1Chapter 17 Work, Heat, and the First Law of Thermodynamics

17.1 Its All about Energy17.2 Work in Ideal-Gas Processes17.3 Heat17.4 The First Law of Thermodynamics17.5 Thermal Properties of Matter17.6 Calorimetry17.7 The Specific Heats of Gases

Internal Energy

2

Heat

3

Work

4

Power

5

Energy & Power Units

6

Energy & Power Question

7

What wattage light bulb gives off about the same amount of heat as a person eats who consumes 2500 Cal/day of food?

Energy & Power Question

8

A student who weighs 70 kg eats a bag of potato chips which have 350 Cal, then decides to work them off by walking up stairs. How many flights of stairs (4 m high) must she climb to use up this number of Calories, assuming the body has an efficiency of 25%?

Heat Capacity

9

Heat Capacity Question

10

A potato has C = 2000 J/K, and starts at T = 20C (room temperature). What is its temperature after it absorbs heat at a rate of 700 W (typical microwave oven) for 2 minutes?

Specific Heat

11

Specific Heat Question

12

A water heater is operated by solar power. If the solar collector has an area of 6.00 m2 and the power delivered by sunlight is 550 W/m2, how long does it take to increase the temperature of 1.00 m3 of water from 20C to 60C ?

Specific Heat Question

13

Two identical pots are placed on identical stove burners. Initially, one pot has one cup of water at 20C and the other pot has two cups at the same temperature. Both burners are turned on at the same time. At the moment when the water in the first pot boils, approximately what is the temperature of the water in the second pot?

1 2

Latent Heat

14

Heat of Fusion (LF)

Heat of Vaporization (LV)

Latent Heats for Water

15

If the same amount of heat necessary to melt 1 kg of ice at 0C were instead applied to warming up 1 kg of liquid water initially at 0C, what would the final temperature of the liquid water be?

What if the heat necessary to boil 1 kg of water at 100C were applied to liquid water initially at 0C?

Latent Heat Question

16

Heat is added at a rate of 200 W to a beaker, which initially contains 200 grams of ice in 300 grams of liquid water. How long will it take for(a) all the ice to melt? (b) then for the water to reach boiling point?(c) then all the water to boil away?

Latent Heat & Specific Heat

17

Heating 0.0010 kg (1.0 cm3) Water

333J 2265J 419J 100

Latent Heat & Specific Heat Question

18

Two identical pots are placed on identical stove burners. Initially, one pot has one cup of water at 20C and the other pot has two cups at the same temperature. Both burners are turned on at the same time. At the moment all of the water in the first pot has boiled away, what can be said about the second pot?

1 2

Latent Heat & Specific Heat Question

19

An ice cube (m1 = 10 g at 0C) is added to a glass of water (200 cm3, m2 = 200 g) at room temperature (20C). Assume the system is thermally isolated (no heat enters from the room). Does all the ice melt? What is the final temperature?

Thermodynamic Systems

20

T

- fixed amount of stuff (gas or other)- in equilibrium a thermodynamic system has certain properties (state variables)

Typical State Variables

p pressure force

areaunit ~

Pa

V volume unit ~ m3

T absolute temperature unit ~ K

An equation of state (relevant to a given thermodynamic system) relates p, V, and T.

Ex. (IGL is equation of state for a gas.)

Other thermodynamic systems have different equations of state.

The cylinder is generally NOT part of system.

A gas in a cylinder is a thermodynamic

system.

amount of stuff, identifies the system

Heat and Work two ways to change the internal energy of a system

21

Adiabatic Process

Q: heat flowing into system (E increases)

W: work done to system (E increases)

Work

22

How can we calculate the work done on a thermodynamic system?

before after

What is the work done at a constant pressure?

Work Questions

23

An air-filled balloon is placed in a warm spot. As a result, it expands. What are the signs ( + or ) of the heat absorbed Q and the work done Won the air in the balloon?

A cup of liquid water with ice cubes in it is left on the table. After a long time the ice has melted and everything is at room temperature. Consider the system to be all of the water (liquid & ice). What are the signs ( + or ) of the heat absorbed Q and the work done W on the system?

Work Question

24

0.200 moles of an ideal gas is enclosed in a cylinder with a movable piston on top. The piston has a mass of 8000 g and an area of 5.00 cm2and is free to slide up and down, keeping the pressure of the gas constant. How much work is done on the gas as the temperature of the gas is raised from 20.0C to 300C?

1 2

Work

25

What is the sign (+ or ) of the work done on the systems below?

Work Question

26

Below are three processes in which a given sample of gas goes from the same original state to the same final state? How does the work done on the system compare?

Work Question

27

How much work is done on the system in case (a) below?

Work Question

28

How much work is done on the system in case (b) below?

Work Question (assume IGL holds)

29

How much work is done on the system in case (c) below if the temperature T is constant?

Work in a Cyclic Process

30

V

p- system starts/stops in same

thermodynamic state

- ex. fridge, AC, engines

- in practical applications often interested in work done by system. Note: This is , not .

Sign Convention for Cyclic Processes

clockwise thermodynamic system does work 0 [like moving a car]

counterclockwise work done to thermodynamic system 0 [refrigerator]

enclosed signed area represents work done by system.

First Law of Thermodynamics

31

Q is heat energy going into system.

W is work energy going into system.

E is internal energy of system. (total KE + PE of system)

Conservation of Energy:

1st Law of Thermodynamics

First Law of Thermodynamics Question

32

A sample of stuff is compressed, which causes it to warm up above room temperature. While keeping it at the new, small volume, it is allowed to cool back to room temperature (by thermal conduction). What relationship does the net change in energy E satisfies?

What if the sample above were an ideal gas?

p

V

First Law of Thermodynamics Question

33

How much work is done by 1.00 mol of water at 100C when it boils and becomes 1.00 mol of steam at 100C and 1.00 atm pressure? Assuming the steam to be an ideal gas, determine the change in internal energy of the steam as it vaporizes.

Adiabatic Process

34

Isovolumetric (Isochoric) Process

- no heat enters/leaves system

- volume constant

Cyclic Process

35

Isothermal Process

- system ends up in same thermodynamic state (p, V, T, E)

- process that occurs at constant temperature 0

Mechanisms of Thermal Energy Transfer Q (or rate P)

36

Conduction

Convection

Radiation

Thermal Conduction

37

xSample Thermal Conductivities

Material k (W/m-K)

rateofheattransferwatts temp. diff. K distancebetweenhot&coldsidesm crosssectionalaream thermalconductivity

(depends on material)

graphene ~5000diamond 1000

steel 50water (20C) 0.6

brick 0.6air (0C) 0.024

Thermal Conduction Question

38

Consider two aluminum rods, with the same cross section, but one rod twice as long as the other one. One end of each rod is maintained at 400C by a flame, while the other ends are maintained at 0C by an ice bath. The rate of heat conduction through the shorter rod is 50 W. What is the rate of conduction through the longer rod?

Convection

39

- Time required for a temperature change to spread by conduction is proportional to distance squared (relatively slow). Its more efficient for the material carrying the heat to do the moving (this is called convection).

- Often differences due to thermal expansion drive convection currents. Most commonly, hot fluid is less dense and so rises (natural convection).

Ex. Weather patterns

- Other times there is a circulator.Ex. Refrigerator or forced heating

Thermal Radiation (Black-Body Radiation)

40

Objects at 0K continuously emit EM radiation (in a spectrum of light)

surfaceareaofobjectm

absolutetemperatureofobjectK

StefanBoltzmannconstant 5.67 10

emissivity, unitlessnumberbetween0and1(depends on object)

Perfect absorber/emitter (black-body) has e = 1.Perfect mirror (reflects all light) has e = 0.

Thermal Radiation Question

41

How many Calories per day must a person (surface area 2 m2, emissivity 0.5) eat to make up for the power he radiates thermally?(a) Compute the answer ignoring absorbed radiation.(b) Compute the answer assuming radiation is also absorbed from surroundings at 20C

Thermal Radiation Question

42

An object is at room temperature. By approximately what percent does the thermal radiation emitted by the object increase if the object is warmed by 3C?