Basic Electrical Engineering MCQs Part 13

Contents

- 1 EMF in a circuit
- 2 A copper wire of resistance Ro is stretched till its length is increased n times of its original length. Its resistance will become
- 3 Certain resistor develops 1000 J of thermal energy in 10 s when a current of 10 A is passed through it. The resistance of resistor is
- 4 A copper wire having area of cross section 5 mm² is 5 meters long and carries a current of 5 A. The number density of free electrons is 8E+28. The time required by an electron to travel the length of wire is
- 5 Two electric bulbs rated for same voltage have powers of 100 W and 200 W. If their resistances are R1 and R2, then which of following relationship is correct

## EMF in a circuit

EMF in a circuit:

- Maintains the potential difference
- Decreases resistance of circuit
- Increases resistance of circuit
- None of these

Correct answer: 1. Maintains the potential difference

## A copper wire of resistance Ro is stretched till its length is increased n times of its original length. Its resistance will become

A copper wire of resistance Ro is stretched till its length is increased n times of its original length. Its resistance will become:

- nRo
- n^2Ro
- Ro/n
- Ro/n^2

Correct answer: 2. n^2Ro

## Certain resistor develops 1000 J of thermal energy in 10 s when a current of 10 A is passed through it. The resistance of resistor is

Certain resistor develops 1000 J of thermal energy in 10 s when a current of 10 A is passed through it. The resistance of resistor is:

- 0.5 Ω
- 1 Ω
- 2 Ω
- 5 Ω

Correct answer: 2. 1 Ω

Solution:

Energy = I² Rt

R = Energy/I²t

R = 1000 J /(10² * 10 ) = 1 Ω

## A copper wire having area of cross section 5 mm² is 5 meters long and carries a current of 5 A. The number density of free electrons is 8E+28. The time required by an electron to travel the length of wire is

A copper wire having area of cross section 5 mm² is 5 meters long and carries a current of 5 A. The number density of free electrons is 8E+28. The time required by an electron to travel the length of wire is:

- 14.32 h
- 17.77 h
- 25.96 h
- 32.33 h

Correct answer: 2. 17.77 h

Solution:

Charge on electron = 1.6*10^-19

Drift velocity = I/nAe = 0.000078125 m/s

Time taken by electron to travel length of wire

t = Length/Drift velocity = 64000 s = 17.77 h

## Two electric bulbs rated for same voltage have powers of 100 W and 200 W. If their resistances are R1 and R2, then which of following relationship is correct

Two electric bulbs rated for same voltage have powers of 100 W and 200 W. If their resistances are R1 and R2, then which of following relationship is correct:

- R1 = R2
- R1 = 2R2
- R1 = 0.5 R2
- None of these

Correct answer: 2. R1 = 2R2

Solution:

R1 = V^2 / 100

R2 = V^2 / 200

R2/R1 = 100/200 = 0.5

Or R1 = 2R2