Potential difference between two points homework

[spottedcow913]

We worked out this problem in class and I'm still confused:
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!

 

[Andrew Mason]

We worked out this problem in class and I'm still confused:
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.

The key is to find the current.

You do not need to use different equations. They are equivalent. You can use P = dE/dt = I²R to determine the power consumption in the resistor and in the battery. (Multiply power by time to get the energy consumed).

AM