First Law of Thermodynamics

Physics 202Professor Lee

CarknerLecture 11

PAL #10 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) Heat Al until it floats

Need = 1000 kg/m3 so V = m/ = 2700/1000 = 2.7 V = VDT, T = V/V = 1.7/((1)(6.9X10-6)) = 24658 C Al would melt before getting to this temperature

Heat Capacity

Put a wooden spoon in a pot of water and boil it

Heat capacity (unit energy per degree temp) is given by:Q = C (Tf - Ti) = C T

Where: Q = heat (J)

C is a property of a specific object

Specific Heat

C=cm 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

One mole = 6.02 X 1023 atoms or molecules

Q = cmol (# of moles) (Tf - Ti) Used for gas

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 liquid to gas

the heat of vaporization, LV is needed 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

Calorimetry1) Write out equation adding all heats in symbolic form and set

equal to zero e.g., Q1 + Q2 + Q3 = 0 or m1c1T1 + m2L2+ m3c3T3 =0

2) Substitute numbers, use correct values for c and L

3) Make sure units for m, c, L and T match

4) Always use T = Tf-Ti (don’t add your own sign!) Temperature decrease results in a negative loss of heat

5) For Lm, make positive if heat is absorbed and negative if heat is given off

Solve for required variable

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?

It must come from the internal energy of the gas Internal energy depends on

temperature

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?

The First Law of Thermodynamics

This conservation of energy is called the First Law of Thermodynamics

Eint = Q - W

Sign convention: If work is done by the system W is positive, if work is

done on the system W is negative

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

The total work is dW integrated over the volume change

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 increases, work is done by the system and the work is positive

No volume change, no work 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 There are many different ways to

produce a cyclical process

Next Time

Read: 18.11-18.12

Which of the following places the temperatures in the correct rank, highest to lowest?

a) 50 X, 50 W, 50 Y b) 50 X, 50 Y, 50 W c) 50 W, 50 X, 50 Y d) 50 Y, 50 W, 50 X e) 50 Y, 50 X, 50 W

Consider 3 aluminum plates, plate A has area of 1X1 m, plate B has area of 2X2 m and plate C has area of 3X3 m. If the temperature of the plates is raised by 10 degrees, which plates has the greatest increase in area?

a) Ab) Bc) Cd) All tiee) You cannot tell with out knowing the

thickness