First Law of Thermodynamics Conservation of Energy for Thermal Systems

First Law of Thermodynamics The increase in the internal energy of a system is equal to the amount of energy added by heating the system minus the amount lost as a result of the work done by the system on its surroundings. So what does this mean?

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

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

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 What happens to the gas if the piston is moved inwards?

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?

1 st Law of Thermodynamics External agent did work in pushing the piston inward. W = Fd =(PA)  x W =P  V xx

1 st Law of Thermodynamics Work done on the gas equals the change in the gases internal energy, W =  U xx

1 st 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?

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

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

1 st 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 +  U F

1 st Law of TD Some conventions: From 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.

1 st Law of TD Final Equation for the first law? E = Q – W The internal energy is equal to the heat put into the system minus the work done by the system. (Positive work done by the system means that energy has left the system).

Adiabatic Processes A gas is compressed or expanded in such a way that no heat enters or leaves the system. –Recall relationship between temperature and pressure for a gas: –Gay-Lussac's law P/T = k Adiabtic = impassible.

Adiabatic Processes As a gas compresses, its temperature increases. As it expands, the temperature decreases. –Example: blow across your hand with your mouth open. Next, blow across your hand while puckering your lips. Do you feel a difference in temperature between the two methods?

Adiabatic Processes Example of the Adiabatic Process – Diesel Engine. tmlhttp:// tml