capacity and battery

[Primarygun]

What's the difference between a battery and capacity that makes the only the former can be the e.m.f. in a normal electric circuit?

[insane_alien]

batteries provide a reasonably constant voltage at whatever level of charge, a capacitor doesn't. a 1.5 volt batter will still proved ~1.4Volts at 1% charge. a capacitor will provide something like 0.05V at the same charge