT^-2A^-2]

D
[MLT^-1A^-1]

Solution

Talking about its dimensional formula first we should know that it is equal to the ratio of the intensity of magnetic field ( simply magnetic field ) to the magnetizing field ( i.e. the to which a magnetic field can magnetize a material ). And a unit of magnetizing field is Ampere per meter.
Hence, the correct option is (C).
Question 6
4 / -1

A heavy nucleus at rest breaks into two fragments, which fly off with velocities in the ratio 8:1. The ratio of the radii of the fragments (assumed spherical) is

A
1 : 2

B
1 : 4

C
4 : 1

D
2 : 1

Solution
Question 7
4 / -1

An escalator is moving downwards with a uniform speed u. A man of mass m is running upwards on it at a uniform speed v. If the height of the escalator is h, the work done by the man is going up the escalator is

A
Zero

B
mgh

C
mghu / (v-u)

D
mghv / (v-u)

Solution

The speed of the escalator is: u

The mass of the man is: m

The speed of the man is: v

The height of the escalator is: h

Calculate the relative velocity of the man with respect to the ground.
Question 8
4 / -1

How will the image formed by a convex lens be affected if the central portion of the lens is wrapped in black paper, as shown in the figure?

A
No image will be formed by the remaining portion of the lens.

B
The central portion of the image will be absent.

C
A full image will be formed, but it will be less bright.

D
There will be two images, one due to each exposed half.

Solution

In the convex lens, the rays are converted to a point on the other side lens when the parallel rays are incident on the surface of the lens. If the central portion of the lens wrapped by black paper, the full image will be formed but with less brightness.

Hence, the correct option is (C).
Question 9
4 / -1

Two rods A and B of different materials are welded together as shown in the figure below. If their thermal conductivities are k₁and k₂ respectively, the thermal conductivity of the composite rod will be

A
2 (k₁ + k₂)

B
(k₁ + k₂)

C
3/2 (k₁ + k₂)

D
1/2 (k₁ + k₂)

Solution
Question 10
4 / -1

A potentiometer has a uniform potential gradient. The specific resistance of the material of the potentiometer wire is 10⁻⁷ ohm-meter and the current passing through it is 0.1 ampere, a cross-section of the wire is 10⁻⁶ m². The potential gradient along the potentiometer wire is:

A
10^-6 V/m

B
10^-4 V/m

C
20^-8 V/m

D
10^-2 V/m

Solution