Physics (Part-V) (Railway)

Total Questions: 50

31. Which of the following statements is/are true for a DC motor? [RRB Group 'D' CBT Exam, 22.08.2022 (Shift-II)]

(i) The function of the split rings is to reverse the flow of current.

(ii) Maximum force is experienced by the arms of the coil aligned parllel to the magnetic field.

(iii) Reversing current after every half rotation leads to continuous rotation of coil.

(1) Both (i) and (ii)
(2) Only (i)
(3) Both (i) and (iii)
(4) Only (ii)

Correct Answer: (3) Both (i) and (iii)

Solution:The split ring in the electrie motor also known as a commutator reverses the direction of current flowing through the coil after every half rotation of the coil.

• Due to this, the coil continues to rotate in the same direction.

• In a D.C. electric motor, the reversing of current in the coil is repeated after every half rotation.

• This causes the coil to continue its rotation as long as the current from the battery is passed through it. The rotating coil can drive a machine connected to it.

32. A convex lens having power 5D is placed in contact with a concave lens having power -3 D. The focal length of the combination will be: [RRB Group 'D' CBT Exam, 22.08.2022 (Shift-II)]

(1)-50 cm
(2)-0.5 cm
(3) 50 cm
(4) 0.5 cm

Correct Answer: (3) 50 cm

Solution:Power of convex lens, P₁ =+5D

Power of concave lens, P₂=-3D

∴ Power of combinotion, P = P₁ +P₂

=5D -3D =2D

now, the power and focal length are related as:

p = 1/f = 1/f 2 = 1/f

f = 1/2 = 0.5m = 50 cm

⇒ F = 50cm

33. The radius of curvature of a given spherical mirror is - 20cm. The focal length of the mirror is: [RRB Group 'D' CBT Exam, 22.08.2022 (Shift-II)]

(1)-10 cm
(2) 40 cm
(3) 10 cm
(4)-40 cm

Correct Answer: (1)-10 cm

Solution:Radius of curvature is equal to twice the focal length for spherical mirrors. Hence R =2f.

• f=R/2 = -20/2 = -10 cm

34. For a ray of light undergoing refraction through a triangular glass prism, the angle of deviationis the angle between: [RRB Group 'D' CBT Exam, 22.08.2020 (Shift-II)]

(1) the incident ray and the normal at the point
(2) the refracted ray and the emergent ray
(3) the incident ry and the refracted ray
(4) the incident ray and the emergent ray

Correct Answer: (4) the incident ray and the emergent ray

Solution:Refraction through a prism occurs when light passes from a lighter medium to the denser medium.

• The angle at which the incident ray passes through the surface of the prism to the normal is called an incident angle and the angle by which the ray emerges out of the prism to the normal is called an emergent ray.

• The angle of deviation d is the angle between the incident ray and emergent ray.

35. A concave mirror having focal length of magnitude 20cm forms a real image at a distance of 60 cm from it. The object distance (in cm) is: [RRB Group 'D' CBT Exam, 22.08.2022 (Shift-II)]

(1)-15
(2) +15
(3) +30
(4) -30

Correct Answer: (4) -30

Solution:From the Mirror formula, 1/v+1/u =i/f

• From the question, f=-20 cm, v=-60 cm, u = ?

• So, 1/u = 1/f - 1/u=1/-20 -1/-60= -2/60= -1/30

• Hence, u= -30 cm.

36. Calculate the total current in the circuit given below: [RRB Group 'D' CBT Exam, 22.08.2022 (Shift-III)]

(1) 4A
(2) 1A
(3) 8A
(4) 2A

Correct Answer: (2) 1A

Solution:The relationship between voltage, current, and resistance is described by Ohm's law. This equation, I = V/R

• From the question, two resistances are connected in series. So, their total resistance = 2 +2=40

• So, I = 4/4 = 1A

37. A convex lens produces a magnification of -3 for an object placed placed at 1.5m from the lens. Find the image distance (with correct sign) [RRB Group 'D' CBT Exam, 22.08.2022 (Shift-II)]

(1) 0.5 m
(2) 4.5 m
(3) -4.5 m
(4)-0.5 m

Correct Answer: (2) 4.5 m

Solution:The magnification of a lens is defined as the ratio of image distance to that of object distance.

• In other words, m = v/u, where m is magnification, v is the image distance and u is the object distance.

• According to the question, m =-3, v =?, u = -1.5m. So, v = m * u=-3 x-1.5 = 4.5m

38. A spherical mirror converges a beam of light, at a given point on the principal axis. Which of the following statement(s) about the mirror is/are true? [RRB Group 'D' CBT Exam, 22.08. 2022 (Shift-III)]

(a) The mirror used is concave

(b) The mirror has positive focal length

(1) only (b)
(2) Both (a) and (b)
(3) Both (a) and (c)
(4) Only (a)

Correct Answer: (4) Only (a)

Solution:All the light rays which are parallel to the principal axis of a concave mirror, converge at the principal focus (F) after reflection from the mirrог.

• Since a concave mirror con verges a parallel beam of light rays, it is also called a converging mirror.

39. A magnet, when moved near a coill, produces an induced potential difference in the cell. What happens when we increase the speed of the mag-net near the coil? [RRB Group 'D' CBT Exam, 22.08.2022 (Shift-III)]

(1) The induced potential difference in the coil increases.
(2) The induced potential difference in the coil decreases.
(3) The induced potential difference remains the same.
(4) The induced current in the coil remains the same.

Correct Answer: (1) The induced potential difference in the coil increases.

Solution:The induced potential difference in the coil increases

• The Induced potential difference depends upon the number of turns and the rate of change of magnetic flux with time.

• The rate of change of magnetic flux depends upon the speed of approach of magnet and magnetic moment of magnet.

• So when magnetic field is increased, the induced potential difference in the coil also increases.

40. An electric heater is rated 2200 W at 220 V. The minimum rating of the fuse wire to be connected to the device is: [RRB Group 'D' CBT Exam, 22.08.2022 (Shift-III)]

(1) 10 A
(2) 20 A
(3) 22 A
(4) 5 A

Correct Answer: (1) 10 A

Solution:According to question,

Power (P) = 2200W

Potential diffrence (V) = 220V

From formula: P = VI

Where I = Current

∴ I = P/V = 2200/200 = 10A

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