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A wire of resistance 5 ohms is connected to a battery whose emf is 2.0 V and whose internal resistance is 1 ohm. In 120 seconds, how much energy is (a) transferred from chemical form in the battery, (b) dissipated as thermal energy in the wire, and (c) dissipated as thermal energy in the battery?

In part a we used

U=E^2(time)/(r+R)

in part b

U=i^2R(time)

if they both are energy and thus in joules, why do we need to use different equations and what does this help us do.

Thanks for any help!