Thursday, 14 August 2014

ELECTRICITY QUESTIONS


                                           ELECTRICITY QUESTIONS


FORMULAE : Current = Charge x Time
Resistance = Voltage/Current
Power = Voltage x Current
Cost of electrical energy = Power (kW) x Time (hours) x 8p

1. Draw a circuit to show two door pushes operating just ONE door bell.











2. What is the cost of the following:

(a) a 4 kW heater running for 3 hours ..................………………………………....

(b) a 200 W stereo on for 5 hours . ...................………………………………...

(c) a 2 kW fan blower on for 30 mins ..........………………………………............

3. Write down THREE safety precautions that you should take when electricity is used in your home.

(a) ....................................………………………………...…...............

(b) ..............................................………………………………....…...

(c) .....………………………………….................................................

4. Draw two circuits, each containing two lamps and two cells.
The first one (A) should have the two lamps in parallel and the two cells in series and the second one (B) should have the two cells in parallel and the lamps in series.

A. B.









Which one will give the brightest lamps? .....…………………………………......

Which one will run the cells down quickest? .......……………………………………

5. A girl measured the current flowing through a lamp filament when the voltage was 12.5 V and the lamp was hardly glowing. The reading she got was 10 A.

The resistance of the lamp was ....…………...…… ohms

She then turned up the voltage and got the following pairs of readings of current and voltage.

Voltage (volts)
Current (amps)
Resistance (ohms)
25
8.3

50
7

75
5.8

100
5

125
4

150
3

175
2.7

200
2


Work out the resistance for each voltage and plot a graph of resistance against current. Plot the resistance on the y axis and the voltage on the x axis. Use ONE LARGE square for 10 ohms and ONE LARGE square for 25 volts.

Draw the best-fit curve on your graph.

The resistance of the lamp filament increases because:

.....................................................……………………………………………………………….........