Electrical Charge and Current

📖 In-Depth Theory

Electric Current — Flow of Charge

ELECTRIC CURRENT is a flow of electrical charge.

For current to flow, two conditions must be met:

1. There must be a CLOSED CIRCUIT — a complete, unbroken conducting path.

2. There must be a SOURCE OF POTENTIAL DIFFERENCE (a battery or power supply) to drive the charge.

Current is measured in AMPERES (A).

1 ampere = 1 coulomb of charge passing a point per second.

In metal conductors, the charge carriers are FREE ELECTRONS.

In solutions (electrolytes), the charge carriers are IONS.

CONVENTIONAL CURRENT flows from + to − around the external circuit.

ELECTRON FLOW is actually from − to + (electrons repelled from the negative terminal).

Conventional current direction was defined before electrons were discovered.

Charge, Current and Time

Q = I × t

Q = charge flow (coulombs, C)

I = current (amperes, A)

t = time (seconds, s)

Rearranging:

I = Q ÷ t

t = Q ÷ I

EXAMPLE 1:

A current of 2 A flows for 30 s:

Q = 2 × 30 = 60 C

EXAMPLE 2:

120 C flows in 4 minutes:

t = 4 × 60 = 240 s

I = 120 ÷ 240 = 0.5 A

One coulomb ≈ 6.24 × 10¹⁸ electrons.

Current in Series and Parallel Circuits

SERIES circuit:

Current is the SAME everywhere — only one path for charge.

I₁ = I₂ = I₃

PARALLEL circuit:

Current SPLITS at junctions — more paths available.

I_total = I₁ + I₂ + I₃

Branches with lower resistance carry more current.

MEASURING CURRENT:

Use an AMMETER connected in SERIES.

Ammeters have very LOW resistance — do not significantly affect the circuit.

⚠️ Common Mistake

Time must be in SECONDS when using Q = It. If given minutes, multiply by 60 first. In a series circuit, current is the SAME at every point — it does not get 'used up' passing through components.

📐 Variables

QCharge (Q) is measured in coulombs (C)

ICurrent (I) is measured in amperes (A)

tTime (t) is measured in seconds (s)

📐 Key Equations

Q = I × t

📌 Key Note

Current = rate of flow of charge. Q = It. Unit: ampere (A). Series: same current everywhere. Parallel: current splits — I_total = sum of branches. Charge carriers in metals = electrons. Conventional current: + to −. Electron flow: − to +.

🎯 Matching Activity — Charge and Current

Match each scenario to the correct value. — drag the symbols on the right to match the component names on the left.

60 C

Drop here

0.5 A

Drop here

Same everywhere

Drop here

Splits at junctions

Drop here

Charge = 90 C, time = 3 minutes — I = 90 ÷ 180

Current at each point in a series circuit

Current = 3 A, time = 20 s — Q = 3 × 20

Current behaviour in a parallel circuit

⚽ FIFA Worked Examples

Charge Calculation

A current of 0.4 A flows through a lamp for 5 minutes. Calculate the charge that flows.

F

Q = I × t

I

I = 0.4 A, t = 5 × 60 = 300 s

F

Q = 0.4 × 300

A

Q = 120 C

🧪 Required Practical

🔬 RP15 (Physics) — Set up series and parallel circuits; measure current with ammeters at different positions to verify series current is constant and parallel currents sum to total.

Know the method, variables, equipment and how to analyse results.

🎯 Test Yourself

Question 1 of 2

1. A charge of 180 C flows through a resistor in 2 minutes. What is the current?

2. In a series circuit, an ammeter before the first lamp reads 0.3 A. What does an ammeter between the two lamps read?

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Electric Current — Flow of Charge

Charge, Current and Time

Current in Series and Parallel Circuits

🔬 RP15 (Physics) — Set up series and parallel circuits; measure current with ammeters at different positions to verify series current is constant and parallel currents sum to total.

Mr. Badmus AI