# Basic Electrical Engineering MCQs Part 13

Basic Electrical Engineering MCQs Part 13

## EMF in a circuit

EMF in a circuit:

1. Maintains the potential difference
2. Decreases resistance of circuit
3. Increases resistance of circuit
4. 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:

1. nRo
2. n^2Ro
3. Ro/n
4. Ro/n^2

## 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:

1. 0.5 Ω
2. 1 Ω
3. 2 Ω
4. 5 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:

1. 14.32 h
2. 17.77 h
3. 25.96 h
4. 32.33 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:

1. R1 = R2
2. R1 = 2R2
3. R1 = 0.5 R2
4. 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