Question

An automobile engine provides 642 Joules of work to push the pistons. In this process the internal energy changes by -2952 Joules. Calculate q for the engine. This represents the amount of heat that must be carried away by the cooling system.Calculate the change in the internal energy of the engine.

Answer 1

Recall from the 1st law of thermodynamic - Energy Balance gives;

assuming : there is no change in kinetic and potential energy, a steady state process, etc.

ΔU = ΔQ + ΔW

For this system, ΔW = +ve (engine does the work)

ΔU = -2952J

ΔQ = -ve? (Heat energy is been released)

Since the heat energy is removed, the internal energy must increase by

-2952J = 642J - ΔQ

ΔQ = (642 + 2952) = 3594 J was released from the engine and the amount that must be cooled off

Now to find the change in the internal energy of the engine?

Note that as as soon as the heat is removed, there would not be any change in the internal energy of the system because all the energy from the burning of the measure of fuel has been removed.

so ΔU = 0J