1. APHY201 5/31/2014 1 15.1 The First Law of Thermodynamics A systems internal energy can be changed by doing work or by the addition/removal of heat: ΔU = Q - W W is negative if work is done on the system Compression of the gas What is the state of the system? Described by P, V, T, m, U

2. APHY201 5/31/2014 2 15.2 Thermodynamic Processes and the First Law Isothermal: T = constant ΔU = 0 W = Q Adiabatic: Q = 0 ΔU = -W

3. APHY201 5/31/2014 3 15.2 Thermodynamic Processes and the First Law If pressure is constant then W = Fd = PAd = P ΔV

4. APHY201 5/31/2014 4 15.2 Thermodynamic Processes and the First Law The total work done during a process is equal to the area under the PV diagram

5. APHY201 5/31/2014 5 15.4 The Second Law of Thermodynamics Heat can flow spontaneously only from a hot object to a cold object. A reversible process is one that is always in equilibrium and can return to its initial conditions along the same path Most natural processes are irreversible Sets an upper limit on efficiency of heat engines

6. APHY201 5/31/2014 6 15.5 Heat Engines Heat engines convert U into other useful forms of energy – mechanical, electrical, … ΔU cycle = 0 Q H = W + Q L Automobile engines

7. APHY201 5/31/2014 7 15.5 Heat Engines The efficiency of a heat engine is Carnot (ideal) engine Reversible processes Too slow for real engines

8. APHY201 5/31/2014 8 15.6 Refrigerators, Air Conditioners and Heat Pumps A heat engine in reverse.

9. APHY201 5/31/2014 9 15.6 Refrigerators, Air Conditioners and Heat Pumps

10. APHY201 5/31/2014 10 2. (a) The work done by a gas at constant pressure is found from Eq. 15-3. (b) The change in internal energy is calculated from the first law of thermodynamics

11. APHY201 5/31/2014 11 26. Find the exhaust temperature from the original Carnot efficiency, and then recalculate the intake temperature for the new Carnot efficiency, using the same exhaust temperature.