Physics 1010: The Physics of Everyday Life. TODAY Heat and Thermodynamics Thermometers, temperature scales; conduction, convection, radiation.

Transcription

1 Physics 1010: The Physics of Everyday Life TODAY Heat and Thermodynamics Thermometers, temperature scales; conduction, convection, radiation. 1

2 Today s topics Heat and thermometers Burning - conversion of chemical energy to thermal (chemical bonds) Heat exchangers - getting the heat into the room: open fires, wood stoves, and furnaces Conduction of heat Convection of heat Radiative heat transport 2

3 Heat Flow Three lead bricks are in contact as shown. Heat flows: A) From the hot to the cold B) From the cold to the medium C) From the hot to the medium D) From the medium to the cold E) All but B 2. Med. Brick 1. Hot Brick 3. Cold Brick 3

4 Heat Flow Three lead bricks are in contact as shown. Heat flows: A) From the hot to the cold B) From the cold to the medium C) From the hot to the medium D) From the medium to the cold E) All but B 2. Med. Brick 1. Hot Brick 3. Cold Brick 4

5 Temperature - something has heat. Caloric. (Why not?) Heat flows from hot to cold Some early physics beliefs: conserved quantity - a fluid called caloric Hot Brick Cold Brick 5

6 Heat is Energy (same a mechanical) Count Rumford (Benjamin Thompson) demonstrated the heating of water by boring cannon for the elector of Munich Joule ( ) measure increase in temperature due to friction Able to equate loss of mechanical energy by friction to heat Famous experiment with weights moving fins in water (he measured the change in temperature of the water) 6

7 Heat flows between 3 bodies of different temperatures, as shown, in contact. When does heat stop flowing? a) When brick 1 has same temp as brick 3 b) When brick 1 has same temp as brick 2 c) When brick 2 has same temp as brick 3 d) When all bricks have the same temperature 2. Med. Brick 1. Hot Brick 3. Cold Brick 7

8 Heat flows between 3 bodies of different temperatures, as shown, in contact. When does heat stop flowing? a) When brick 1 has same temp as brick 3 b) When brick 1 has same temp as brick 2 c) When brick 2 has same temp as brick 3 d) When all bricks have the same temperature 2. Med. Brick 1. Hot Brick 3. Cold Brick 8

9 Fundamental Principle of Thermodynamics (used for measuring temperature) Bodies in thermodynamic contact will eventually all have the same temperature Hot Brick Med. Brick Med. Brick Cold Brick Med. Brick Med. Brick 9

10 Origin of the Fahrenheit scale He <Fahrenheit> proposed (rather arbitrarily) a zero for the freezing point of a brine solution, a value of 32 for the melting point of ice, and the body temperature at 100 units (this should be about 96, 100 is feverish) Actually, water freezes at a lower temperature with salt. Fahrenheit chose the salt solution to make water freeze at the coldest possible temperature. This was the coldest he could get something stable. Fahrenheit Scale: Water freezes at 32 o F, boils at 212 o F 10

11 Celsius temperature scale (1742) Scale based on freezing and boiling points of water. Boiling point depends on pressure Must set the pressure for boiling 1. Put the cylinder AB of the thermometer (i.e. the bulb) in thawing snow and mark the freezing point of water C, which should be at such a height over the cylinder at A that the distance AC is half the distance between C and the water boiling point mark D 2. Mark the boiling point of water D at a pressure of "25 tum 3 linier" (approximately 755 mm) 3. Divide the distance in 100 equal parts or degrees; so that 0 degree corresponds to the boiling point of water D, and 100 to the freezing point of water C. When the same degrees have been continued below C all the way down to A the thermometer is ready. 4. Celsius Scale: Water freezes at 0 o C, boils at 100 o C 11

12 Kelvin (absolute) scale Increase in temperature gives increase in pressure, so decrease enougn and no pressure! Increase in density (compression) gives increase in pressure Can have ideal gas law: P = nkt if the temperature is measured relative to something very cold: T kelvin = T celsius

13 Temperature - the various scales and conversions T k = T c T f = (9/5)T c + 32 NOTATION 0 C = 32 F = 273 K 13

14 Production of heat from chemical energy - burning, activation energy, exothermic reactions Carbon Oxygen Mass falling to earth, lower potential energy C Converts to kinetic energy Bounces until friction changes energy to heat Carbon Monoxide Carbon and oxygen like to join Carbon Monoxide or CO Carbon Dioxide or CO 2. Carbon Dioxide O O O C C O Microscopic view: atoms falling together, means they gave up potential energy Kinetic energy (HEAT) acquired by these and nearby molecules 14

15 In most burning, weaker bonds (less potential energy) broken Methane and oxygen molecules are more weakly bound Come out of a shallow valley, go into a deeper one Carbon C Oxygen O Hydrogen H H H C H + 5 O O O C H O + 4 O H O Activation energy: energy needed to get first reaction going The released energy of first one (or more) reactions gets others going 15

16 Lennard-Jones Potential Same shape for all molecules Distance, change in potential energy different for different substances 16

17 Lennard-Jones Potential also explains Hook s Law Force on atom is the negative of the slope of graph: For small displacements from minimum (slope=force=0) force is the same for compression and extension. 17

18 Heating, Cooling Wood contains resins - SMOKE Burning creates carcinogens Need to move heat without the smoke -> Heat Exchangers To heat (or cool) our homes, we move heat in three ways: Convection Conduction Radiation 18

19 The furnace and heat exchangers Combustion air brought into heat room (not through house) Hot gas burns, heating metal tubes Fans push house air through tubes and to rest of house To house To outside 19

20 Transport of heat Conduction: The heat is carried by molecules that move, and then move those next to them, but there is no net movement of molecules Convection: Hot molecules are carried to a cold region Radiation: electromagnetic radiation carries the heat 20

21 Conduction Heat one end, the other gets hot. Diffusion: nothing moves, but heat gets there ( like sound, but not a wave) Hot pad Ice pack 21

22 Convection: bulk movement of hot fluid Hot air Hot air rises Pushes cold air down Convective cell Cold air 22

23 Radiative transport: All bodies radiate electromagnetic energy over a range of frequencies. Light - for very hot objects (10,000 K) More radiation for larger temperature Room temperature (300 K), light at wavelengths longer by 30x are emitted. Infra-red light Works even in vaccum! COLD Radiation HOT Less More 23

24 Summary Heat flows from hot to cold Bodies in thermodynamic contact will all reach the same temperature HEAT IS ENERGY (same as mechanical energy) Burning releases chemical (electrostatic) potential energy We use convection, conduction, and radiation to heat (or cool) a house 24

25 Exam Tonight 7:30 pm, in this room. Closed Book. One 3x5 note card with own notes allowed. Calculators allowed. Cumulative, but emphasis on new material. Multiple choice plus one essay. 25

26 Practice Questions; Work You are using a frictionless ramp to move a 200kg filing cabinet onto a truck. The bed of the truck is 2m above the ground, and the ramp is 8m long. How much work will you do moving the cabinet onto the truck A) 4000N B) 2000N C) 4000J D) 2000J E) 200N 26

27 Practice Questions; Work You are using a frictionless ramp to move a 200kg filing cabinet onto a truck. The bed of the truck is 2m above the ground, and the ramp is 8m long. How much work will you do moving the cabinet onto the truck A) 4000N B) 2000N C) 4000J D) 2000J E) 200N Anwer is C: W=mgh=200kg*9.8m/s 2 *2m=3920J 27

28 Practice Questions; Work You are using a frictionless ramp to move a 200kg filing cabinet onto a truck. The bed of the truck is 2m above the ground, and the ramp is 8m long. Once you get the cabinet moving at a constant speed, how much force will you exert to move the cabinet onto the truck A) 200N B) 200kg C) 500N D) 25kg E) 25N 28

29 Practice Questions; Work You are using a frictionless ramp to move a 200kg filing cabinet onto a truck. The bed of the truck is 2m above the ground, and the ramp is 8m long. Once you get the cabinet moving at a constant speed, how much force will you exert to move the cabinet onto the truck A) 200N B) 200kg C) 500N D) 25kg E) 25N Anwer is C: mgh=fl, L=mg(h/L)= 200kg*9.8m/s 2 *(2m/8m)=490N 29

30 Practice Questions; Oscillators A mass of 10kg is hooked to a spring with spring constant k=100n/m. What is the period of the oscillator A) 20s B) 1s C) 5s D) 10s E) 2s 30

31 Practice Questions; Oscillators A mass of 10kg is hooked to a spring with spring constant k=100n/m. What is the period of the oscillator A) 20s B) 1s C) 5s D) 10s E) 2s Anwer is E: T=2π sqrt(m/k)= =2 π sqrt(10kg/100n/m)=1.99 s 31

32 Practice Questions; Oscillators A mass of 10kg is hooked to a horizontal spring with spring constant k=100n/m. If I stretch the spring horizontally by 0.2m and let it go, how fast will the mass be moving as it crosses the equilibrium point? A) 0.6m/s B) 6m/s C) 20m/s D) 1.3m/s E) 13m/s 32

33 Practice Questions; Oscillators A mass of 10kg is hooked to a horizontal spring with spring constant k=100n/m. If I stretch the spring horizontally by 0.2m and let it go, how fast will the mass be moving as it crosses the equilibrium point? A) 0.6m/s B) 6m/s C) 20m/s D) 1.3m/s E) 13m/s Anwer is A: (1/2)mv 2 =(1/2)kx 2 so v=x*sqrt(k/m) v= 0.2m sqrt(100n/m / 10kg)=0.63 m/s 33

34 Practice Questions; Newton s Laws David (in red, with sling) is trying to hit Goliath (in blue). The velocity of the stone is shown by the arrows. He should release the stone at approximately: B A C D 34

35 Practice Questions; Newton s Laws David (in red, with sling) is trying to hit Goliath (in blue). The velocity of the stone is shown by the arrows. He should release the stone at approximately: B A C D 35

36 Practice Questions; Kinematics v start The car is subjected to a constant force in the direction away from the motion detector. Sketch your predictions for the velocity and acceleration of the cart moving toward the motion detector, slowing down at a steady rate, and then reversing direction and speeding up. (Start your graph after the push that gets the cart moving; + is to the right) 36

37 Velocity A time Velocity B time Sketch your predictions for the velocity and acceleration of the cart moving toward the motion detector, slowing down at a steady rate, and then reversing direction and speeding up. + is to the right Acceleration Velocity C time #1 time #1 time Acceleration Velocity Acceleration D time #1 time #1 time 37

38 Velocity time time #1 # Velocity time Acceleration time #1 # Velocity time time Velocity time Acceleration time A D C B Acceleration Answer is D Acceleration is constant, and positive

39 A skier goes down a frictionless slope, decending 5m. She hits a level stretch with friction, and the friction force is 50N. How far will she travel before she stops. The mass of the skier is 50kg. A 4.9m B 49m C 98m D 9.8 m 39

40 A skier goes down a frictionless slope, decending 5m. She hits a level stretch with friction, and the friction force is 50N. How far will she travel before she stops. The mass of the skier is 50kg. A 4.9m B 49m C 98m D 9.8 m Concserv of energy: mgh=fl L=(mgh)/F 40