It is the electrical strength or “pressure” developed by a source of electrical energy such as a battery or generator. The emf represents energy per unit charge (voltage) which has been made obtainable by the generating mechanism and is not “energy”. The electric charge that has been separated creates an electric potential difference that can be measured with a voltmeter between the terminals of the device. The magnitude of the emf for the battery (or other sources) is the value of this ‘open circuit’ voltage. When the battery is charging or discharging, the emf itself cannot be measured directly using the external voltage because some voltage is lost inside the source.
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