Current is a flow of charge. Potential difference drives it. Resistance opposes it. Power is the rate at which the circuit transfers energy.
The idea
Current is measured in amperes and is charge passing a point per second. Potential difference, in volts, is the energy transferred per coulomb. Resistance, in ohms, is potential difference divided by current. For a metal wire at constant temperature, potential difference and current stay in proportion. That is Ohm's law. An ammeter goes in series. A voltmeter goes in parallel across the component.
Electrical power is potential difference times current, in watts. Energy transferred is power times time, in joules, if the power stays constant. A 12 V lamp carrying 2 A has a power of 24 W. In 10 s it transfers 240 J. The same relations let you find a current if you know the power and the voltage. Keep the seconds. A time in minutes left as minutes makes the energy sixty times too small or too large, depending on which way you forget.
What earns the marks
State the relation you use. V = IR, P = IV and E = Pt are different marks. Units: W for power, J for energy, unless the question asks for kilowatt hours. Series and parallel rules still apply if the question joins resistors. This page's numbers are one component, so you do not need them. A candidate who multiplies voltage by resistance to get current has inverted Ohm's law.
Practice questions
These are practice questions in the style of the paper, with a model answer and a mark scheme. They are not a promise of what your paper will ask.
1Question 1
6 marks
A lamp operates at 12 V and carries a current of 2 A.
(a) Calculate the power. (2 marks)
(b) Calculate the energy transferred in 10 s. (2 marks)
(c) Calculate the resistance of the lamp. (2 marks)
Model answer
(a) P = IV = 12 × 2 = 24 W.
(b) E = Pt = 24 × 10 = 240 J.
(c) R = V/I = 12/2 = 6 Ω.
Mark scheme
Uses power = current × potential difference1 mark
24 W1 mark
Uses energy = power × time1 mark
240 J1 mark
Uses resistance = potential difference / current1 mark
6 Ω, or 6 ohms1 mark
2Question 2
2 marks
State where an ammeter and a voltmeter are placed to measure the current through the lamp and the potential difference across it. (2 marks)
Model answer
The ammeter is in series with the lamp. The voltmeter is in parallel across the lamp.
Mark scheme
Ammeter in series1 mark
Voltmeter in parallel across the lamp1 mark
3Question 3
1 mark
The lamp is left on for 2 minutes. Explain the mistake in calculating the energy as 24 × 2. (1 mark)
Model answer
The 2 is in minutes, and the watt times a minute is not a joule. The time must be 120 s, so the energy is 24 × 120 J.
Mark scheme
Time must be in seconds for the energy in joules. 2 minutes is 120 s1 mark
Practise Physics with Anna
More questions on this topic are inside GLearn, marked as you answer, with the reason written out. The first ones need no account.
Power is potential difference times current, in watts. Energy is power times time in seconds, in joules. A 12 V lamp carrying 2 A uses 24 W, and in 10 s it transfers 240 J.
What does Ohm's law say?+
For a metal conductor at constant temperature, potential difference is proportional to current. Resistance is potential difference divided by current. The 12 V lamp carrying 2 A has a resistance of 6 ohms while it is operating at that current. All WASSCE Physics past questions