Physics · Both
Electricity – Current, Voltage & Circuits
Current, potential difference, resistance, Ohm’s law, series and parallel circuits.
Electric circuits transfer energy. You must use the relationships between current, voltage and resistance, and know how series and parallel circuits behave.
Key quantities
• **Current (I)** – flow of charge, measured in amperes (A), ammeter in series
• **Potential difference / voltage (V)** – energy per unit charge, volts (V), voltmeter in parallel
• **Resistance (R)** – opposition to current, ohms (Ω)
**Ohm’s law:** V = I R
(R = V / I, I = V / R)
Worked example
A current of 2 A flows through a resistor when 10 V is applied. What is the resistance?
Solution
R = V / I = 10 / 2 = **5 Ω**
Series vs parallel
**Series** (one loop):
• Same current through all components
• Total V = sum of voltages across components
• Total R = R₁ + R₂ + …
**Parallel** (branches):
• Same voltage across each branch
• Total current = sum of branch currents
• Adding more branches decreases total resistance
Power
Power (W) = energy transferred per second
P = V I
Also P = I² R
Units: watts (W)
Practice questions
Try these without looking at the answers first.
Q1.V = 12 V, R = 4 Ω. Current?
Show answer
3 A
Q2.In a series circuit, is current the same everywhere?
Show answer
Yes
Q3.P = V I. Power when V = 230 V and I = 2 A?
Show answer
460 W