Imagine a simple circuit: a battery, a wire, a resistor. A positive charge leaves the terminal, crosses the resistor losing energy (dissipated as heat), arrives at the terminal at lower potential, then enters the battery and must return to the terminal.
The delicate point is precisely this last stretch. Inside the battery the charge is moving against the internal electric field: the electrostatic field alone would push it back, towards the terminal. Something, inside the battery, must do positive work on it to bring it back to the positive pole.
Principle — A non-electrostatic force is needed
To sustain the current, inside the source the positive charges must climb back up the potential, that is, move against the electrostatic field. Only a non-electrostatic force can perform the necessary positive work.
This “something” is a non-electrostatic force, whose origin varies with the type of source:
- chemical in a battery;
- mechanical in a dynamo generator;
- thermal in a thermocouple;
- light-driven in a photovoltaic cell.
The effect, whatever its origin, is the same: to push the positive charges towards the terminal, compensating for the internal electrostatic “drop”. It is this force that sustains the potential difference at the terminals and hence the current in the external circuit.
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Topics: Circuiti elettrici Concepts: Forza elettromotrice
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