Current, Voltage and Resistance

This lesson introduces the three fundamental electrical quantities — current, voltage (potential difference) and resistance — as required by the Edexcel GCSE Combined Science specification (1SC0). You must understand what each quantity means, how it is measured and how the three are related.

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Electric Current

Electric current is the rate of flow of electric charge around a circuit.

$I = \frac{Q}{t}$I=tQ​

where:

• I = current in amperes (A)
• Q = charge in coulombs (C)
• t = time in seconds (s)

Quantity	Symbol	Unit	Measuring Instrument
Current	I	Ampere (A)	Ammeter (connected in series)

Key Points

• Current is carried by electrons in metal wires.
• Electrons are negatively charged and flow from the negative terminal to the positive terminal of the battery.
• Conventional current is defined as flowing from positive to negative — this is the direction used in circuit diagrams and calculations.

Exam Tip: Do not confuse electron flow with conventional current. In exam answers, unless specifically asked about electrons, use conventional current (positive to negative).

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Charge and Current