Railway Science (Physics-Electric Current and its Effects) (Part-VII)

Total Questions: 50

31. The resistivity of a material of length 5 m is 1.6 × 10⁻⁸ ohm-m. If the length of the material is increased by 10 m, then the value of resistivity is_________. [RRC Group D 06/10/2022 (Morning)]

Correct Answer: (d) 1.6 × 10⁻⁸
Solution:

Resistivity depends only on the temperature of the conductor and not on the length or density of the material. Hence, the resistivity remains the same. Resistivity (ρ) is a property that describes the extent to which a material opposes the flow of electric current through it. ρ = 𝑅𝐴/𝑙 where, R =  Resistance, A = Cross sectional area, 𝑙 = Length. Resistivity of copper = 1.6×10⁻⁸ Ωm and that of aluminium =2.6 ×10⁻⁸ Ω m.

32. If the length of a conductor is doubled and the area of cross section is halved, the resistance will be_________ [RRC Group D 06/10/2022 (Afternoon)]

Correct Answer: (a) four times of its original value
Solution:

33. When two resistors are connected in series, their total effective resistance is 120 Ω. When the same two resistors are connected in parallel then their effective resistance is 30 Ω. The resistances of the two resistors are: [RRC Group D 07/10/2022 (Morning)]

Correct Answer: (d) 60 Ω and 60 Ω each
Solution:

34. The ratio of currents flowing through two electric circuits is 1 : 4. If the ratio of resistances is 2 : 1, what is the ratio of their powers? [RRC Group D 07/10/2022 (Morning)]

Correct Answer: (c) 1 : 8
Solution:

35. An electric lamp of 40 Ω, a toaster of resistance 200 Ω and a water filter of resistance 400 Ω are connected in parallel to a 220 V source. Suppose an electric iron when connected to a 220 V source draws the same current as that of the three appliances in the above, then the resistance and current flowing through the iron are: [RRC Group D 07/10/2022 (Afternoon)]

Correct Answer: (b) 30.77 Ωand 7.15 A, respectively
Solution:

36. A copper wire has radius 0.5 mm and length 100 m. If the resistivity of the material is 2.5 × 10⁻⁸ ohm-m, then the resistance of the wire is ________. [RRC Group D 11/10/2022 (Afternoon)]

Correct Answer: (d) 10/π ohm
Solution:

37. The power of an electric bulb of resistance 18 ohm if no voltage is applied across it is ________. [RRC Group D 11/10/2022 (Evening)]

Correct Answer: (a) 0 W
Solution:

Relation between Power and Resistance expressed as P = 𝑉²/R . ;
where P = power, R = resistance, V = voltage.
Given : R = 18 Ω , V = 0 volts , P = ?
P = 𝑉²/R = 0/18 = o w.

38. Three wires of resistance 3 ohm, 6 ohm and 9 ohm are connected in parallel. What will be the total resistance of the circuit? [RRB NTPC CBT - I (10/02/2021) Morning]

Correct Answer: (b) ohms 18/11 ohms
Solution:

39. What is the resultant resistance of 3 Ω and 6 Ω resistances connected in series? [RRB JE 23/05/2019 (Morning)]

Correct Answer: (c) 9 Ω
Solution:

Given two resistance connected in series
According to the question, R₁ = 3 Ω and R₂ = 6 Ω.
Then, Resultant resistance (R) = R₁ + R₂ ⇒ 3 + 6 = 9 Ω

40. If a current of 3 Amperes flows for 1 minute, how much charge flows in this time? [RRB JE 23/05/2019 (Morning)]

Correct Answer: (d) 180 Coulombs
Solution:

Given that : Current (I) = 3A, Time (t) = 1 min = 60 sec The formula for charge is Q = I × t ⇒ 3 × 60 , electric charge flown (Q) = 180 C.