Question

A closed system’s internal energy changes by 178 J as a result of being heated with 658 J of energy.

The energy used to do work by the system is ____________ J.

Answer 1

480 J

Long Explanation:

The energy used to do work by the system is 480 joules.

This is because the first law of thermodynamics states that

Heat (Q) = The change in internal energy (ΔU) + work done by the system (W)

This is because the heat or thermal energy transferred into the system does not disappear, and therefore, is transformed and made up of the output of energy.

Basically, it means that the input (heat) = the output. (work done & change in potential and kinetic energy)

We can rearrange this formula, (Q(heat) = ΔU(change in internal energy) + W (work)) to suit this equation:

W = Q - ΔU

The work done ( one part of the output) is equal to the heat (input) - the change in internal energy (another part of the output)

W = 658 J - 178 J

W = 480 J

Short Explanation:

W = Q - ΔU

The work done is equal to the heat - the change in internal energy

W = 658 J - 178 J

W = 480 J

Answer 2

Internal energy of the system changes by ΔE = 178 J.
Heat given to the system = Q = +658 J.

According to the first law of thermodynamics,
ΔE = Q + W
178 = 658 + W
∴ W = 178-658 = -480 J

Minus sign indicates that work is done by the system.