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How Simple Cells Produce Electricity

Every time you turn on a wireless mouse or a torch, a chemical reaction is quietly pushing electrons around a circuit. A simple cell is a system containing chemicals that react to produce a potential difference (voltage).

To construct a simple cell, you need two different metals to act as electrodes and a liquid called an electrolyte. An electrolyte is a solution containing ions that conducts electricity by allowing charge to flow between the electrodes.

The electricity is generated through a step-by-step mechanism:

First, the two metals have different tendencies to lose electrons based on their position in the reactivity series.
Then, the more reactive metal loses electrons more easily, becoming oxidised and acting as the negative electrode.
Next, the electrons flow through the external wire (the circuit) from the more reactive metal to the less reactive metal (the positive electrode).
Finally, ions move through the electrolyte to complete the circuit.

Factors Affecting Cell Voltage

Why do some combinations of metals produce a tiny spark, while others can power a motor? The voltage produced by a cell depends heavily on the difference in reactivity between the two metal electrodes.

If you look at the reactivity series, the greater the difference in reactivity between the two metals, the higher the voltage produced. For example, a cell using magnesium and copper will produce a higher voltage than a cell using zinc and copper, because magnesium is much more reactive than zinc.

Other factors also affect the output, such as the type of electrolyte and the concentration of ions within it. However, if both electrodes are made of the exact same metal, the potential difference is exactly $0\text{ V}$ because there is no difference in their tendency to lose electrons.

Feature	Non-rechargeable Cells	Rechargeable Cells
Reaction Type	Feature an irreversible reaction (it only goes one way).	Feature a reversible reaction.
Why they stop	The chemical reactions stop when one of the reactants is used up.	They stop producing electricity when reactants are depleted, but can be restored.
How they restart	Cannot be restarted; they must be disposed of.	Can be recharged by applying an external electrical current to reverse the reactions.
Common Examples	Alkaline batteries.	Lithium-ion batteries, lead-acid batteries.

Students often say a battery stops working because it 'runs out of energy' or 'runs out of charge' — you must explicitly state that a chemical reactant is 'used up'.

When describing how simple cells work, always specify that electrons flow through the external wire, while ions move through the electrolyte. Confusing these two will lose you marks.

Pay close attention to terminology in exam questions: do not use the word 'battery' to describe a circuit diagram or setup that only contains a single 'cell'.

You may be asked to identify the most reactive metal from a table of experimental voltages; look for the metal that produces the highest voltage when paired with a fixed, less reactive metal like copper.

A system containing chemicals that react to produce a potential difference (electricity).

A measure of the energy transferred per unit charge, driven by the chemical reactions in the cell. AQA often uses this interchangeably with 'voltage'.

Solid conductors (usually different metals) through which electricity enters or leaves a cell.

A liquid or solution containing ions that allows the flow of charge between electrodes to complete a circuit.

Two or more cells connected together in series to provide a greater overall voltage.

A cell in which the chemical reactions stop when one of the reactants has been used up and cannot be reversed.

A chemical reaction that can only proceed in one direction (towards the products).

Common examples of non-rechargeable cells that must be discarded once a reactant is used up.

A cell in which the chemical reactions can be reversed by applying an external electrical current.

A chemical reaction that can proceed in both the forward and backward directions.

An electrical supply applied to a rechargeable cell to force the chemical reactions to reverse and regenerate the reactants.