Basic Electrical Engineering MCQs Part 18

Basic Electrical Engineering MCQs Part 17

Contents

1 If potential difference across conductor of material having resistivity ρ remains constant, then heat produced is proportional to:
2 Once the kettle is switched on, the water in electric kettle boils in 15 minutes. Using same mains supply, if one wants to boil the same quantity of water in 10 minutes, the length of wire used for heating element should be
3 A 3°C rise of temperature is observed in a conductor when certain amount of current is passed through it. If amount of current is doubled, the rise of temperature will be
4 For a given resistance wire of length 50 cm and a battery having negligible internal resistance, which of the below mentioned case has maximum amount of heat generated
5 Certain motor draws 20 A from 230 V source. If motor is 90 efficient, the horsepower rating is

If potential difference across conductor of material having resistivity ρ remains constant, then heat produced is proportional to:

1/ρ
Ρ
ρ²
1/√ρ

Correct answer: 1. 1/ρ

Once the kettle is switched on, the water in electric kettle boils in 15 minutes. Using same mains supply, if one wants to boil the same quantity of water in 10 minutes, the length of wire used for heating element should be

Once the kettle is switched on, the water in electric kettle boils in 15 minutes. Using same mains supply, if one wants to boil the same quantity of water in 10 minutes, the length of wire used for heating element should be:

Increases
Decreased
Kept the same
None of these

Correct answer: 2. Decreased

A 3°C rise of temperature is observed in a conductor when certain amount of current is passed through it. If amount of current is doubled, the rise of temperature will be

A 3°C rise of temperature is observed in a conductor when certain amount of current is passed through it. If amount of current is doubled, the rise of temperature will be:

1°C
3°C
6°C
12°C

Correct answer: 4. 12°C

Solution:

Heat produced H ∝ I²

Temperature rise Δt ∝ H

Δt ∝ I²

For first case Δt1 ∝ I²

For second case Δt2 ∝ (2I)²

Now, Δt2/ Δt1 = (2I)²/ I²

Or Δt2/3 = 4

Δt2 = 4 * 3 =12°C

For a given resistance wire of length 50 cm and a battery having negligible internal resistance, which of the below mentioned case has maximum amount of heat generated

For a given resistance wire of length 50 cm and a battery having negligible internal resistance, which of the below mentioned case has maximum amount of heat generated:

When wire is divided in four equal parts and all are connected in parallel across battery
When wire is divided in two equal parts and all are connected in parallel across battery
When wire is directly connected across battery
None of these

Correct answer: 1. When wire is divided in four equal parts and all are connected in parallel across battery

Certain motor draws 20 A from 230 V source. If motor is 90 efficient, the horsepower rating is

Certain motor draws 20 A from 230 V source. If motor is 90 efficient, the horsepower rating is:

3 HP
6 HP
12 HP
20 HP

Correct answer: 2. 6 HP

Solution:

Motor input = VI = 230 * 20 = 4600 W

————

Motor output = η * Motor input = 0.9 * 4600 = 4140 W