A level Physics electricity

By The STEM-Tutor team · Updated October 2026

Electricity is a core A level Physics topic and a common source of lost marks. Here are the key ideas and how to apply them.

Charge, current and potential difference

• Current is the rate of flow of charge: I = ΔQ/Δt.
• Potential difference is energy transferred per unit charge: V = W/Q.
• Resistance is defined by R = V/I.

Resistivity

R = ρL/A. Resistance increases with length and decreases with cross-sectional area. Resistivity ρ is a property of the material.

I-V characteristics

• Ohmic conductor: a straight line through the origin at constant temperature.
• Filament lamp: resistance increases as it heats up, so the graph curves.
• Diode: conducts in one direction only, above a threshold voltage.
• Thermistors and LDRs: resistance falls as temperature or light intensity rises.

Series and parallel circuits

• Series: same current; potential differences add; R_total = R₁ + R₂ + …
• Parallel: same potential difference; currents add; 1/R_total = 1/R₁ + 1/R₂ + …

Kirchhoff's laws come from conservation of charge and energy.

Potential dividers

Two resistors in series split the supply voltage in the ratio of their resistances. With a thermistor or LDR, they make sensor circuits.

Emf and internal resistance

ε = I(R + r). Some energy is lost inside the source, so the terminal potential difference is less than the emf when current flows.

Common mistakes

• Confusing emf with terminal potential difference.
• Adding parallel resistances directly.
• Unit slips with mA, kΩ and mm².

Thinking about circuits clearly

Most electricity questions become simple once you hold on to two conservation laws. Kirchhoff's first law says charge is conserved: the current flowing into a junction equals the current flowing out. Kirchhoff's second law says energy is conserved: around any closed loop, the sum of the emfs equals the sum of the potential differences. Every rule about series and parallel circuits follows from these two ideas, so if you forget a rule, you can rebuild it.

It also helps to picture potential difference as energy transferred per coulomb of charge. A 6 V battery gives each coulomb 6 J of energy, and that energy is shared between the components in a loop. In a parallel circuit, each branch is a separate loop across the same supply, so each branch gets the full potential difference.

Worked examples

Example 1: charge and current

A current of 0.25 A flows for 2 minutes. How much charge passes, and how many electrons is that?

• Q = It = 0.25 × 120 = 30 C.
• Number of electrons = 30 / (1.6 × 10⁻¹⁹) ≈ 1.9 × 10²⁰.

Example 2: resistivity

A wire is 2.0 m long with cross-sectional area 1.0 × 10⁻⁷ m² and resistivity 4.9 × 10⁻⁷ Ωm. Find its resistance.

• R = ρL/A = (4.9 × 10⁻⁷ × 2.0) / (1.0 × 10⁻⁷) = 9.8 Ω.

Example 3: combining resistors

A 6 Ω resistor is in series with a parallel pair of 3 Ω and 6 Ω. Find the total resistance.

• Parallel pair: 1/R = 1/3 + 1/6 = 1/2, so R = 2 Ω.
• Total = 6 + 2 = 8 Ω.

Example 4: a potential divider

A 12 V supply is connected across 2 kΩ and 4 kΩ resistors in series. Find the potential difference across the 4 kΩ resistor.

• V_out = V_in × R₂/(R₁ + R₂) = 12 × 4/6 = 8 V.

Example 5: emf and internal resistance

A cell of emf 1.5 V and internal resistance 0.5 Ω is connected to a 2.5 Ω resistor. Find the current and the terminal potential difference.

• I = ε/(R + r) = 1.5/3.0 = 0.5 A.
• Terminal pd V = ε − Ir = 1.5 − 0.25 = 1.25 V. The 0.25 V difference is the ‘lost volts’ inside the cell.

Example 6: power

A 230 V kettle draws 10 A. Find its power, its resistance and the energy used in 3 minutes.

• P = IV = 2300 W. R = V/I = 23 Ω.
• E = Pt = 2300 × 180 = 414 kJ.

Sensors in potential dividers

A common question puts a thermistor or a light-dependent resistor (LDR) into a potential divider. Use the rule that the bigger resistance takes the bigger share of the voltage. As temperature rises, a negative temperature coefficient thermistor's resistance falls, so the voltage across it falls and the voltage across the fixed resistor rises. As light gets brighter, an LDR's resistance falls in the same way. Always say which component the output is taken across before explaining the change.

More mistakes to avoid

• Adding resistances in parallel as if they were in series.
• Forgetting to convert mm² to m² in resistivity questions (1 mm² = 10⁻⁶ m²).
• Confusing emf with terminal potential difference.
• Describing a filament lamp's I-V graph without explaining that its resistance rises because it heats up.

Practice questions

Try each one before opening the answer.

Frequently asked questions

Why does a filament lamp's resistance increase?

As current rises, the filament heats up. The metal ions vibrate more, so electrons collide with them more often, which increases resistance.

What is a superconductor?

A material whose resistance drops to zero below a critical temperature. It is useful for strong electromagnets and loss-free power cables.

Key takeaways

• Charge and energy are conserved: these give Kirchhoff's laws.
• Series resistances add; parallel reciprocals add.
• R = ρL/A links resistance to the shape of a wire.
• Terminal pd is emf minus the lost volts, Ir.
• In a potential divider, the bigger resistance takes the bigger share.

Where to go next

Electricity links to fields, capacitors and the practical skills you will be tested on.

• A level Physics required practicals
• A level Physics formula sheet
• A level Physics revision

Electricity rewards careful, step-by-step working. Label every current and voltage on your circuit diagram before you calculate, and most questions become routine.