Why is voltage called potential difference?
"Potential" refers to potential energy -- for example, a test charge at rest in an electric field has potential energy. In a similar way, a brick held in the air has gravitational potential energy.
Between two points at different heights above the ground, there is a difference in potential. Between two points in a circuit with different voltages, there is a difference in potential. A voltage is a difference in potential, or a potential difference.
But it's not potential is it because there isn't a voltage before you switch the circuit on. If this statement is wrong then I still don't understand and then could someone else please try explaining it to me.
If there's no voltage, then there is no potential difference, that's correct. "Potential difference" is synonymous with "voltage."
The two ends of a AA battery have two different electric potentials; therefore, the battery presents a potential difference (or voltage). When you connect it to a circuit, the potential difference coaxes charges to move.
In a similar manner, when you let go of the brick you're holding in mid-air, the brick is coaxed to move by the difference in gravitational potential.
The fact is circiut source voltage is present across an open switch. The way to find an open in a live circiut is to measure voltages, looking for a wire or connection that measure source voltage.
There is your potetial.
Try thinking of it as pressure. A good analogy is water in a pipe. Voltage is the pressure whether the switch or valve is open or not. Current or amperage is the amount of water or electrons that flow. Obvisously when the switch is open there is no current flow but the pressure i.e. voltage/potenial is still there.
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