Practise Electrical charge and current. 12 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
What an electric current is, why a circuit needs a source of potential difference, and the equation Q = I t. Expect definitions, questions on the current being the same all round a series loop, and 2 to 3 mark calculations, often with a time to convert into seconds.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
State that current is a flow of chargeElectric current is a flow of electrical charge around a closed circuit.
3
Give the units of charge, current, timeCharge in coulombs (C), current in amperes (A) and time in seconds (s).
4
State what a circuit needs for currentA closed circuit (complete loop) that includes a source of potential difference, such as a cell.
4
Calculate charge using Q = I tMultiply the current in amps by the time in seconds to get the charge in coulombs.
5
State that current is the same around a loopIn a single closed loop the current has the same value at every point.
6
Rearrange Q = I t and convert unitsUse I = Q ÷ t or t = Q ÷ I, converting minutes to seconds and mA to A first.
Notes
What current is
An electric current is a flow of electrical charge.
The size of the current is the rate of flow of charge: how much charge passes a point each second.
In metal wires the charge is carried by electrons, which are negatively charged.
Charge is measured in coulombs (C). Current is measured in amperes (A), usually called amps.
What a circuit needs
For charge to flow, the circuit must be a closed loop, with no gaps.
The circuit must include a source of potential difference, such as a cell, a battery or a power supply. The potential difference is what makes the charge flow.
Opening a switch makes a gap, so the current stops everywhere in that loop.
Current in a single loop
In a single closed loop (a series circuit) the current is the same at every point.
Charge is not used up by the components. The charge that leaves the cell each second is the same as the charge that returns to it. Energy is transferred to the components, but no charge is lost.
So two ammeters placed anywhere in the same loop give the same reading.
The equation Q = I t
charge flow = current × time: Q = I t, with Q in coulombs, I in amperes and t in seconds.
A current of 1 A means 1 coulomb of charge passes a point every second.
Charge flows only in a closed circuit with a source of potential difference
Current is the same at every point in a single closed loop
1 mA = 0.001 A; 1 minute = 60 s; 1 hour = 3600 s
How to answer each type of question
Calculate the charge that flows
2 marks4
Write Q = I t.
Check the time is in seconds and the current is in amps.
Substitute, calculate and give the unit C (coulombs).
Example. A current of 0.40 A flows through a lamp for 90 s. Calculate the charge that flows through the lamp.
Show the model answer
Q = 0.40 × 90 (1) Q = 36 C (1)
Rearrange Q = I t, with a unit conversion
3 marks6
Convert the time to seconds (or mA to A) first.
Rearrange: I = Q ÷ t or t = Q ÷ I.
Substitute and give the answer with its unit.
Example. A charge of 720 C flows through a heater in 4.0 minutes. Calculate the current in the heater.
Show the model answer
t = 4.0 × 60 = 240 s (1) I = Q ÷ t = 720 ÷ 240 (1) I = 3.0 A (1)
State and explain the current at different points in a loop
2 marks5
Check whether the ammeters are in the same single loop.
If they are, the readings are equal.
Give the reason: the current is the same at every point in a single closed loop; charge is not used up.
Example. A cell, a lamp and a resistor are connected in series. Ammeter X is between the cell and the lamp. Ammeter Y is between the lamp and the resistor. Ammeter X reads 0.25 A. (a) State the reading on ammeter Y. (b) Give a reason for your answer.
Show the model answer
(a) 0.25 A (1) (b) The current is the same at every point in a single closed loop / charge is not used up by the lamp (1)
Shortcuts and memory tricks
Formula triangle: Q on top, I and t underneath. Cover the one you want to find.
Think of current as coulombs per second: 3 A means 3 C passes every second.
Sense check: a bigger current or a longer time must give more charge.
Where marks are lost
Leaving the time in minutes or hours instead of converting to seconds.
Forgetting to change mA to A (divide by 1000).
Saying the current is 'used up' by a lamp, so it is smaller after the lamp. In a single loop it is the same everywhere.
Defining current as 'the amount of charge' or 'the speed of the electrons'. It is the RATE of flow of charge.
Exam technique
Write the equation, then substitute, then give the answer with its unit. Each step can earn a mark.
Q = I t is one of the equations you must learn: it is not on the equation sheet.
When asked to define current, use the words 'rate of flow of (electrical) charge'.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
The current in the lamp is 0.40 A. Calculate the charge that flows through the lamp in 60 s. Use the equation: charge flow = current × time
24 C
Write down the equation that links charge flow (Q), current (I) and time (t).
Q = I t
State the instrument used to measure current, and how it is connected in a circuit.
An ammeter, connected in series.
A wire carries a current of 2 A. Explain what this means, in terms of charge.
2 coulombs of charge pass a given point in the wire every second.
State the unit of electric current.
Ampere (amp, A)
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy5 marks
A student sets up a simple circuit containing a cell, a switch and a lamp.
(a) Complete the sentence. Electric current is a flow of electrical ... Tick (✓) one box.[1]
charge.
energy.
power.
resistance.
(b) Which component in the circuit is the source of potential difference? Tick (✓) one box.[1]
the cell
the lamp
the switch
the wires
(c) The current in the lamp is 0.40 A. Calculate the charge that flows through the lamp in 60 s. Use the equation: charge flow = current × time[2]
(d) Give the unit of charge.[1]
Show the answer and mark scheme
(a)Answer: charge.
(b)Answer: the cell
(c)Answer: 24 C
Q = 0.40 × 60
Q = 24 (C)
(d)Answer: coulomb (C)
coulomb / C
Question 2Medium6 marks
A mobile phone battery is being charged.
(a) Write down the equation that links charge flow (Q), current (I) and time (t).[1]
(b) In 30 minutes, 2700 C of charge flows into the battery. Calculate the mean charging current.[3]
(c) In a different circuit, a lamp and a resistor are connected in series with a battery. The current in the lamp is 0.25 A. State the current in the resistor. Give a reason for your answer.[2]
Show the answer and mark scheme
(a)Answer: Q = I t
Q = I × t / charge flow = current × time
(b)Answer: 1.5 A
t = 1800 s
2700 = I × 1800
I = 1.5 (A)
(c)Answer: 0.25 A — the current is the same at every point in a single closed loop. A
0.25 (A)
the current is the same at any point in a single closed loop / series circuit
Question 3Hard8 marks
The current in a metal wire is a flow of electrons. The charge on one electron is 1.6 × 10−19 C.
(a) A current of 0.20 A flows in a wire for 5.0 minutes. Calculate the number of electrons that pass a point in the wire in this time.[4]
(b) The capacity of a torch battery is 3000 mA h. This means it can supply a current of 3000 mA for 1 hour. Calculate the total charge the battery can supply.[2]
(c) The torch draws a current of 0.45 A. Calculate how long a fully charged battery can power the torch. Give your answer in hours.[2]