An electric current is a flow of charge. A complete circuit gives that charge a closed path.
The idea
Current is measured in amperes with an ammeter connected in series. Potential difference, often called voltage, is measured in volts with a voltmeter connected in parallel across a component. Resistance, measured in ohms, is the opposition to current. They are linked by the relation voltage equals current times resistance. A conductor, such as a metal wire, lets charge flow. An insulator, such as plastic or rubber, does not.
In a series circuit there is one path. The current is the same all the way round, and the voltages across the components add up to the battery voltage. In a parallel circuit each branch is a separate path. The voltage across each branch equals the battery voltage, and the currents in the branches add up to the current from the battery. A fuse is a thin wire that melts and opens the circuit if the current is too high. A switch opens or closes the path.
What earns the marks
Say where the meter goes. An ammeter is in series. A voltmeter is across the component, in parallel. Do not swap series and parallel. In series the current is the same and the voltages add. In parallel the voltage is the same and the currents add. Show the working when you use voltage = current × resistance, with units.
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
4 marks
A lamp draws a current of 0.5 A from a 6 V battery.
(a) Calculate the resistance of the lamp. (2 marks)
(b) State how an ammeter should be connected to measure this current. (1 mark)
(c) State how a voltmeter should be connected to measure the voltage across the lamp. (1 mark)
Model answer
(a) Resistance = voltage divided by current = 6 divided by 0.5 = 12 ohms.
(b) In series with the lamp.
(c) In parallel with the lamp, across its two ends.
Mark scheme
Uses resistance = voltage divided by current1 mark
12 ohms, or 12 Ω1 mark
Ammeter in series1 mark
Voltmeter in parallel across the lamp1 mark
2Question 2
4 marks
Two bulbs are connected in series to a battery. State what happens to the other bulb if one bulb blows and breaks the filament, and explain why. Then state what would happen if the same bulbs were in parallel. (4 marks)
Model answer
In series the other bulb goes out, because there is only one path and the break stops the current everywhere. In parallel the other bulb stays on, because its branch is still a complete path.
Mark scheme
Series: the other bulb goes out1 mark
Because the single path is broken, so current stops in the whole circuit1 mark
Parallel: the other bulb stays lit1 mark
Because each bulb has its own path back to the battery1 mark
3Question 3
4 marks
(a) What is a fuse for? (2 marks)
(b) Name one conductor and one insulator used in a household cable. (2 marks)
Model answer
(a) A fuse melts when the current is too high and so it opens the circuit, which protects the cable and the appliance.
(b) The copper wire is a conductor. The plastic cover is an insulator.
Mark scheme
Melts or breaks when current is too large1 mark
Opens the circuit and protects the appliance or prevents overheating1 mark
A correct conductor, such as copper or another metal1 mark
A correct insulator, such as plastic or rubber1 mark
Practise Science with Kevin
More questions on this topic are inside GLearn, marked as you answer, with the reason written out. The first ones need no account.
What is the difference between series and parallel?+
Series has one path, so the current is the same in each component and the voltages add. Parallel has branches. The voltage across each branch is the same, and the branch currents add to the current from the battery.
Where do you put an ammeter and a voltmeter?+
An ammeter goes in series so the current passes through it. A voltmeter goes in parallel, across the component whose voltage you want. Swapping them gives a wrong reading and can damage the ammeter. All BECE Science past questions