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