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

This guide follows the OCR GCSE (9-1) Physics A (Gateway Science) specification (J249), Version 5.0, July 2026, for first assessment in 2018. It covers learning outcomes P3.1e, P3.1f and P3.1g from section P3.1 (Static and charge) and P3.2a and P3.2b from section P3.2 (Simple circuits), all inside Topic P3, Electricity. None of these statements is printed in bold in the specification, so all of them can be tested in both Foundation Tier and Higher Tier papers. Topic P3 content sits in the papers that assess Topics P1 to P4.

Related pages: revision notes, practice questions, the OCR GCSE Physics hub, the printable checklist, the free 10-minute diagnostics and the exam preparation guide.

What this unit covers

Statement	What you must be able to do
P3.1e	Recall that current is the rate of flow of charge (electrons), and the conditions needed for charge to flow: a source of potential difference and a closed circuit
P3.1f	Recall that, in a single closed loop, the current is equal wherever you measure it
P3.1g	Recall and use the relationship between charge, current and time
P3.2a	Describe the differences between series and parallel circuits, including where to place measuring instruments and how energy, current and potential difference behave
P3.2b	Represent d.c. circuits using the conventions for positive and negative terminals and the symbols for common circuit elements

1. Current is a flow of charge (P3.1e)

Charge is a property of matter. It is measured in coulombs (C). In a metal wire, the particles that move are electrons, which carry negative charge. Electric current is the rate of flow of charge: how much charge passes a point every second. Current is measured in amperes (A). One ampere means one coulomb of charge passing a point each second.

A larger current means more charge passes a point in each second. It does not mean charge is used up.

Conditions for charge to flow

Two conditions must both be met:

1. A source of potential difference. A cell, a battery of cells or a power supply pushes the charge round. Without it, nothing drives the electrons in a particular direction.
2. A closed circuit. There must be a complete conducting path from one terminal of the source, through the components, back to the other terminal. A gap, such as an open switch or a loose wire, stops the flow everywhere in that loop.

An insulator such as plastic in the loop has the same effect as a gap.

Conventional current

The specification refers to the measurement of conventional current. Conventional current is drawn as flowing from the positive terminal of the source, round the circuit, to the negative terminal. In a metal, the electrons actually move the other way, from negative to positive. Arrows on circuit diagrams show conventional current. When a question asks about the direction in which electrons move, say it is opposite to the arrows.

2. Same current everywhere in a single loop (P3.1f)

In a single closed loop, with one path and no branches, the current has the same value at every point. An ammeter placed before a lamp, after it, or beside the cell reads the same value.

The reason is that charge is not used up or stored by components. Each electron that enters a component is matched by one leaving it in the same time. What a lamp or a motor changes is the energy the charge carries, not the amount of charge that gets through each second. Energy is transferred to the surroundings; charge is not.

Common misconception: “The lamp uses up some of the current, so the reading after the lamp is smaller.” This is wrong for a single loop. Both readings are equal.

Worked example 1: ammeters in a loop

A cell, a switch, a lamp and a motor are connected in a single loop. Three ammeters are placed in the loop: one between the cell and the lamp, one between the lamp and the motor, and one between the motor and the cell. The first reads 0.24 A. State the other two readings and explain.

Answer: both read 0.24 A. There is one path and no branches, so each reading is equal, wherever the meter sits in the loop. Charge is not used up by the lamp or motor.

3. Charge, current and time (P3.1g)

If a steady current flows, the charge that passes grows in proportion to the time:

charge flow (C) = current (A) × time (s)

Q = I t

Here Q is charge in coulombs, I is current in amperes and t is time in seconds. The rearrangements are:

• I = Q / t
• t = Q / I

Three habits prevent most errors:

1. Convert time to seconds first. 2 minutes is 120 s, not 2.
2. Convert milliamperes to amperes by dividing by 1000. 250 mA is 0.250 A.
3. Write the unit with the answer: C, A or s.

Worked example 2: finding charge

A steady current of 0.35 A flows through a lamp for 2.0 minutes. Calculate the charge that flows.

t = 2.0 min = 2.0 × 60 = 120 s
Q = I t
Q = 0.35 × 120
Q = 42 C

Worked example 3: finding current

540 C of charge passes through a heater element in 15 minutes. Calculate the current.

t = 15 × 60 = 900 s
I = Q / t
I = 540 / 900
I = 0.60 A

Worked example 4: finding time, with a milliampere value

A current of 250 mA delivers 90 C to a battery. How long does this take? Give the answer in minutes.

I = 250 mA = 0.250 A
t = Q / I
t = 90 / 0.250
t = 360 s
t = 360 / 60 = 6.0 minutes

4. Series and parallel circuits (P3.2a)

The specification asks you to describe how series and parallel circuits differ. It highlights three things: where measuring instruments go, and how energy, current and potential difference behave.

Series circuits

In a series circuit, components are connected one after another in a single loop.

• Current: the same at every point (section 2). There is only one path.
• Potential difference: the potential difference of the supply is shared between the components in the loop. The more components in the loop, the smaller the share each receives.
• Energy: the energy supplied by the source is shared between the components in the single loop.
• If one component fails: the loop is broken, so every other component stops working as well.

Parallel circuits

In a parallel circuit, components are connected on separate branches that join at junctions.

• Current: the current from the supply splits at a junction, one part going down each branch, and the parts join again after the branches. The current in the supply equals the sum of the currents in the branches.
• Potential difference: each branch is connected directly across the source, so each branch has the same potential difference as the supply.
• Energy: each branch gets energy from the source directly. A component in one branch does not share its energy with a component in another branch.
• If one component fails: only its own branch is broken. The other branches keep working. This is why household lighting circuits and similar systems use parallel connection.

Feature	Series	Parallel
Paths for charge	One	Two or more
Current	Same everywhere	Splits between branches, then rejoins
Potential difference	Shared between components	Same across each branch
One component removed	Everything stops	Other branches keep working

Positioning measuring instruments

• Ammeter: measures current. Connect it in series with the component, so that the same current passes through the ammeter and the component. This means breaking the circuit and putting the ammeter into the gap.
• Voltmeter: measures potential difference. Connect it in parallel with the component, with its two terminals on either side of the component. You do not break the circuit to do this.

Worked example 5: current in a parallel circuit

Two lamps are connected in parallel to a power supply. The ammeter in the supply lead reads 0.80 A. An ammeter in the branch with lamp X reads 0.50 A.

(a) What is the current in the branch with lamp Y?

The supply current equals the sum of the branch currents:

0.80 = 0.50 + I(Y)
I(Y) = 0.80 - 0.50 = 0.30 A

(b) Calculate the charge that passes through lamp Y in 40 s.

Q = I t = 0.30 × 40 = 12 C

(c) Calculate the charge that passes through the power supply in the same 40 s.

Q = 0.80 × 40 = 32 C

The charge through the supply (32 C) equals the charge through X (0.50 × 40 = 20 C) plus the charge through Y (12 C). Charge is not lost at the junction.

5. Circuit diagrams and symbols (P3.2b)

Circuit diagrams use standard symbols drawn with straight lines and right angles. The specification lists these elements: cells, power supply, diodes, LDRs, NTC thermistors, filament lamps, ammeter, voltmeter, fixed and variable resistors and a switch. Learn to recognise and draw each one.

Component	How the symbol looks
Cell	Two parallel lines, one long and thin, one short and thick. The long line is the positive terminal.
Battery	Two or more cells drawn in a row, joined end to end
d.c. power supply	A symbol with marked positive and negative terminals; the terminals are labelled so that you know which way conventional current leaves
Switch	A gap in the line with a lever resting on one side. Closed when the lever touches both ends.
Filament lamp	A circle with a cross inside
Fixed resistor	A rectangle with a wire at each end
Variable resistor	A rectangle crossed by a sloping arrow
Diode	A triangle pointing at a vertical bar; conventional current passes in the direction the triangle points
LDR	A rectangle inside a circle, with two arrows pointing in to show light
NTC thermistor	A rectangle with a diagonal line through it that ends in a short horizontal hook
Ammeter	A circle containing the letter A
Voltmeter	A circle containing the letter V

Terminal conventions matter on a diagram. The positive terminal of a cell or supply is marked with a plus sign or by the longer line. Conventional current leaves the positive terminal. A diode conducts in one direction only, so the way round it is drawn matters.

Reading a diagram

Find the source and its positive terminal, then trace conventional current from it. If the path divides, the circuit has parallel branches. Check that each ammeter is inside a loop and each voltmeter is across a component.

Worked example 6: describing a circuit

A student wants to measure the current through a filament lamp and the potential difference across it. Describe how to connect the meters.

Connect a cell, a switch and the lamp in series. Place the ammeter in series, in the same loop, so the current in the lamp passes through it. Connect the voltmeter in parallel with the lamp, with one terminal on each side of the lamp.

Common errors

• Writing that “current is used up” by a lamp. Energy is transferred by the lamp; the current in a single loop is the same on both sides.
• Using minutes in Q = I t without converting to seconds.
• Using milliamperes as amperes.
• Wiring a voltmeter into the loop with a component, or an ammeter beside one on its own branch.
• Saying that a parallel circuit shares the potential difference between branches. Each branch has the same potential difference as the supply.
• Drawing the long and short cell lines the wrong way round.

Next steps

Try the practice questions, then use the revision notes for the final weeks. Check your coverage on the printable checklist and the OCR GCSE Physics hub, or take a 10-minute diagnostic.

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.