Charge, Energy & Power

IGCSE Edexcel Physics
2.4–2.5, 2.14–2.21 Charge, current, energy and electrical power
Key Concepts: Charge: $Q = It$ (coulombs). Energy transferred: $E = QV = IVt$. Power: $P = IV = I^2R = V^2/R$ (watts). Current = rate of flow of charge.

Section A — Charge and Current

1. Define electric current in terms of charge. [2]
2. Write the equation linking charge ($Q$), current ($I$) and time ($t$). State the unit of charge. [2]
3. A current of $0.8\,\text{A}$ flows for 50 s. Calculate the charge transferred. [2]
4. A charge of 120 C flows through a wire in 4 minutes. Calculate the current. [2]

Section B — Energy Transferred

5. Write the equation linking energy transferred ($E$), charge ($Q$) and voltage ($V$). [1]
6. A charge of 2 C passes through a component with 6 V across it. Calculate the energy transferred. [2]
7. A charger transfers 1800 C of charge at 5 V. Calculate the energy transferred. [2]

Section C — Electrical Power

8. Write three equations for electrical power. [3]
9. A device uses a current of 3 A at 10 V. Calculate: [4]

a) the power

b) the energy transferred in 120 s

10. A 60 W lamp is connected to a 12 V supply. Calculate the current and the charge transferred in 30 s. [3]
11. A resistor of $5\,\Omega$ carries a current of $4\,\text{A}$. Calculate the power dissipated using $P = I^2R$. [2]
12. A 230 V supply delivers 2 kW of power. Calculate the current drawn. [2]

Total marks: 27

Mark Scheme

1. Electric current is the rate of flow of electric charge [1]; measured in amperes (A) [1] [2]
2. $Q = It$ [1]; unit: coulomb (C) [1] [2]
3. $Q = It = 0.8 \times 50 = 40\,\text{C}$ [2]
4. Convert: 4 min = 240 s [1]; $I = Q/t = 120/240 = 0.5\,\text{A}$ [1] [2]
5. $E = QV$ [1]
6. $E = QV = 2 \times 6 = 12\,\text{J}$ [2]
7. $E = QV = 1800 \times 5 = 9000\,\text{J}$ [2]
8. $P = IV$ [1]; $P = I^2R$ [1]; $P = V^2/R$ [1] [3]
9. a) $P = IV = 3 \times 10 = 30\,\text{W}$ [2]; b) $E = Pt = 30 \times 120 = 3600\,\text{J}$ [2] [4]
10. $I = P/V = 60/12 = 5\,\text{A}$ [1]; $Q = It = 5 \times 30 = 150\,\text{C}$ [2] [3]
11. $P = I^2R = 4^2 \times 5 = 16 \times 5 = 80\,\text{W}$ [2]
12. $I = P/V = 2000/230 \approx 8.7\,\text{A}$ [2]