Current, Charge and Potential Difference

This lesson covers the fundamental electrical quantities — current, charge and potential difference — as required by AQA GCSE Combined Science Trilogy (8464, section 6.2.1). These three quantities and the relationships between them form the foundation for every electricity topic at GCSE.

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Electric Current

Electric current is the rate of flow of electric charge. In metallic conductors (such as copper wires), the charge carriers are electrons. In electrolytes (solutions that conduct), the charge carriers are ions.

Current is measured in amperes (A) using an ammeter, which is always connected in series with the component being measured.

The Charge Equation

$Q = I \times t$Q=I×t

Where:

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

Rearranging: $I = \dfrac{Q}{t}$I=tQ​ and $t = \dfrac{Q}{I}$t=IQ​

Quantity	Symbol	Unit	Unit Symbol
Charge	Q	Coulombs	C
Current	I	Amperes	A
Time	t	Seconds	s

Exam Tip (AQA 8464): When defining electric current, say "the rate of flow of electric charge." Do not say "the flow of electrons" — current can also be carried by ions in solutions. The AQA mark scheme requires the precise definition involving charge.

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Conventional Current vs Electron Flow

• Conventional current flows from positive to negative (this was defined before electrons were discovered).
• Electron flow is from negative to positive (electrons are negatively charged, so they move toward the positive terminal).
• In your AQA exam, use conventional current direction unless the question specifically asks about electron flow.

flowchart LR
    subgraph "Direction of flow"
        direction LR
        A["(+) Terminal"] -->|"Conventional current ➜"| B["(−) Terminal"]
        B -->|"Electron flow ➜"| A
    end

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Potential Difference (Voltage)

Potential difference (p.d.) is the energy transferred per unit of charge that passes between two points in a circuit. It is sometimes called voltage.

Potential difference is measured in volts (V) using a voltmeter, which is always connected in parallel across the component being measured.

The Energy–Charge Equation

$E = Q \times V$E=Q×V

Rearranging: $V = \dfrac{E}{Q}$V=QE​

Where:

• $V$V = potential difference in volts (V)
• $E$E = energy transferred in joules (J)
• $Q$Q = charge in coulombs (C)

Quantity	Symbol	Unit	Unit Symbol
Potential difference	V	Volts	V
Energy transferred	E	Joules	J
Charge	Q	Coulombs	C

Exam Tip: A common mistake is confusing potential difference with current. Remember: potential difference tells you how much energy each coulomb of charge transfers. Current tells you how much charge flows per second.

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What Does 1 Volt Mean?

A potential difference of 1 V means that 1 joule of energy is transferred for every 1 coulomb of charge that passes through the component.

$1 \text{ V} = 1 \text{ J/C}$1 V=1 J/C

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Worked Examples

Worked Example 1 — Calculating Charge

A current of 3 A flows through a lamp for 2 minutes. Calculate the charge that flows.

Step 1: Convert time to seconds: $t = 2 \times 60 = 120 \text{ s}$t=2×60=120 s

Step 2: Use $Q = I \times t$Q=I×t

$Q = 3 \times 120 = 360 \text{ C}$Q=3×120=360 C

Worked Example 2 — Calculating Potential Difference

A charge of 50 C passes through a resistor and transfers 150 J of energy. Calculate the potential difference.

$V = \frac{E}{Q} = \frac{150}{50} = 3 \text{ V}$V=QE​=50150​=3 V

Worked Example 3 — Calculating Energy Transferred

A 12 V battery transfers charge of 200 C. How much energy is transferred?

$E = Q \times V = 200 \times 12 = 2400 \text{ J}$E=Q×V=200×12=2400 J

Worked Example 4 — Calculating Current

A charge of 900 C flows through a heater in 5 minutes. What is the current?

Step 1: Convert time: $t = 5 \times 60 = 300 \text{ s}$t=5×60=300 s

Step 2: $I = \dfrac{Q}{t} = \dfrac{900}{300} = 3 \text{ A}$I=tQ​=300900​=3 A

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Common Mistakes

Mistake	Correction
Forgetting to convert minutes to seconds	Always convert to seconds before using $Q = It$Q=It
Confusing current and voltage	Current = rate of charge flow; voltage = energy per unit charge
Saying current is "used up"	Current is not used up — it is the same at all points in a series circuit
Using electron flow direction for conventional current	Use positive-to-negative unless asked specifically about electron flow

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Summary

• Electric current is the rate of flow of charge: $Q = I \times t$Q=I×t.
• Current is measured in amperes (A) with an ammeter in series.
• Potential difference is the energy transferred per coulomb: $E = Q \times V$E=Q×V.
• Potential difference is measured in volts (V) with a voltmeter in parallel.
• Conventional current flows from positive to negative; electrons flow from negative to positive.
• 1 V = 1 J per coulomb of charge.
• Always convert time to seconds before calculating.

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Extended Worked Examples

Worked Example 1 — Charge in a Torch

A torch is switched on for 5 minutes. The current in the bulb is 0.30 A. Calculate the total charge that flows.

Step 1 — Convert time: 5 minutes × 60 s = 300 s. Step 2 — Use the equation: $Q = I \times t$Q=I×t. Step 3 — Substitute: $Q = 0.30 \times 300 = 90$Q=0.30×300=90 C.

The bulb transfers 90 coulombs of charge in 5 minutes.

Worked Example 2 — Energy Transferred by a Battery

A 12 V car battery drives a current of 5.0 A through a starter motor for 3.0 s. Calculate the energy transferred.

Step 1 — Charge moved: $Q = I \times t = 5.0 \times 3.0 = 15$Q=I×t=5.0×3.0=15 C. Step 2 — Energy transferred: $E = Q \times V = 15 \times 12 = 180$E=Q×V=15×12=180 J.

Alternatively, using $E = I \times V \times t = 5.0 \times 12 \times 3.0 = 180$E=I×V×t=5.0×12×3.0=180 J — the same answer.

Worked Example 3 — Reverse Calculation

A heater transfers 9 600 J of energy when 40 C of charge passes through it. Calculate the potential difference.

Using $E = QV$E=QV, $V = E/Q = 9600 \div 40 = 240$V=E/Q=9600÷40=240 V.