three 1.5v cells are connected in series in a circuit. what is the total potential difference?

The total potential difference is 4.5 V.

In a series circuit, the voltages of cells simply add together, because charge gains energy from each cell in turn as it moves around the circuit.

Here you have three cells, each of 1.5 V, so the total is
1.5 V+1.5 V+1.5 V=4.5 V1.5\text{ V}+1.5\text{ V}+1.5\text{ V}=4.5\text{ V}1.5 V+1.5 V+1.5 V=4.5 V.

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