Electricity and Circuits

IGCSE Edexcel Physics
2.1–2.21 Electricity and Circuits
Key Concepts: Current is the rate of flow of charge. Voltage is energy per charge. Series and parallel circuits behave differently. Power and energy can be calculated using $P = IV$ and $E = IVt$.

Section A: Units and Safety

1. State the SI units for current, charge, voltage, resistance, power, energy and time. [7]
2. Describe how insulation, double insulation, earthing, fuses and circuit breakers protect users. [5]
3. Explain why current in a resistor transfers energy and causes a temperature increase. Give one domestic use. [3]
4. State the difference between AC and DC. [2]

Section B: Circuits and Current

5. Explain why series or parallel circuits are used for different applications (e.g. household lighting). [3]
6. A resistor has 12 V across it and current of 3 A. Calculate its resistance. [2]
7. Describe how current and voltage behave in series and parallel circuits. Include current at a junction and voltage across parallel components. [5]
8. Explain how current in a series circuit depends on the supply voltage and the components. [2]

Section C: Charge and Current

9. Explain what electric current is in metals and calculate the charge for 2 A flowing for 3 s. [3]
10. State why lamps and LEDs can indicate the presence of current. [2]
11. Define voltage as energy transferred per unit charge and state the unit definition of the volt. [2]

Section D: I–V Characteristics and Sensors

12. Describe the I–V characteristics of a resistor, filament lamp and diode. [4]
13. Explain how resistance changes in an LDR and in a thermistor. [2]
14. Describe qualitatively how changing resistance affects current for a fixed voltage. [2]

Section E: Power and Energy

15. A device uses 4 A at 12 V. Calculate the power and energy used in 3 minutes. [4]
16. A charge of 0.5 C passes through a component with 9 V across it. Calculate the energy transferred. [2]
17. Two resistors of 2 ohm and 4 ohm are connected in series to a 12 V supply. Calculate the current and the voltage across each resistor. [4]
Total marks: 48

Mark Scheme

1. A, C, V, ohm, W, J, s [7]
2. Insulation prevents contact; double insulation uses two layers; earthing provides low-resistance path; fuse melts; circuit breaker switches off [5]
3. Electrical energy transfers to thermal energy in resistor; temperature rises; e.g. kettle/toaster [3]
4. AC changes direction; DC flows one direction [2]
5. Series: same current, one failure breaks circuit; parallel: same voltage, components work independently [3]
6. $R = V/I = 12/3 = 4\,\Omega$ [2]
7. Series: current same, voltage shared; parallel: voltage same, current splits; current conserved at junction [5]
8. Increasing voltage increases current; adding components increases resistance so current falls [2]
9. Current is flow of electrons; $Q = It = 2 \times 3 = 6\,C$ [3]
10. Lamps/LEDs emit light when current flows so show circuit is complete [2]
11. Voltage is energy transferred per unit charge; $1\,V = 1\,J/C$ [2]
12. Resistor: straight line; filament lamp: curve; diode: conducts one direction only [4]
13. LDR resistance decreases with light; thermistor resistance decreases with temperature (NTC) [2]
14. Higher resistance means lower current for fixed voltage [2]
15. $P = IV = 48\,W$; $E = IVt = 48 \times 180 = 8640\,J$ [4]
16. $E = QV = 0.5 \times 9 = 4.5\,J$ [2]
17. $R_{total} = 6\,\Omega$, $I = 12/6 = 2\,A$; $V$ across $2\,\Omega = 4\,V$, across $4\,\Omega = 8\,V$ [4]