First Law of Thermodynamics

Physics 102Professor Lee

CarknerLecture 5

(Session: 104884)

PAL #5 Phase Change Final temperature of melted Frosty Four heats:

Warm up Frosty to 0 C: micecice(0-(-5)) Melt Frosty: miceLice

Warm up melted Frosty: mwatercwater(Tf-0) Cool down air: maircair(Tf-20)

(100)(2100)(5)+(100)(333000)+(100)(4186)(Tf)+(9700)(837)(Tf-20) = 0

1.05X106+3.33X107+4.19X105Tf+8.12X106Tf-1.62X108 = 0 8.54X106Tf = 1.28X108

Tf = 15 C

Energy We know that in mechanics energy is

conserved

In what ways can energy be expressed?

Heat can flow in or out

Related to expansion or compression The internal energy might change

Related to temperature

Consider a piston of gas with weight on the top and a thermal reservoir at the bottom

Weight can be added or subtracted so that the system does work on the weight or the weight does work on the system

If we add weight and do 6 J of work we either increase the internal energy by 6 J or produce 6 J of heat or some combination that adds up to 6

The First Law of Thermodynamics

This conservation of energy is called the First Law of Thermodynamics

U = Q - W

If work is done by the system W is positive, if work is done on the system W is negative Positive work is the useful work we get out

Heat flow in is +, heat flow out is -

PV Diagram

How much work is done if a gas expands and raises a piston?

Depends on: Pressure

Change in volume

The relationship between P and V can be complicated, but Work equals area under curve in PV diagram

The P-V Curve

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)

P-V Diagrams

Internal Energy and Temperature Internal energy (U) is directly related to temperature

High T, large U Low T, small U

KE = (3/2)kT for one molecule U = (3/2)nRT for n moles

Note that everything has some internal energy, we want to know about the change in internal energy (U)

Types of Processes

We want to understand 5 basic types of thermodynamic processes

For each you should know: PV diagram

Isobaric In an isobaric process

the pressure does not change

Since the area under the PV curve is a rectangle: W=PV

Isochoric

e.g. a sealed hollow cylinder

W = 0 soU = Q

If any heat is applied to the system it goes directly into internal energy

Isothermal An isothermal process

happens at constant temperature

Since T = 0: U = 0 so Q=W

We can use calculus to find the area under the curve W = nRTln(Vf/Vi)

Adiabatic

Adiabatic processes are ones in which no heat is transferred

Since there is no heat:Q=0 so U = -W

P-V Diagram

P

V

Isobaric (P=const.)

Isochoric (V=const)

Isothermal (T=const)

Adiabatic (Q=0)

Cyclical Process

A cyclical process returns to its initial state

U = 0 so Q=W There are many different ways to

produce a cyclical process

Next Time

Read: 15.4-15.6 Homework : Ch 15: P 7, 10, 17, 29

Test 1 next Friday About 10 multiple choice (~25%) About 4 problems (~75%) Equation and constant sheet given I have posted equation sheet and

practice problems

As a pot of water boils, the temperature of the water,

A) IncreasesB) DecreasesC) Stays the sameD) Fluctuates unpredictablyE) It depends on the temperature of the

stove

Water condenses out of the air onto a cold piece of metal. Due to this condensation, the temperature of the air around the metal,

A) IncreasesB) DecreasesC) Stays the sameD) Fluctuates unpredictablyE) It depends on the temperature of the

metal

The temperature of a solid is held constant and the pressure is lowered. When the pressure gets very close to zero the solid will,

A) Become a gasB) Become a liquidC) Stay a solidD) Be at the triple pointE) Be at the critical point