# Warm-Up Exercise for lecture 23

## Due 8:00 am, Tues, 17 Nov 2009

Physics 105, Fall 2009

A gas has its pressure reduced while its volume is kept constant. What does this look like on a PV diagram?
☐ a horizontal line going to the right
☐ a horizontal line going to the left
☐ a vertical line going up
☑ a vertical line going down

Same situation. How did the temperature of the gas change during that process?
☐ the temperature increased
☑ the temperature decreased
☐ the temperature stayed the same
☐ the temperature change cannot be determined from the information given

The second law of thermodynamics says for a heat engine:
☐ You get more work energy out than you put in as heat
☐ You get the same work energy out as you put in as heat
☑ You get less work energy out than you put in as heat

Ralph is confused because he knows that when you compress gases, they tend to heat up. Think of bicycle pumps, for example: compressing the air heats up the nozzle of the pump. Yet section 12.3 (8th edition) talks about "isothermal" processes where the temperature doesn't change. How are such processes possible?  How can you compress a gas without its temperature increasing?
Just take away some heat at the same time, by (for example) surrounding the gas with something cold.