什么是电动势
电动势是反映电源把其他形式的能转换成电能的本领的物理量。电动势使电源两端产生电压。在电路中，电动势常用δ表示。电动势的单位和电压的单位相同，也是伏。
电源的电动势可以用电压表测量。测量的时候，电源不要接到电路中去，用电压表测量电源两端的电压，所得的电压值就可以看作等于电源的电动势。如果电源接在电路中，用电压表测得的电源两端的电压就会小于电源的电动势。这是因为电源有内电阻。在闭合的电路中，电流通过内电阻r有内电压降，通过外电阻R有外电压降。电源的电动势δ等于内电压Ur和外电压UR之和，即δ=Ur+UR 。严格来说，即使电源不接入电路，用电压表测量电源两端电压，电压表成了外电路，测得的电压也小于电动势。但是，由于电压表的内电阻很大，电源的内电阻很小，内电压可以忽略。因此，电压表测得的电源两端的电压是可以看作等于电源电动势的。
干电池用旧了，用电压用测量电池两端的电压，有时候依然比较高，但是接入电路后却不能使负载（收音机、录音机等）正常工作。这种情况是因为电池的内电阻变大了，甚至比负载的电阻还大，但是依然比电压表的内电阻小。用电压表测量电池两端电压的时候，电池内电阻分得的内电压还不大，所以电压表测得的电压依然比较高。但是电池接入电路后，电池内电阻分得的内电压增大，负载电阻分得的电压就减小，因此不能使负载正常工作。为了判断旧电池能不能用，应该在有负载的时候测量电池两端的电压。有些性能较差的稳压电源，有负载和没有负载两种情况下测得的电源两端的电压相差较大，也是因为电源的内电阻较大造成的。
其它的解析
The potential difference between the two electrodes in a cell. The cell emf is the cell voltage measured when no current is flowing through the cell. It can be measured by means of a pH meter with high input impedance.
www.flw.com/define_e.htm A measure of voltage in an electrical circuit.
www.omega.com/literature/transactions/volume1/glossary.html The electrical force that exists across the terminals of an electrical generator, or battery. When connected to a load in a closed circuit, this force produces a voltage across the load and causes current to flow in that circuit. EMF is measured in volts and designated with the letter E (supply voltage) or V (load voltage).
www.nuhorizons.com/Glossary/BasicElecConcepts.html The force that causes the movement of electrons through an electrical circuit.
library.thinkquest.org/10401/vocab.html Difference of potential produced by sources of electrical energy which causes current to flow in a circuit.
www.birnbach.com/TechnicalInformation/glossary/E.htm An electrical potential difference which produces or tends to produce an electrical current. The unit of measure is the volt.
www.simpsonelectric.com/e.htm That force which determines the flow of electricity; a difference of electric potential. VOLTAGE RATING The highest voltage that may be continuously applied to a mated contact pair in conformance with set standards or specifications.
www.bimltd.com/terms.html Force that makes an electrical current flow. Voltage.
www.peakagents.ca/glossary/e3.htm (emf) - the voltage difference between two electrodes in an electrochemical cell.
wblrd.sk.ca/~chem30_dev/appendix/glossary.htm voltage: the rate at which energy is drawn from a source that produces a flow of electricity in a circuit; expressed in volts wordnet.princeton.edu/perl/webwn
Electromotive force (emf) is a measure of the strength of a source of electrical energy. The unit of emf is the volt (energy per unit electric charge) and so the term 'force' is misleading. Thus, the expansion of the acronym is considered obsolete or at best, an embarrassing historical artifact. (The term is attributed to Alessandro Volta.). ...
en.wikipedia.org/wiki/Electromotive_force
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