Physics Test 57

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Physics Test 57

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Weekly Quiz Competition

Question 1
4 / -1

The electrical conductivity of a semiconductor increases when electromagnetic radiation of wavelength shorter than 2480 nm is incident on it. The band gap (in eV) for the semiconductor is

A
0.9

B
0.7

C
0.5

D
1.1

Solution
Question 2
4 / -1

The primary of a transformer has 400 turns, while the secondary has 2000 turns. If the power output from the secondary at 1000V is 12kW, then what is the primary voltage?

A
200V

B
300V

C
400V

D
500V

Solution

Turns of the primary transformer =400

Turns of the secondary transformer =2000

The relation between voltage drop of a coil and number in terms is given as −N₁/N₂ = V₁/V₂

Where, N is the number of turns in the coil =400, and

V is the voltage across the coil =2000

Therefore,

1000/V₂= 2000/400

V=200

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

A smooth sphere "A" is moving on a frictionless horizontal surface with angular speed ω and centre of mass velocity v. It collides elastically and head-on with an identical sphere B at rest. Neglect friction everywhere. After the collision, their angular speeds are ω_A and ω_B, respectively. Then,

A
ω_A < ω_B

B
ω_A = ω_B

C
ω_A = ω

D
ω_B = ω

Solution

As the spheres are smooth there will be no friction (no torque) and therefore there will be no transfer of angular momentum.

Thus A, after collision will remain with its initial angular momentum

i.e., ω_A= ω

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

A block of mass M is resting on an inclined plane as shown in the figure. The inclination of the plane to the horizontal is gradually increased. It is found that when the angle of inclination is θ, the block just begins to slide down the plane. What is the minimum force F applied parallel to the plane that would just make the block move up the plane?

A
M q sinθ

B
M g cosθ

C
2M a cosθ

D
2M q sinθ

Solution

The component of weight mg of the block along the inclined plane =Mgsinθ

The minimum frictional force to be overcome is also =Mgsinθ

To make the block just move up the plane the minimum force applied must overcome the component Mgsinθ of gravitational force as well as the frictional force Mgsin⁡θ = 2Mgsin⁡θ

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

An equilateral triangular prism is made of glass (μ=1.5). A ray of light is incident normally on one of its faces. The angle between the incident and emergent rays is

A
180⁰

B
120⁰

C
60⁰

D
30⁰

Solution
Question 6
4 / -1

An a-particle of energy 5MeV is scattered through 180^∘ by a fixed uranium nucleus. The distance of the closest approach is of the order of

A

B
10^-10 cm

C
10^-12 cm

D
10^-15 cm

Solution
Question 7
4 / -1

A body of mass m is dropped from a certain height. It has a velocity v when it is at a height h above the ground. Which of the following will remain constant during the free fall?

A
v² = 2gh

B
v² - 2gh

C

D
v - 2√gh

Solution

This is the final speed of an object which falls from a height h with no initial speed to the ground, provided there is no air resistance. So, in this case, it is true.

g is the gravitaitional acceleration of the planet the object is on. Of course, the heavier the planet (or a moon, or a star, or even you), so, objects fall faster towards it. That is why objects fall

much faster on Earth than on the Moon.

Since, there is no air resistance, the object doesn't lose or gain energy. So the total energy remains the same.

At the height h, the object has a potential energy = m×g×h

Where, m is the mass of the object

On the ground, the whole potential energy is converted into kinetic energy = (m×v²) / 2

Then, since the total energy is conserved, we have m×g×h =(m×v²) / 2

So, v²= 2×g×h

Hence, the correct option is (A).
Question 8
4 / -1

When the plane surface of a plano-convex lens of refractive index 1.5 is silvered, it behaves like a concave mirror of focal length 30cm. When its convex surface is silvered, it will behave like a concave mirror of focal length

A
10cm

B
20cm

C
30cm

D
45cm

Solution
Question 9
4 / -1

A vessel contains a mixture of one mole of oxygen and two moles of nitrogen at 300 K. The ratio of the rotational kinetic energy per O₂ molecule to that per N₂ molecule is

A
1 : 1

B
1 : 2

C
2 : 1

D
It depends on the moment of inertia of the two molecules

Solution

The law of euipartition of energy states that 'For a dynamical system in thermal equilibrium, the energy of a system is equally distributed among its various degrees of freedom and the

energy associated with each degree of freedom per molecule is 1 / 2k × T

In this case, O₂ and N₂ both have two degrees of rotational kinetic energy and since the temperature is also same, the ratio of the average rotational kinetic energy is 1:1

Hence, the correct option is (A).
Question 10
4 / -1

Two particles X and Y, having equal charges, after being accelerated through the same potential difference, enter a region of uniform magnetic field and describe circular paths of radii R₁ and R₂, respectively. The ratio of mass of X to that of Y is

A
(R₁/R₂)^1/2

B
R₁/R₂

C
(R₁/R₂)²

D
R₁/R₂

Solution