AQA 7408 · Section 3.5.1
Current, charge and electrical energy
Connect charge flow to potential difference, energy and power. Predict the effect of each change, then use the live model to check your reasoning.
Active revision
Predict, test, explain
Predict
Commit to a direction or value before moving a control.
Test
Change one variable and compare the evidence with your prediction.
Explain
Use an equation and a physical reason, then attempt the exam questions.
Key knowledge
Follow the charge and the energy
Current
Current is the rate of flow of charge. A current of 1 A means 1 C passes a point each second. Charge is conserved: components transfer energy, not charge.
Potential difference
Potential difference is the work done per unit charge. One volt is one joule per coulomb.
Energy and power
Power is the rate of energy transfer. Select the power equation that uses known quantities; do not treat the three forms as unrelated facts.
Interactive investigation
Electrical energy transfer laboratory
Predict before changing the controls
How will doubling voltage affect power if resistance stays constant? Test it, then explain using P = V²/R.
Calculation method
A reliable four-step method
1. Identify
Write the quantity required and its unit.
2. Convert
Convert minutes, mA and kΩ before substitution.
3. Select
Choose the equation containing the known quantities.
Original exam-style practice
Questions and worked answers
1. A 0.40 A current flows for 3.0 minutes. Find the charge transferred. [2 marks]
t = 180 s. Q = It = 0.40 × 180 = 72 C.
2. A motor transfers 540 J when 45 C passes through it. Find the p.d. [2 marks]
V = W/Q = 540/45 = 12 V.
3. A 24 Ω heater is connected to 12 V for 5.0 minutes. Calculate the energy transferred. [4 marks]
P = V²/R = 144/24 = 6.0 W. E = Pt = 6.0 × 300 = 1.8 × 10³ J.
4. Explain why the current entering a lamp equals the current leaving it. [2 marks]
Charge is conserved and does not steadily accumulate inside the lamp. Therefore the rate of charge flow into it equals the rate out.
Specification checklist
AQA 7408 coverage
- 3.5.1.1(a) Define electric current as the rate of flow of charge, I = ΔQ/Δt.
- 3.5.1.1(b) Define potential difference as work done per unit charge, V = W/Q.
- 3.5.1.4(c) Use electrical energy equation E = IVt.
- 3.5.1.4(d) Use power equations P = IV, P = I²R and P = V²/R.
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Written against AQA Physics 7408 section 3.5.1. All questions are original and designed for active revision.