B.5.2—Electrical energy sources

Two ways to supply emf

Chemical and solar cells both supply energy per unit charge, but they obtain that energy differently.

Feature	Chemical cell	Solar cell
Input energy	chemical potential energy	photon/radiant energy
Availability	works without illumination while reactants remain	output depends on illumination and cell area
Storage	primary cells are finite; secondary cells can be recharged	converts energy but does not itself store it
Electrical output	provides emf, normally dc	provides emf, normally dc

Boundary

Compare the energy source and operating conditions, not just the external circuit. A separate battery may store energy produced by a solar cell.

How it is assessed

The evidence uses comparison and classification: identify a power source operating on a different principle, or recognize an incorrect statement about photovoltaic cells, especially the claim that a photovoltaic cell generates alternating current.

Command terms

Identify / Distinguish / Explain

What earns marks

Identify the source type and connect its energy conversion to the electrical output. For a solar-cell question, check whether the statement concerns photon absorption, cell area, output power, storage, or current type; do not import generator behaviour into a photovoltaic cell.

Watch for

Confusing photovoltaic cells with rotating generators and therefore claiming that their direct electrical output is alternating current.

Representative question

[Maximum number: 1]

What is not correct about a photovoltaic cell?

It has an output power that is related to the surface area of the cell.

It generates an alternating current.

It absorbs energy over a range of photon frequencies.

It can be used to store energy in a secondary cell.

Source and transfer

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

Current and circuit laws

Conventional current is the direction positive charge would move, with $I=\Delta q/\Delta t$ . 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 $R=V/I$ for a component, $R= ho L/A$ 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, $arepsilon=I(R+r)$ 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.

B.5.2—Electrical energy sources

Compare Chemical and Solar Cells

B.5.2 Exam Analysis

Assessment in practice

Retrieve the B.5 Current and Circuits Model

Question 1