Physics Test 218

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Physics Test 218

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Question 1
4 / -1

Hydrogen atom is exited from ground state to another state with principal quantum number equal to 4. Then, the number of spectral lines in the emission spectra will be

A
3

B
5

C
6

D
2

Solution

Number of spectral lines in the emission spectra,
Question 2
4 / -1

The current I drawn from a 5 volt source will be

A
0.4 A

B
0.33 A

C
0.5 A

D
0.67 A

Solution

The equivalent circuit is a balanced Wheatstone's bridge. Hence, no current flows through arm BD.
Question 3
4 / -1

In the circuit shown in figure, A and B are two cells of the same emf, E and of internal resistances r_A and r_B, respectively. L is an ideal inductor and C is an ideal capacitor. The key K is closed. When the current in the circuit becomes steady, what should be the value of R so that the potential difference across the terminals of cell A is zero?

A
R = r_A – r_B, if r_A > r_B

B

C

D
The potential difference between the terminals of cell A will be equal to zero for no value of R.

Solution

When the key, K is inserted, the current starts growing and after some time it acquires a steady value. At this stage, no current flows through the capacitor (because an ideal capacitor offers an infinite resistance to a steady current). All the current flows through the inductor (because an ideal inductor offers zero resistance to a steady current). Now, the network of resistors is a balanced wheatstone bridge. Hence, no current flows through the resistance 2R. Therefore, this resistance can be ignored. The net resistance between points X and Y = resistance of the parallel combination of 2 R and 2

Hence, the current in the circuit is
Question 4
4 / -1

A coil of inductance 0.20 H is connected in series with a switch and a cell of emf 1.6 V. The total resistance of the current is 4.0Ω. What is the initial rate of growth of the current when the switch is closed?

A
0.05 As^-1

B
0.4 As^-1

C
0.13 As^-1

D
8.0 As^-1

Solution

When the switch is closed at(t = 0s), no current flows, voltage drop across the inductor is the same as the supply voltage of 1.6 V.

Voltage equation for the circuit, we have

Where i is the current drawn from the source , at t = 0s, i = 0, we thus have
Question 5
4 / -1

Mass of the earth is 81 times the mass of the moon and the distance between the earth and moon is 60 times the radius of the earth. If R is the radius of the earth, then the distance between the moon and the point on the line joining the moon and earth where the gravitational force becomes zero is

A
30R

B
15R

C
6R

D
5R

Solution

Let d be the distance between the moon and the point at force on mass m is zero then
Question 6
4 / -1

A magnet of magnetic moment M and length 2l is bent at its mid-point such that the angle of bending is 60°. Now, the magnetic moment is

A
M

B
2M

C
M/2

D
M/√2

Solution
Question 7
4 / -1

Three identical uniform rods of the same mass M and length L are arranged in xy plane as shown in the figure. A fourth uniform rod of mass 3 M has been placed as shown in the xy plane. What should be the value of the length of the fourth rod such that the centre of mass of all the four rods lie at the origin?

A
3 L

B
2 L

C

D

Solution

Let 'x' be the length of the 4^th rod and the centre of the mass of all the rods lies at origin then X_cm = 0
Question 8
4 / -1

A metallic wire with tension T and at temperature 30°C vibrates with its fundamental frequency of 1 kHz. The same wire with the same tension but at 10°C temperature vibrates with a fundamental frequency of 1.001 kHz. The coefficient of linear expansion of the wire is

A
2 × 10^-4 °C^-1

B
1.5 × 10^-4 °C^-1

C
1 × 10^-4°C^-1

D
0.5 × 10^-4 °C^-1

Solution
Question 9
4 / -1

For a gas, the difference between the two specific heats at constant pressure and constant volume is 4150 J kg^-1 K^-1 and their ratio is 1.4. What is the specific heat of the gas at constant volume in units of J kg^-1 k^-1?

A
8475

B
5186

C
1660

D
10375

Solution

Given: C_p - C_v = 4150 Jkg-1K_-1 and C_p/C_v = 1.4 Or C_p= 1.4 C_v.

Therefore,

1.4 C_v - C_v = 4150

Or C_V = 4150/0.4 = 10375 J kg^-1 K^-1
Question 10
4 / -1

Among the following pairs, which one does not have identical dimensions?

A
Moment of inertia and Moment of a force

B
Work and Torque

C
Angular momentum and Planck's constant

D
Impulse and Momentum

Solution

Moment of inertia (I) = mr²

[I] = [ML²]

Moment of force (C) = rF

[C] = [r][F] = [L][MLT^-2] or [C] = [ML²T^-2]

Moment of inertia and moment of a force do not have identical dimensions.