how much energy is gained by an electron in a battery?
Depends on the battery. Batteries range from tiny watch batteries to car batteries to even larger specialized ones.
The energy gained is proportional to the voltage of the battery; instead of joules think of electron volts (eV).
The enrgy is the charge of the electron times the voltage of the battery.
Note that in terms of units, 1 volt = 1 joule / coulomb, so 1 volt * 1 coulomb = 1 joule.
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