Edexcel IGCSE Physics · Spec 2.1-2.3
Current, Charge & Circuits
Electric current, charge, and the basics of electrical circuits.
Physics revision video
Current, Charge & Circuits
Prefer to watch on YouTube? Open this video on YouTube.
Explained
Charge, current, and what a circuit is doing
Electric current is the rate of flow of electric charge. In a metal wire the charge is carried by delocalised electrons drifting through the lattice.
Current is measured in amperes with an ammeter, connected in series with the component so that the current being measured passes through it.
Charge, current and time
Charge equals current multiplied by time, Q equals I times t. Charge is in coulombs, current in amperes, time in seconds.
Read it as a definition of the ampere: one ampere is one coulomb of charge passing each second. So a current of 2 A flowing for 30 s delivers 60 C.
Time must be in seconds. A time given in minutes needs multiplying by 60 first, and that conversion is the commonest arithmetic slip here.
Potential difference
Potential difference is the energy transferred per unit of charge passing. One volt means one joule of energy transferred for each coulomb.
So energy transferred equals charge multiplied by potential difference, E equals Q times V. A charge of 60 C moving through a potential difference of 12 V transfers 720 J.
A voltmeter is connected in parallel, across the component, because potential difference is a difference between two points rather than something at a point.
Current is not used up
This is the misconception that causes most trouble in circuits. Charge is conserved, so the current leaving a component is the same as the current entering it. A bulb does not consume current; it transfers energy from the charge passing through.
In a series circuit there is one path, so the current is identical everywhere. In a parallel circuit the current divides between the branches and the branch currents add back up to the total.
What the mark scheme accepts and rejects
An Edexcel International GCSE Physics mark scheme asks for the definition of potential difference and credits energy transferred per unit charge passed. It allows work done in place of energy transferred and coulomb in place of unit charge.
Then it adds two notes worth studying. The formula V equals E over Q is allowed only if all the terms are defined, and joules per coulomb is ignored.
Both refuse to accept a restatement of the unit as a definition of the quantity. Joules per coulomb is what a volt is; saying it does not explain what potential difference means. Writing the symbols without saying what they stand for is the same problem.
The same paper marks the ammeter and voltmeter positions strictly, requiring the ammeter in series with the component and the voltmeter in parallel with it, and rejecting a voltmeter drawn across the component and something else together.
Direct and alternating current
Direct current flows in one direction only, as from a cell or a battery.
Alternating current reverses direction repeatedly, as from the mains, which in the UK does so 50 times a second. An oscilloscope shows the difference immediately: a straight horizontal line for direct current, a repeating wave for alternating.
Circuit symbols
Learn the standard symbols properly, because diagram questions mark them individually. A cell is one long and one short line; a battery is several cells together. An ammeter is an A in a circle and a voltmeter a V in a circle. A resistor is a plain rectangle; a variable resistor is a rectangle with an arrow through it.
Drawing a component recognisably but not correctly loses the mark for that component, and the rest of the circuit is marked independently, so one wrong symbol does not spoil the whole diagram.
Spec 2.1-2.3
What you need to know
- Describe electric current as a flow of charge
- Tell series and parallel circuits apart
- Link current, charge and time
Active recall
Quick check
Answer each question before opening the answer.
What is the equation linking charge, current and time?
Charge = current × time (Q = I × t).
What is electric current?
The rate of flow of electric charge, measured in amperes (A).
Physics revision app
Take this topic further in the app
275 guided topics with questions marked as you answer them, section checkpoints and timed practice papers. Fifteen topics are free to try, with no card needed.