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Electricity and magnetism

Batteries and cells

A lemon batteryA lemon battery Batteries store chemical energy and change it into electrical energy. Batteries are the common name for cells: units that produce electricity. They have three main parts: a negative electrode, a positive electrode and a mixture of chemicals in a moist paste (in dry batteries) or liquid form (in wet batteries), called an electrolyte. An electrolyte is a liquid that conducts electricity: it allows ions—atoms that have become electrically charged—to flow through the battery. A chemical reaction inside the battery causes electrons to flow from the negative electrode along a circuit, which includes the device that is being powered (the "load"), and back to the positive electrode. The points at which the circuit connects to the battery are called terminals.


Button cellsButton cells

Primary or secondary?

Primary batteries can only be used once. They transform chemical energy into electrical energy but cannot do the reverse. When the supply of reacting chemicals is exhausted, the battery must be discarded. Most alkaline batteries are primary batteries.

Batteries that can be recharged are called secondary batteries. The chemical reactions that produce electricity can be reversed by supplying electrical energy to the battery. Examples include the lead-acid batteries used in cars and lithium-ion batteries used for portable electronic devices such as mobiles and laptops.

The electric eel is like a living, swimming battery. Muscular organs, which run along most of its body, create electricity by the movement of ions (electrically charged atoms) inside it. When ready, the organs send out powerful blasts of electricity into the water—enough to stun or kill nearby fish. Some eels can produce up to 650 volts.

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