IGCSE §2: Electricity · IGCSE
Current, Charge and Voltage
Revision notes on Current, Charge and Voltage for the OCR A-level Physics specification (H556). Free to read, with 4 practice questions in the app.
Electric current — the rate of flow of electric charge:
I = Q / t
with current in amperes (A), charge in coulombs (C) and time in seconds. Rearranged, Q = I t.
Charge — measured in coulombs. One coulomb is the charge that passes when a current of one ampere flows for one second.
What is actually moving. In a metal, the current is a flow of free electrons, which are negative and drift from the negative terminal towards the positive one. Conventional current is drawn the other way — from positive to negative — because it was defined before anyone knew electrons existed. Both are correct; questions expect conventional current on circuit diagrams unless they say otherwise.
Potential difference (voltage) — the energy transferred per unit charge between two points:
V = E / Q
with p.d. in volts (V) and energy in joules. One volt is one joule per coulomb.
e.m.f. and p.d. — a cell or supply provides the energy, and the voltage across it is its e.m.f.; a component uses the energy, and the voltage across it is a potential difference. The distinction matters in wording, not in units.
Measuring them:
- An ammeter measures current and is connected in series, because the current it reads must pass through it. An ideal ammeter has zero resistance.
- A voltmeter measures p.d. and is connected in parallel with the component, because it compares two points either side. An ideal voltmeter has infinite resistance so it draws no current.
Wiring an ammeter in parallel across a supply short-circuits it. This is worth remembering for both the exam and the laboratory.
Conductors and insulators — a conductor has free charges that can move, so current flows through it easily; metals and graphite are conductors. An insulator has no free charges, so current does not flow; plastic, glass and rubber are insulators.
Example: a current of 0.50 A flows for 2 minutes. The charge passing is Q = It = 0.50 × 120 = 60 C. Substituting 2 rather than 120 for the time gives 1 C, which is wrong by a factor of sixty.
4 questions on Current, Charge and Voltage
Reading is not revising. This topic has 4 questions in the app — multiple choice, calculations, and written answers marked against the mark scheme, point by point. Miss one and it comes back later with its options reshuffled, so you have to know it rather than remember where the answer sat.
- Every topic of Edexcel International GCSE Physics (4PH1)
- Flashcards and every equation, linked to the topic it comes from
- Full mock papers in the real paper structure
20 minutes a day is free. No card, nothing to cancel — and the paid version never renews itself either.
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