1. First Law of Thermodynamics
Conservation of Energy for Thermal Systems

2. Joule Equivalent of Heat
James Joule showed that mechanical energy could be converted to heat and arrived at the conclusion that heat was another form of energy.
He showed that 1 calorie of heat was equivalent to J of work.
1 cal = J

3. Energy Mechanical Energy: KE, PE, E Work is done by energy transfer.
Heat is another form of energy.
Need to expand the conservation of energy principle to accommodate thermal systems.

4. 1st Law of Thermodynamics
Consider an example system of a piston and cylinder with an enclosed dilute gas characterized by P,V,T & n.

5. 1st Law of Thermodynamics
What happens to the gas if the piston is moved inwards?

6. 1st Law of Thermodynamics
If the container is insulated the temperature will rise, the atoms move faster and the pressure rises.
Is there more internal energy in the gas?

7. 1st Law of Thermodynamics
External agent did work in pushing the piston inward.
W = Fd
=(PA)Dx
W =PDV Dx

8. 1st Law of Thermodynamics
Work done on the gas equals the change in the gases internal energy,
W = DU Dx

9. 1st Law of TD Let’s change the situation:
Keep the piston fixed at its original location.
Place the cylinder on a hot plate.
What happens to gas?

10. Heat flows into the gas.
Atoms move faster, internal energy increases.
Q = heat in Joules
DU = change in internal energy in Joules.
Q = DU

11. 1st Law of TD F
What if we added heat and pushed the piston in at the same time?

12. 1st Law of TD
Work is done on the gas, heat is added to the gas and the internal energy of the gas increases!
Q = W + DU F

13. 1st Law of TD Some conventions: For the gases perspective:
heat added is positive, heat removed is negative.
Work done on the gas is positive, work done by the gas is negative.
Temperature increase means internal energy change is positive.