Starter quiz
- Which of the following are ways of writing the definition of power?
- rate of energy transfer ✓
- amount of energy used up per second
- amount of energy transferred per second ✓
- time taken to use up one joule of energy
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- A mains circuit includes a fuse. Which of the following makes a fuse ‘blow’?
- a circuit current that is too high ✓
- a circuit resistance that is too high
- a room temperature that is too high
-
What is the potential difference across the resistor in the circuit shown?- 0.75 V
- 1.0 V
- 1.5 V ✓
- 2.5 V
- 3.0 V
-
What is the missing current in the circuit shown?- 1.0 A ✓
- 1.5 A
- 3.5 A
- 5.0 A
-
- A cell is connected to a lamp and a current flows. What happens to the current through the cell if another lamp is added in parallel with the first lamp?
- It stays the same.
- It increases. ✓
- It decreases.
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- An ammeter shows that the current through a resistor is 0.40 A and a voltmeter shows that the p.d. across it is 1.5 V. A pupil calculates the resistance. Which of these answers should they give?
- 3 Ω
- 4 Ω
- 3.7 Ω
- 3.8 Ω ✓
- 3.75 Ω
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Exit quiz
- A lamp has power of 7 W. Which of the following explains what this means?
- The lamp transfers 1 J in 7 s.
- The lamp transfers 7 J in 1 s. ✓
- The lamp transfers 1 W in 7 s.
- The lamp transfers 7 W in 1 s.
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- When an iron is working correctly, the current through it varies between 1.6 A and 3.3 A. Which of the following fuse sizes should be used in the plug of this iron?
- 3 A
- 5 A ✓
- 13 A
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- Which of the following could cause a circuit breaker to switch off a circuit?
- no appliances being used
- too many appliances being connected in series
- too many appliances being connected in parallel ✓
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- The current through a lamp is 1.5 A and the p.d. across it is 12 V. The power of the lamp is ______ W.
- '18' ✓
- What is the current through a 600 W lawnmower that is plugged into the mains?
- 0.38 A
- 2.6 A ✓
- 4.7 A
- 6.0 A
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- A 1300 W iron and a 1000 W hairdryer are plugged into a mains circuit, in parallel. What is the current through a circuit breaker in the consumer unit?
- 0.10 A
- 0.77 A
- 1.3 A
- 5.3 A
- 10 A ✓
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Worksheet
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Presentation
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Video
Lesson Details
Key learning points
- Power is the amount of energy transferred per second and is measured in watts (W).
- Power can be calculated using the equation: power = current × potential difference (P = I V).
- At a constant p.d., a larger electric current transfers energy more quickly than a smaller current.
- Domestic appliances plugged into the mains supply in the UK have a p.d. of 230 V across them.
- The fuse with the smallest rating for the required current (calculated using the equation: P = I² R) should be used.
Common misconception
Pupils often use the terms power and energy interchangeably.
Use concrete examples to show pupils that a greater power transfers energy more quickly.
Keywords
Power - the amount of energy transferred each second
Watts (w) - the unit of power; equal to the number of joules of energy transferred each second
Kilowatts (kw) - a unit of power; 1 kilowatt is equal to 1000 watts
Fuse - a thin wire that melts when a dangerously large current flows through it
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