Download Electrical Power 1

Electrical Power - Formal Homework 1
1. Appliances convert electrical energy into other forms of energy.
State the useful energy output from the following appliances.
(i) Lamp: electrical energy ............................. energy
(ii) Kettle: electrical energy ............................. energy
(iii) Hair dryer: electrical energy ............................. energy
(iv) Food processor: electrical energy ............................. energy
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2. A circuit is set up as shown.
The power supplied to the resistor is
A. 1.20 × 10 -4 W
B. 1.44 × 10 -3 W
C. 1.44 W
D. 694 W
E. 1.20 × 10 6 W.
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3. Parking sensors are fitted to the rear bumper of some cars. A buzzer emits
audible beeps, which become more frequent as the car moves closer to an
object.
Ultrasonic pulses are emitted from the rear of the car. Objects behind the car
reflect the pulses, which are detected by sensors. Ultrasonic pulses travel at
the speed of sound.
The sensor operates at a voltage of 12 V and has a current range of
20–200 mA. Calculate the maximum power rating of the sensor.
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4. The MP3 player put on a docking station with external speakers.
Calculate the resistance of a 10 W speaker when the voltage across it is
9V.
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5. Calculate the power which is dissipated in the following resistors:
a) a 1kΩ resistor carrying 50 mA
b) a 5MΩ resistor carrying 2.5 mA
c) a 680 Ω resistor carrying 3x10 -2 A
d) a 390 Ω resistor carrying 27 mA
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6. Calculate the resistance of the resistor when:
a) a rated power of 500W requires a current of 0.05 A
b) a rated power of 4.5 kW requires a current of 300mA
c) a rated power of 10 mW requires a current of 250µA
d) a rated power of 5MW requires a current of 2x102 A
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7. Calculate the current in the following resistors when used in their maximum
power rating:
a) a 0.5Ω resistor, rated at 4.5W
b) a 1kΩ resistor, rated at 640 W
c) a 0.5Ω resistor, rated at 3kW
d) a 3x 10 4Ω resistor, rated at 7200W.
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Total 23 marks