First Law of Thermodynamics
Physics 202Professor Lee
CarknerLecture 13
PAL #12 Temperature How does the Galileo thermometer work?
The water in the tube changes density with T
Water gets less dense as T rises and so the balls fall (64 to 80)
Galileo thermometer’s limitations
Not much range (64-80 F) Only works upright and in a gravitational field
Heat Capacity
Put a wooden spoon in a pot of water and boil it Which has the higher temperature?
Heat capacity (unit energy per Kelvin) is given by:
Where: Q = heat (J)
C is a property of a specific object
Specific Heat The heat capacity depends on the mass
of the object and so is normally written:
Where m is the mass and c is the specific heat Each type of substance has a c (e.g. water,
iron, etc.)
Q = cm(Tf - Ti) Amount of heat (Q) needed to change the
temperature of m kg of a certain type of substance from Ti to Tf
Molar Specific Heat
Instead of per kilogram it is sometimes more convenient to use specific heat per mole (cmol)
We need to use a modified version of the last formula:
Good for gases
Heat of Transformation
Boiling water stays at 100 deg C, the added energy goes into transforming the water from liquid to gas
Q = L m Where L is the heat of transformation
Vaporization and Fusion
For the phase change from solid to liquid the heat of fusion, LF is needed
The total heat necessary to change temperature and state is the sum of heats required for each
Phase Curve for Water
Calorimetry To find total heat, add heats from all temperature and
phase changes
Make sure units for m, c, L and T match Always use T = Tf-Ti
Temperature decrease results in a negative loss of heat
For a isolated system the sum of all heats is zero
Work and Internal Energy
No heat can travel in or out
If weight is removed from the piston head the remaining weight will rise Where does the energy come from?
Internal Energy and Work
Work and Heat
The thermal reservoir can add or subtract heat from the system
What happens to the internal energy of the system as heat is applied or work is done?
Heat and Work
Work, Heat and Internal Energy
We find in every situation that the change in internal energy is equal to the change in heat minus the change in work
Energy is conserved!
The First Law of Thermodynamics
This conservation of energy is called the First Law of Thermodynamics
Eint = Q - W
Sign convention:
Work, Pressure and Volume How does work change the system?
The amount of work done in moving something a
small distance ds is:
However, F = pA and dV = A ds
W = dW = p dV (integrated from Vi to Vf) Work is the area under the curve on a p-V diagram
p-V Diagrams
The p-V Curve Pressure must be non-zero in order for work to
be done
If the volume decreases, work is done on the system and the work is negative
If the process is cyclical and returns to the same point by two different paths the area between the paths is equal to the work (and also equal to the heat)
Adiabatic
Q=0 so Eint = -W Consider a thermally isolated
(insulated) piston with weight on top
By changing the weight, the gas is compressed or expands
Constant Volume
Eint = Q If any heat is applied to the system
it goes directly into internal energy
The gas cannot expand
Cyclical Process
The final pressure, volume and internal energy are the same as the initial
Eint = 0 so Q=W