GCSE Physics · Physics
Electricity: a clear study guide
Circuit questions become clearer when you separate charge flow, energy transfer and resistance. Draw the circuit, write the relevant equation and keep units consistent.
Current, charge and time
Current is the rate of flow of charge. Use Q = I t, where charge is measured in coulombs, current in amperes and time in seconds. Rearrange the equation only after identifying the quantity required.
In a series circuit the current is the same at every point. In a parallel circuit the current splits between branches and the total current is the sum of the branch currents.
- Q = I t
- Charge is measured in coulombs (C).
- Current is measured in amperes (A).
Potential difference and resistance
Potential difference measures the energy transferred per unit charge. Resistance describes how strongly a component opposes current. Use V = I R, with voltage in volts, current in amperes and resistance in ohms.
In series, potential differences add and the total resistance is the sum of the component resistances. In parallel, the potential difference is the same across each branch and the total resistance is lower than the resistance of the smallest branch.
A safe exam method
Label the values on the circuit, identify whether it is series or parallel, and write the equation before substituting. Check that the unit in the final answer matches the quantity asked for and use the component graph if the resistance changes with temperature or light.
Quick check
What is the current if 12 C flows in 3 s?
Answer: 4 A, using I = Q รท t.
What happens to potential difference across components in parallel?
Answer: The potential difference is the same across each branch.
Common questions
What is the difference between current and charge?
Charge is the amount of electrical charge transferred; current is the rate at which that charge flows.
Why does a filament lamp's resistance change?
As the filament heats, its resistance increases. Use the current-potential difference graph to describe the relationship accurately.