# 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

Correct answer: 2. 0.64

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

Correct answer: 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

Correct answer: 1. Length

## 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

Correct answer: 2. 5 A

Solution:

According to maximum power transfer theorem, the power dissipated in external resistor is maximum when the value of external resistance is equal to the internal resistance of source. In order to achieve this the external resistnace should also be 3 Ω.

Required current (I) = E/(R+r) = 30/(3 + 3) = 5 A