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电动势（EMF）的定义

1800

Alessandro Volta

电动势（[latex]\mathcal{E}[/latex]）是指电池或发电机等非电电源对单位电荷所做的功。尽管名称如此，但它并不是一种机械力，而是一种以伏特为单位的电动势。它代表电荷通过电源时从另一种形式（化学、机械）转换成电能的能量。

Electromotive force, or EMF, is a fundamental concept in electromagnetism that quantifies the energy conversion process within a source. When a charge [latex]q[/latex] moves through a device that provides EMF, the work done [latex]W[/latex] on the charge is given by [latex]W = \mathcal{E}q[/latex]. This work is performed by a non-conservative field, often called an impressed field [latex]\mathbf{E}_{im}[/latex], which is distinct from the conservative electrostatic field generated by static charges. The EMF is formally defined as the line integral of this impressed field around a 闭环: [latex]\mathcal{E} = \oint \mathbf{E}_{im} \cdot d\mathbf{l}[/latex].

This distinction is crucial. An electrostatic field, described by Coulomb’s law, is conservative, meaning the work it does on a charge around any closed loop is zero. This is why a simple configuration of static charges cannot sustain a continuous current in a circuit. An EMF source, however, provides the “push” to drive charges against the electrostatic field’s potential gradient, maintaining a current. In a battery, this impressed field arises from chemical processes at the electrodes. In a generator, it’s due to a changing magnetic field (Faraday’s law) or the motion of a conductor in a magnetic field (motional EMF). The EMF represents the maximum potential difference the source can provide when no current is flowing, also known as the open-circuit voltage. When current flows, the terminal voltage across the source is typically lower than the EMF due to internal resistance within the source itself.