Physics (Part-IX) (Railway)

Total Questions: 50

31. The unit of resistance is _________. [RRB Group 'D' CBT Exam, 29.09.2022 (Shift-III)]

(1) Ampere
(2) Coulomb
(3) Ohm
(4) Volt

Correct Answer: (3) Ohm

Solution:The SI unit of resistance is ohm, which is defined as a volt per ampere.

Ω = V/A

32. A material of length I and area of cross section A has a resistance R. What should be changes in the area of cross section A if its length is tripled to produce the same resistance R? [RRB Group 'D' CBT Exam, 29.09.2022 (Shift-III)]

(1) The area of cross section should be decreased by 9 times.
(2) The area of cross section should be increased by 9 times.
(3) The area of cross section should be increased by 3 times.
(4) The area of croos section should be decreased by 3 times.

Correct Answer: (2) The area of cross section should be increased by 9 times.

Solution:Given,

I₂ = 31₁

Using,

I₁A₁ = I₂A₂

I₁A₁ = 31₁A₂

A₂ = A₁/3

R₂ = ρ l₂/A₂ = ρ 3l₁/A₁/3 = 9ρ l₁/A₁

R₂ = = 9R

33. Fleming's right hand rule gives the direction of current induced in a conductor moving in a_________. [RRB Group 'D' CBT Exam, 30.09.2022 (Shift-II)]

(1) electromagnetic field
(2) gravitational field
(3) electric field
(4) magnetic field

Correct Answer: (4) magnetic field

Solution:Fleming's right-hand rule shows the direction of the induced current when a conductor moves in a magnetic field.

• It can be used to determine the direction of current in a generator's winding.

• The thumb is pointed in the direction of motion of the conductor.

34. The length of a conducting wire is 1 km and its radius of cross section is 7mm. A resistor of resistance 10Ω is made out of this wire. The resistivity of this conductor will be_________. [RRB Group 'D' CBT Exam, 30.09.2022 (Shift-II)]

(1) 7×10-⁶Ω-m
(2) 1.54 × 10-⁶Ω-m
(3) 2.22 × 10-⁶Ω-m
(4) 1×10-⁶Ω-m

Correct Answer: (3) 2.22 × 10-⁶Ω-m

Solution:R = p L/A

⇒ R = p L/πr²

P = Rπr²/L

⇒ P = 10 * 3.14 * (0.227)²/1000

= = 1.54 × 1.0-⁶Ω -m

35. If two parallel straight conductors are carrying current in the same direction, then the directions of the magnetic fields produced will be: [RRB Group 'D' CBT Exam, 30.09.2022 (Shift-II)]

(1) opposite to each other
(2) perpendicular to each other
(3) in the same direction
(4) in random directions

Correct Answer: (1) opposite to each other

Solution:When two parallel wires carry current in the same direction, they exert equal and opposite attractive forces on each other and repel each other when the current is in opposite direction.

• This can be verified using Fleming's left-hand rule.

• The directions of the magnetic fieldsproduced by the two conductors will be opposite to each other.

36. The rate at which electrical energy is dissipated in an electric circuit is called_________. [RRB Group 'D' CBT Exam, 30.09.2022 (Shift-II)]

(1) electric work
(2) electric heat
(3) electric force
(4) electric power

Correct Answer: (4) electric power

Solution:The rate of work done is called power. Rate of electric work done is called electric power.

• Electric power is the rate at which electric energy is dissipated or consumed in an electric circuit.

37. The element used as a fuse________ when a huge current flows through it. [RRB Group 'D' CBT Exam, 30.09.2022 (Shift-II)]

(1) cools
(2) heats up
(3) boils
(4) melts

Correct Answer: (4) melts

Solution:Fuse (Made of Tin (Sn) and lead (Pb) is a safety device consisting of a wire that melts and breaks an electric circuit if the current exceeds a safe level.

• So, Fuse wire should have high resistance and low melting point.

• Low melting point lets it break the circuit when high current passes through it. It melts. thus, saving electric devices from damage.

38. A concave lens has focal length of 12 cm. At what distance should the object from the lens be placed so that if forms an image at 8 cm from the lens? [RRB Group 'D' CBT Exam, 30.09.2022 (Shift-II)]

(1) 12 cm
(2) 24 cm
(3) -24 cm
(4) 18 cm

Correct Answer: (3) -24 cm

Solution:f = - 12cm

v = - 8cm

u=?

1/v - 1/u = 1/f ;1/u = 1/v - 1/f

= 1/- 8 - 1/- 12 = - 1/8 + 1/12

1/u = - 3 + 2/24 ; 1/u = - 1/24

u = - 24cm

39. Which of the following statements is/are correct with regard to the principal axis of a spherical mirror? [RRB Group 'D' CBT Exam, 30.09.2022 (Shift-II)]

(a) A principal axis is an imaginary line extended on both sides of the mirror.

(b) A principal axis passes through the centre of curvature, principal focus and pole.

(1) Only (a) is correct
(2) Only (b) is correct
(3) (a), (b) and (c) are correct
(4) Only (a) and (b) are correct

Correct Answer: (4) Only (a) and (b) are correct

Solution:The center of the spherical mirror that forms the curved mirror is called the center of curvature.

• The center of the aperture of the curved mirror is called the pole of the curved mirror.

• The imaginary line that joins the pole to the curved mirror to the center of curvature of the curved mirror is called the principal axis.

40. The resistance of a conductor is directly proportional to its________. [RRB Group 'D' CBT Exam, 06.10.2022 (Shift-II)]

(1) area of corss section
(2) length
(3) volume
(4) temperature

Correct Answer: (2) length

Solution:Resistance of a conductor Is a property by virtue of which It opposes the flow of electric current in it.

• It is directly proportional to length and inversely proportional to area of cross-section of the substance.

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