4.1.1Electrons, Waves and Photons

Electric Current and Charge — OCR A Level Physics A H556 Revision Notes & Topical Questions

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📚 OCR A Level Physics A H556 Hub / Topical Revision

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Lesson summary

4.1.1 Electric Current and Charge — OCR A Level Physics A notes: Define current and charge, use I = ΔQ/Δt and Q = It, and state the conservation of charge.Matter contains positive protons, negative electrons and neutral neutrons.

Electric charge
Kirchhoff's first law
Worked example 1
Worked example 2
Worked example 3
Elementary charge

Part 1 of 7Charge

Matter contains positive protons, negative electrons and neutral neutrons. Electric charge Q is measured in coulombs (C).

The charge of a proton is +e and of an electron −e, where the elementary charge e = 1.60 × 10⁻¹⁹ C.

Quantised

All charge on an object is an integer multiple of e (charge is quantised).

Because 1 C is a very large charge, equal to about 6.2 × 10¹⁸ electrons, microcoulombs and nanocoulombs are common.

Like charges repel and opposite charges attract.

A body becomes charged when electrons are transferred to or from it, for example by rubbing an insulator,

and then has a net charge ±ne for an integer n.

Part 2 of 7Electric current

Electric current I is the rate of flow of charge: I = ΔQ/Δt, where ΔQ is the charge in coulombs passing a point in time Δt seconds.

The unit is the ampere (A), which is one coulomb per second, and the ampere is an SI base unit.

Rearranging, Q = It: the charge that has passed in a given time is the current multiplied by the time.

On a graph of charge against time, the gradient gives current; on a graph of current against time, the area under the curve gives the charge passed.

A current of 2.0 A for 30 s moves 60 C.

Current is measured with an ammeter connected in series, so all of the current passes through it. An ideal ammeter has zero resistance.

Part 3 of 7Conventional current and charge carriers

Conventional current is defined as flowing from the positive terminal of a source through the circuit to the negative terminal, the direction in which positive charge would move.

Conventional current and charge carriers

In metals the charge carriers are actually free (delocalised) electrons that move in the opposite direction, from negative to positive.

In electrolytes and gases, both positive and negative ions move, in opposite directions, and both contribute to the current.

The direction of the arrow on a circuit diagram is always the conventional one.

Part 4 of 7Conservation of charge

Charge is never created or destroyed, so in an electric circuit the total charge entering a point equals the total charge leaving it.

1Kirchhoff's first law

This leads to Kirchhoff's first law: the sum of the currents entering a junction equals the sum of the currents leaving it.

In a series circuit the current is the same everywhere because charge does not accumulate.

In a parallel circuit the currents in the branches add up to the current in the main wire.

If 5.0 A enters a junction and 2.0 A leaves in one branch, the other branch carries 3.0 A.

This law means that an ammeter reads the same wherever it is placed in a series loop, and that current is not "used up" by components; energy is transferred, but the charge flow continues around the circuit.

Part 5 of 7Units and quantities

Useful relations

1 A = 1 C s⁻¹;
- 1 mA = 10⁻³ A;
- 1 μA = 10⁻⁶ A.

Units and quantities

A battery rated at 2000 mA h can supply 2.0 A for 1 h, a charge of 2.0 × 3600 = 7200 C.

The number of electrons passing per second in a current I is I/e; for 1.0 mA this is 10⁻³/1.60 × 10⁻¹⁹ = 6.3 × 10¹⁵ s⁻¹.

Part 6 of 7Applications and problem approach

When solving problems, first list what you know and convert all quantities to base units: minutes to seconds, milliamperes to amperes.

Applications and problem approach

Use Q = It to link charge to current and time, Q = Ne to link charge to the number of elementary charges,

and the junction law to find missing currents.

Graphs of current against time appear often: calculate the area as a sum of rectangles and triangles, or count squares.

Charge storage in capacitors, covered later, builds on these ideas,

as does the energy transferred per unit charge in the next topic on potential difference.

Part 7 of 7Worked examples

A current of 0.80 A flows through a lamp for 2.0 minutes. Find the charge that passes and the number of electrons.
A current–time graph is a straight line from 0 A at t = 0 to 4.0 A at t = 10 s. Find the charge passed.
At a junction 3.5 A enters along wire A and 1.2 A leaves along wire B. Find the current in wire C, which also leaves.

Key terms

Term	Meaning
Charge	Property of matter that causes electric forces, in coulombs.
Current	Rate of flow of charge, I = ΔQ/Δt.
Ampere	One coulomb per second.
Elementary charge	1.60 × 10⁻¹⁹ C, the size of the electron charge.
Conventional current	Direction positive charge would flow, from + to −.
Kirchhoff's first law	Total current into a junction equals total current out.

Avoid these mistakes

Saying current is used up in a bulb. Charge keeps flowing; energy is transferred.

Mixing up the direction of conventional current and electron flow.

Using minutes in Q = It; time must be in seconds.

Confusing charge (C) and current (A).

Placing an ammeter in parallel across a component.

Last updated: 18 October 2026