Thermodynamics Chapter 10

Work, Heat and Internal Energy Work: Force applied over a distance  W = Fd or W = F H d Work: The transfer of energy through motion  W = ΔKE Heat: The energy transferred between 2 objects that have different temperatures.  Q = mC p ΔT Internal energy: Energy associated with atomic motion. Symbolized by the letter U.  ΔPE + ΔKE + ΔU = 0

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 1 kcal = 4184 J 1 kcal = 1 Calorie (food calorie)

Energy Mechanical Energy: ME = KE + PE –Discussed in chapter 5 Work is done by energy transfer. –Work energy theorem from chapter 5 Heat is another form of energy. –Heating curve chapter 9 Internal energy. –Discussed this chapter

Energy We need to revisit energy and how all these forms relate to each other. This is where the 1 st Law of Thermodynamics comes in. Sometimes the 1 st Law of Thermodynamics is called the law of conservation of energy.

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

1 st Law of Thermodynamics If there is no change in temperature then ΔT = 0, so ΔU = 0 so Q = W  Isothermal process Constant internal energy

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

1 st 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? Yes, Do you know why?

1 st Law of Thermodynamics External agent did work in pushing the piston inward. W = Fd W =(PA)  x W =P  V xx  F = PA from pressure equation  Rearrange for volume V = lwh or ΔV = AΔx  and substitute Substitute PA for F

1 st Law of Thermodynamics Work done on the gas equals the change in the gases internal energy, W =  U xx  Adiabatic process (NO heat energy, Q = 0)

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

Heat flows into the gas. Atoms move faster, internal energy increases. Q = heat in Joules  U = change in internal energy in Joules. Q =  U  Isovolumetric (constant volume)

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

1 st Law of Thermodynamics Work is done on the gas, heat is added to the gas and the internal energy of the gas increases! Q = W +  U F  1 st Law of Thermodynamics

1 st Law of Thermodynamics Some conventions: For the gases perspective: Heat added is positive (Q > 0), heat removed is negative (Q < 0). Work done on the gas is positive (W > 0), work done by the gas is negative (W < 0). Temperature increase means internal energy change is positive (  T= +ΔU).