OCR GCSE Physics A: Current, charge and circuit conventions (J249) – Revision Notes

These notes condense the OCR GCSE (9-1) Physics A (Gateway Science) (J249) specification, Version 5.0, July 2026, statements P3.1e, P3.1f, P3.1g, P3.2a and P3.2b. All five are assessed in both tiers; none is Higher Tier only. For full explanations, read the study guide. When you have revised, test yourself with the practice questions, tick off statements on the checklist, or find weak spots with the free diagnostics. The course hub is the OCR GCSE Physics hub.

Key definitions

Term	Meaning
Charge (Q)	A property of matter; measured in coulombs (C)
Current (I)	The rate of flow of charge; measured in amperes (A)
Time (t)	Measured in seconds (s) in calculations
Conventional current	Drawn from the positive terminal round to the negative terminal
Electron flow	In a metal, electrons move from negative to positive, opposite to conventional current
d.c.	Direct current: flows in one direction only

P3.1e: what makes charge flow

Both conditions are needed:

1. A source of potential difference (cell, battery or power supply).
2. A closed circuit: a complete conducting loop.

Missing either one means no current. An open switch, a break in a wire or an insulator in the loop all give no current anywhere in that loop.

Must-know distinction: electrons move opposite to the arrows on a circuit diagram. The arrows show conventional current.

P3.1f: same current round a single loop

• One path, no branches: every point carries an equal current.
• An ammeter before a component and an ammeter after it read the same.
• Components transfer energy; they do not use up charge.

Method in steps for a question about ammeter readings in a loop:

1. Check there are no branches.
2. State that the current is equal everywhere in the loop.
3. Give the same value for each ammeter.

P3.1g: Q = I t

Quantity	Symbol	Unit
Charge	Q	coulomb, C
Current	I	ampere, A
Time	t	second, s

Three forms:

• Q = I × t
• I = Q / t
• t = Q / I

Method in steps:

1. Write down what you know, with units.
2. Convert minutes to seconds (multiply by 60) and mA to A (divide by 1000).
3. Choose the form that has your unknown on its own.
4. Substitute, calculate, and add the unit.

Worked reminders

Find charge. Current 2.5 A for 30 s.

Q = I t = 2.5 × 30 = 75 C

Find current. 72 C in 4.0 minutes.

t = 4.0 × 60 = 240 s
I = Q / t = 72 / 240 = 0.30 A

Find time. 33 C at 0.55 A.

t = Q / I = 33 / 0.55 = 60 s

Milliamperes. 280 mA for 50 s.

I = 0.280 A
Q = 0.280 × 50 = 14 C

P3.2a: series against parallel

Feature	Series	Parallel
Layout	Components one after another in one loop	Components on separate branches between the same two points
Current	Same everywhere	Splits at a junction; branch currents add up to the supply current
Potential difference	Shared between the components	Same across every branch
Energy	Source energy shared between the components in the loop	Each branch takes energy from the source directly
If one component breaks	The whole circuit stops	Other branches keep working

Meters

Meter	Measures	Connected
Ammeter	Current	In series, in the loop
Voltmeter	Potential difference	In parallel, across the component

Reminder of why: an ammeter has to have the current pass through it. A voltmeter compares the two sides of a component.

P3.2b: symbols and conventions

Component	Symbol in words
Cell	Long thin line (positive) and short thick line (negative)
Power supply	Marked with positive and negative terminals
Switch	A gap with a lever; closed when the lever touches both ends
Filament lamp	Circle with a cross
Fixed resistor	Rectangle
Variable resistor	Rectangle crossed by a sloping arrow
Diode	Triangle pointing at a bar; conventional current passes the way the triangle points
LDR	Rectangle in a circle, with arrows pointing in
NTC thermistor	Rectangle with a diagonal line ending in a hook
Ammeter	Circle with A
Voltmeter	Circle with V

Draw with a ruler, use right angles at corners and leave no gaps in the loop (unless a switch is meant to be open).

Fast links between the ideas

• Current needs a closed loop, so a break anywhere in a single loop stops the current everywhere in it.
• The same current in a single loop (P3.1f) is why Q = I t gives the same charge through every component in that loop in the same time.
• In a parallel circuit, apply Q = I t to each branch separately. Branch charges add up to the charge from the supply in the same time.
• Series: one loop, one current, potential difference shared. Parallel: several loops, current shared, potential difference the same across each branch.

Worked reminder: parallel branches

A supply current of 1.00 A splits into branch currents of 0.60 A and 0.40 A. In 20 s the branches carry 0.60 × 20 = 12 C and 0.40 × 20 = 8 C. The supply provides 1.00 × 20 = 20 C, and 12 + 8 = 20 C. The numbers agree.

Worked reminder: two-step question

A lamp carries 0.50 A. How many minutes does it take for 450 C to pass?

t = Q / I = 450 / 0.50 = 900 s
900 / 60 = 15 minutes

Writing good answers

• For “explain” questions about a series loop, use the pattern: one path, so the same current everywhere, so the readings are equal.
• For “describe how to connect” questions, give the position and the reason: ammeter in series so the same current passes through it; voltmeter in parallel so it compares the two sides of the component.
• For symbols, label the plus and minus of a cell and show the arrow of a diode in the direction of conventional current.
• Show each step of a calculation: equation, substitution, answer with unit.

Quick self-test

1. State the two conditions needed for charge to flow round a circuit.
2. In which direction do electrons move in a wire compared with conventional current?
3. Write the equation linking charge, current and time, with units.
4. Calculate the charge that flows when a current of 1.8 A passes for 25 s.
5. A current of 0.15 A flows for 3.0 minutes. Calculate the charge.
6. 66 C of charge flows in 2.0 minutes. Calculate the current.
7. A current of 20 mA flows. How long does it take for 72 C to pass? Give the answer in minutes.
8. In a series circuit, an ammeter before a lamp reads 0.40 A. What does an ammeter after the lamp read?
9. How should an ammeter be connected, and how should a voltmeter be connected?
10. In a parallel circuit the supply current is 1.20 A. One branch carries 0.45 A. What current is in the other branch (two branches only)?
11. Describe what happens to the other lamps when one lamp is removed from a series circuit and from a parallel circuit.
12. Describe the symbol for a diode and say which way conventional current flows through it.

Answers

1. A source of potential difference and a closed circuit.
2. They move in the opposite direction, from negative to positive.
3. Q = I t, with charge Q in coulombs (C), current I in amperes (A) and time t in seconds (s).
4. Q = 1.8 × 25 = 45 C.
5. t = 3.0 × 60 = 180 s; Q = 0.15 × 180 = 27 C.
6. t = 120 s; I = 66 / 120 = 0.55 A.
7. I = 0.020 A; t = 72 / 0.020 = 3600 s = 60 minutes.
8. 0.40 A: the current is the same everywhere in a single loop.
9. Ammeter in series with the component; voltmeter in parallel with the component.
10. 1.20 - 0.45 = 0.75 A.
11. Series: the loop is broken so the other lamps go out. Parallel: the other lamps stay lit because their branches are unaffected.
12. A triangle pointing at a bar; conventional current flows in the direction the triangle points.

Where marks are usually lost

• Giving only one condition for charge to flow when two are needed.
• Saying that current is “used up” or “lost” in a lamp.
• Forgetting to change minutes to seconds before using Q = I t.
• Treating 250 mA as 250 A.
• Leaving off the unit, or writing the wrong one (for example A for charge).
• Drawing arrows on a diagram from the negative terminal.
• Wiring a voltmeter into the loop, or an ammeter beside a component on its own branch.
• Describing parallel branches as sharing the potential difference, when each branch has the full supply value.
• Drawing a circuit with a gap that was not meant to be there, so no current could flow.

An examiner reading an explanation is looking for the physics idea written in a full sentence. A bare “it is a series circuit” gains little; “there is only one path, so the current is the same at each point” is the idea the question is after.

Official syllabus

OCR, GCSE (9-1) Physics A (Gateway Science) J249, Version 5.0, July 2026, for first assessment in 2018, Topic P3: Electricity, statements P3.1e to P3.1g and P3.2a to P3.2b.