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Answer: the cells. A cell contains chemicals which are a store of energy. They have to do work to push the current around the circuit. By doing this work, the chemicals get used up and, eventually, the cell will need to be replaced. Chemcial energy has been transferred to light.
On page 4, we saw that an electric current is like a moving bicycle chain. The cell is like your muscles. Your muscles use chemical energy (from food) to do work to push the chain round. Eventually, you would have to eat some more food to keep pedalling!
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|Picture 1.10. A cell produces a voltage using a chemical reaction inside the casing.|