Electric current is the rate of flow of charge: I = Q/t. Potential difference is energy transferred per unit charge: V = W/Q. The units are ampere for current, coulomb for charge and volt for potential difference. Keep symbols and units visible throughout a calculation.
Ohm’s law and combinations
For an ohmic conductor under constant physical conditions, V = IR. Resistance R is measured in ohms, Ω. If temperature or other physical conditions change substantially, do not assume the same resistance or a linear V–I relationship.
For resistors in series, R = R₁ + R₂ + … and the current through each is the same. In parallel, 1/R = 1/R₁ + 1/R₂ + … and the potential difference across each branch is the same.
- A series equivalent resistance exceeds any single positive resistor in the chain.
- A parallel equivalent resistance is smaller than the smallest positive branch resistance.
Power and energy
Electrical power P = VI. For a resistor obeying V = IR, this also gives P = I²R = V²/R. Power is measured in watts. Energy transferred over a time t is E = Pt; watts multiplied by seconds give joules.
A kilowatt-hour is a unit of energy, not power. 1 kWh = 3.6 × 10⁶ J. Convert watts to kilowatts before multiplying by a time in hours to calculate kWh.
- Use circuit questions for practice; never experiment with household mains wiring.
Put it into practice
A worked example
A 3 Ω resistor has 6 V across it. Find its current and power.
- I = V/R = 6/3 = 2 A.
- P = VI = 6 × 2 = 12 W.
- Check with I²R = 2² × 3 = 12 W.
Answer: Current = 2 A; power = 12 W.
Try it yourself
Have a go before revealing the answer.
Find the equivalent resistance of 6 Ω and 3 Ω in parallel.
Show answer
1/R = 1/6 + 1/3 = 1/2, so R = 2 Ω.
Answer: 1/R = 1/6 + 1/3 = 1/2, so R = 2 Ω.
A 100 W device runs for five hours. How much energy does it use?
Show answer
0.1 kW × 5 h = 0.5 kWh.
Answer: 0.1 kW × 5 h = 0.5 kWh.
Watch out for these
- Do not add resistances directly when the resistors are in parallel.
- kW is power; kWh is energy.
Continue with official resources
Match these concept notes to your current textbook and syllabus. They are independent explanations, not official board publications.