# Basic Electrical Engineering MCQs Part 23

Basic Electrical Engineering MCQs Part 23

## Certain amplifier develops 100 W of output power

Certain amplifier develops 100 W of output power. A DC power supply furnishes the amplifier 5 at 50 V. The power supply draws 3 A form 130 V source. The efficiency of power supply is:

1. 0.32
2. 0.64
3. 0.78
4. 0.94

Solution:

Input to power supply = 130 * 3 = 390 W

Output of power supply = 50 * 5 = 250 W

——————-

Efficiency of power supply = 250/390 = 0.64

## Two bulbs 100 W, 250 V and 200 W, 250 V are connected in series to a 500 source. Which bulb is likely to fuse

Two bulbs 100 W, 250 V and 200 W, 250 V are connected in series to a 500 source. Which bulb is likely to fuse:

1. 100 W bulb
2. 200 W bulb
3. Both of these
4. None of these

Correct answer: 1. 100 W bulb

Solution:

From formula R = V²/P

Bulb1 R = 625 ohms

Bulb2 R = 312.5 ohms

———————

Voltage across Bulb 1 = 333 V

Voltage across Bulb 2 = 167 V

Since voltage across bulb 1 exceeds rated voltage, it will be fused.

## A constant voltage is applied between the two ends of a metallic wire of uniform area of cross-section. Certain heat is developed. The heat will be doubled if

A constant voltage is applied between the two ends of a metallic wire of uniform area of cross-section. Certain heat is developed. The heat will be doubled if:

1. Radius of wire is doubled
2. Length of wire is doubled
3. Both length and radius are halved
4. Both length and radius are doubled

Explanation: For constant applied voltage, heat produced per second H ∝ 1/R.

H ∝ a/ ρl

If both radius and length are doubled, H becomes doubled.

## Which of following is immaterial for a fuse

Which of following is immaterial for a fuse:

1. Length
3. Resistivity
4. None of these

## A 30 V battery having internal resistance 3 Ω is connected to variable resistor. For what value of current drawing from battery, the heat produced from resistor is maximum

A 30 V battery having internal resistance 3 Ω is connected to variable resistor. For what value of current drawing from battery, the heat produced from resistor is maximum:

1. 2 A
2. 5 A
3. 7 A
4. 10 A