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B.5.1—Cells provide a source of emf

Syllabus
First assessment 2025
Objective
—
Level
SL

Explain How Cells Provide emf

A cell as an energy source

A cell transfers energy from a non-electrical source, such as chemical or solar energy, to charge carriers. The energy source establishes an electromotive force (emf) that can drive charge around a circuit.

Meaning of emf

The emf is the energy supplied by the source per unit charge when charge passes through the source. Its unit is the volt, 1 V=1 J C−11\,\mathrm V=1\,\mathrm{J\,C^{-1}}.

Follow the energy

The cell is not a reservoir of charge that gets used up. Charge circulates; the cell supplies energy that is transferred in circuit components such as lamps, motors and resistors.

Common trap

Emf is not the same as current. Emf is energy per charge supplied by the source; current is charge flow per unit time.

B.5.1 Exam Analysis

Assessment in practice

1–2 marks
How it is assessed

The evidence tests short definitions and identification of emf in a circuit, including selecting the source quantity and distinguishing it from a potential difference across a component.

Command terms

State / Define / Identify

What earns marks

Define emf as energy supplied by the cell per unit charge, then distinguish it from terminal potential difference when current flows. If a numerical relationship is required, identify the charge or energy quantity first, use consistent units, and state the unit of the result.

Watch for

Treating emf as the same quantity as terminal voltage in every situation.

Representative question

Question 1

[Maximum number: 1]

State the emf of the cell.

Retrieve the B.5 Current and Circuits Model

Source and transfer

Cells provide emf arepsilonarepsilon, the energy transferred per unit charge by the source. Electrical energy transferred in a circuit is E=VItE=VIt, and power is P=VI=I2R=V2/RP=VI=I^2R=V^2/R. Keep emf, terminal potential difference, energy and power distinct.

Current and circuit laws

Conventional current is the direction positive charge would move, with I=Δq/ΔtI=\Delta q/\Delta t. In DC, the direction is constant; in AC, it reverses periodically. Apply Kirchhoff’s junction rule to charge conservation and the loop rule to energy conservation.

Resistance model

Use R=V/IR=V/I for a component, R=hoL/AR= ho L/A for a uniform conductor, and the correct series or parallel combination rule. Ohmic behaviour means constant resistance at constant physical conditions; non-ohmic behaviour requires reading the gradient or ratio from the graph at the stated point.

Real and variable components

For a real cell, arepsilon=I(R+r)arepsilon=I(R+r) and V= arepsilon-Ir. A variable resistor changes circuit resistance; LDRs and thermistors use a stimulus-dependent resistance. Before calculating, draw or inspect the circuit, identify the fixed quantity, and state the relevant assumption.