Electric CurrentCambridge IGCSE Physics: Revision notes
Section 1
What Is Electric Current?
Electric current is the flow of electric charge around a circuit. In a metal wire this charge is carried by moving electrons.
- Current is measured using an ammeter, connected in series with the component under test.
- Ammeters can be analogue (needle and scale) or digital (numerical display), and are available with different current ranges so the correct range can be chosen for the size of current expected.
Always connect an ammeter in series, never in parallel — it has very low resistance and connecting it in parallel would short-circuit the component.
Section 2
How Do Metals Conduct Electricity?
Metals conduct electricity because they contain free electrons (also called delocalised electrons) that are not bound to any particular atom.
- When a potential difference is applied across a metal, these free electrons drift through the fixed lattice of positive metal ions, carrying charge from one end of the wire to the other.
- This movement of free electrons is what is measured as electric current in a metallic conductor.
Think of free electrons like marbles already filling a tube — push one in at one end and one immediately pops out the other end, even though no single marble travels the whole tube instantly.
Section 3
Direct Current and Alternating Current
Electric current can flow in two different ways.
| Type | Description | Example source |
|---|---|---|
| Direct current (d.c.) | Charge flows in one direction only | Cells and batteries |
| Alternating current (a.c.) | Charge repeatedly reverses direction | Mains electricity supply |
Section 4
Defining and Calculating Current
Electric current (I) is defined as the rate of flow of charge — the amount of charge passing a point per unit time.
I = Q / t
where I is current in amperes (A), Q is charge in coulombs (C), and t is time in seconds (s).
Direction matters:
- Conventional current is defined as flowing from positive to negative around a circuit.
- The actual flow of free electrons is from negative to positive — the opposite direction to conventional current.
Don't mix up the two directions — conventional current (+ to −) and electron flow (− to +) go opposite ways. Exam questions often test this directly.
Must Know
- Electric current is the flow of charge, measured with an ammeter connected in series.
- Metals conduct because free electrons drift through the fixed positive ion lattice.
- Direct current (d.c.) flows one way; alternating current (a.c.) keeps reversing direction.
- I = Q / t — current equals charge divided by time.
- Conventional current flows from + to −; electrons actually flow from − to +.
That's the notes covered.
Carry on to the next subtopic.