delta E (change in internal energy)?

Question

Which of the following changes always results in a negative delta E (change in internal energy)?

a.) The system absorbs heat and does work.
b.) The system gives off heat and does work.
c.) The system absorbs heat and has work done on it.
d.) The system gives off heat and has work done on it.

Answer 1

Hi Brooke B

This is a question about the first law of thermodynamics, which you might recall takes the following general form:

dE = dQ - dW

for E the energy of the system, Q the heat absorbed and W the work done by the system. If dE is going to be definitively negative (regardless of the relative sizes of dQ and dW) then both dQ and dW must also be negative. Does this make sense? If not, look at the equation above - the only way to be sure that dE is negative is if it is the sum of two negative numbers. So dQ and dW must be negative. Negative dQ means the system loses heat to its surroundings. Negative dW means the system does work on its surroundings. So the answer is (b): the system gives off heat and does work.


Hope this helps!
The Chicken