Difference between electromotive force and potential difference
Electromotive force and potential difference (voltage) are two concepts that are easily confused. The electromotive force is the ratio of the work done by the non-electrostatic force to move the unit positive charge from the negative electrode through the power supply to the positive electrode. The potential difference indicates that the electrostatic force moves the unit positive charge from a certain point in the electric field to another point. The ratio of work to charge. They are two completely different concepts.
Although the electromotive force is different from the potential difference (voltage), the electromotive force is the same as the potential difference. For a given power supply, regardless of the external resistance, the electromotive force of the power supply is always constant, and the voltage of the power supply terminal varies with the external resistance, which is a physical quantity that characterizes the external circuit.