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Physics 1010:The Physics of Everyday Life

TODAY• Heat and ThermodynamicsThermometers, temperature scales; conduction,convection, radiation.

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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

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Heat Flow

Three lead bricks are in contact as shown.Heat flows:

A) From the hot to the coldB) From the cold to the mediumC) From the hot to the mediumD) From the medium to the coldE) All but B

1. Hot Brick

3. Cold Brick2. Med. Brick

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Heat Flow

Three lead bricks are in contact as shown.Heat flows:

A) From the hot to the coldB) From the cold to the mediumC) From the hot to the mediumD) From the medium to the coldE) All but B

1. Hot Brick

3. Cold Brick2. Med. Brick

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Temperature - something has heat.Caloric. (Why not?)

• “Heat flows” from hot tocold

• Some early physicsbeliefs: conservedquantity - a fluid calledcaloric

Hot Brick

Cold Brick

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Heat is Energy (same a mechanical)

• Count Rumford (BenjaminThompson) demonstrated theheating of water by boring cannonfor the elector of Munich

• Joule (1818 - 1889) measure increase intemperature due to friction

• Able to equate loss of mechanical energy byfriction to heat

• Famous experiment with weights moving fins inwater (he measured the change in temperature ofthe water)

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Heat flows between 3 bodies of differenttemperatures, as shown, in contact. When

does heat stop flowing?

a) When brick 1 has sametemp as brick 3

b) When brick 1 has sametemp as brick 2

c) When brick 2 has sametemp as brick 3

d) When all bricks havethe same temperature

1. Hot Brick

3. Cold Brick2. Med. Brick

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Heat flows between 3 bodies of differenttemperatures, as shown, in contact. When

does heat stop flowing?

a) When brick 1 has sametemp as brick 3

b) When brick 1 has sametemp as brick 2

c) When brick 2 has sametemp as brick 3

d) When all bricks have thesame temperature

1. Hot Brick

3. Cold Brick2. Med. Brick

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Bodies in thermodynamic contact willeventually all have the same temperature

Hot Brick

Cold BrickMed. Brick

Fundamental Principle of Thermodynamics (used for measuring temperature)

Med. Brick Med. Brick

Med. Brick

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Origin of the Fahrenheit scale

• “He <Fahrenheit> proposed (rather arbitrarily) azero for the freezing point of a brine solution, avalue of 32 for the melting point of ice, and the bodytemperature at 100 units (this should be about 96,100 is feverish)”

• Actually, water freezes at a lower temperature withsalt. Fahrenheit chose the salt solution to makewater freeze at the coldest possible temperature.This was the coldest he could get something stable.

• Fahrenheit Scale: Water freezes at 32oF, boils at 212oF

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Celsius temperature scale (1742)

1. Put the cylinder AB of the thermometer (i.e. the bulb) inthawing snow and mark the freezing point of water C, whichshould be at such a height over the cylinder at A that thedistance AC is half the distance between C and the waterboiling point mark D

2. Mark the boiling point of water D at a pressure of "25 tum 3linier" (approximately 755 mm)

3. Divide the distance in 100 equal parts or degrees; so that 0degree corresponds to the boiling point of water D, and 100to the freezing point of water C. When the same degreeshave been continued below C all the way down to A thethermometer is ready.

4. Celsius Scale: Water freezes at 0oC, boils at 100oC

Scale based on freezing and boiling points of water.Boiling point depends on pressureMust set the pressure for boiling

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Kelvin (absolute) scale

• Increase in temperature givesincrease in pressure, so decreaseenougn and no pressure!

• Increase in density (compression)gives increase in pressure

• Can have ideal gas law: P = nkT ifthe temperature is measuredrelative to something very cold:

Tkelvin = Tcelsius + 273

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Temperature - the various scales andconversions

Tk = Tc + 273Tf = (9/5)Tc + 32

NOTATION0 °C = 32 °F = 273 °K

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Production of heat from chemical energy -burning, activation energy, exothermic

reactions

• Carbon and oxygen like to joinCarbon Monoxide or COCarbon Dioxide or CO2.

• Microscopic view: atoms fallingtogether, means they gave uppotential energy

• Kinetic energy (HEAT) acquiredby these and nearby molecules

C

Carbon

O

Oxygen

Mass falling to earth, lower potential energyConverts to kinetic energyBounces until friction changes energy to heat

CO

O

COCarbon Monoxide

Carbon Dioxide

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In most burning, weaker bonds (lesspotential energy) broken

• Methane and oxygen molecules are more weakly bound• Come out of a shallow valley, go into a deeper one

C

Carbon

O

Oxygen

CO

O

H

Hydrogen

C

H

H

H

H O O+ 5

+ 4 OOH

Activation energy: energy needed toget first reaction goingThe released energy of first one (ormore) reactions gets others going

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Lennard-Jones Potential

Same shape for all moleculesDistance, change in potential energydifferent for different substances

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Lennard-Jones Potential also explainsHook’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.

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• To heat (or cool) our homes, we move heat inthree ways:

• Convection• Conduction• Radiation

Heating, Cooling

Wood contains resins - SMOKEBurning creates carcinogensNeed to move heat without the smoke-> Heat Exchangers

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The furnace and heat exchangers• Combustion air brought

into heat room (notthrough house)

• Hot gas burns, heatingmetal tubes

• Fans push house airthrough tubes and torest of house

To houseTo outside

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Transport of heat

• Conduction: The heat is carried bymolecules that move, and then move thosenext to them, but there is no netmovement of molecules

• Convection: Hot molecules are carried to acold region

• Radiation: electromagnetic radiationcarries the heat

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Conduction

• Hot pad• Ice pack

Heat one end, the other gets hot. Diffusion: nothing moves, but heat gets there (“like” sound, but not a wave)

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Convection: bulk movement of hot fluid

• Hot air rises• Pushes cold air

down• “Convective cell”

Hot air

Cold air

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Radiative transport:

• All bodies radiate electromagnetic energy over arange of frequencies. Light - for very hotobjects (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!

HOTCOLDRadiation

Less More

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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

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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.

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Practice Questions; Work

You are using a frictionless ramp to move a 200kgfiling cabinet onto a truck. The bed of the truck is 2mabove the ground, and the ramp is 8m long.How much work will you do moving the cabinet ontothe truckA) 4000N B) 2000N C) 4000J D) 2000J E) 200N

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Practice Questions; Work

You are using a frictionless ramp to move a 200kgfiling cabinet onto a truck. The bed of the truck is 2mabove the ground, and the ramp is 8m long.How much work will you do moving the cabinet ontothe truckA) 4000N B) 2000N C) 4000J D) 2000J E) 200N

Anwer is C: W=mgh=200kg*9.8m/s2*2m=3920J

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Practice Questions; Work

You are using a frictionless ramp to move a 200kg filingcabinet onto a truck. The bed of the truck is 2m abovethe 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 ontothe truckA) 200N B) 200kg C) 500N D) 25kg E) 25N

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Practice Questions; Work

You are using a frictionless ramp to move a 200kg filingcabinet onto a truck. The bed of the truck is 2m abovethe 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 ontothe truckA) 200N B) 200kg C) 500N D) 25kg E) 25N

Anwer is C: mgh=FL, L=mg(h/L)= 200kg*9.8m/s2*(2m/8m)=490N

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Practice Questions; Oscillators

A mass of 10kg is hooked to a spring with springconstant k=100N/m.What is the period of the oscillatorA) 20s B) 1s C) 5s D) 10s E) 2s

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Practice Questions; Oscillators

A mass of 10kg is hooked to a spring with springconstant k=100N/m.What is the period of the oscillatorA) 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

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Practice Questions; Oscillators

A mass of 10kg is hooked to a horizontal spring withspring 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 theequilibrium point?A) 0.6m/s B) 6m/s C) 20m/s D) 1.3m/s E) 13m/s

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Practice Questions; Oscillators

A mass of 10kg is hooked to a horizontal spring withspring 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 theequilibrium point?A) 0.6m/s B) 6m/s C) 20m/s D) 1.3m/s E) 13m/s

Anwer is A: (1/2)mv2=(1/2)kx2 so v=x*sqrt(k/m) v= 0.2m sqrt(100N/m / 10kg)=0.63 m/s

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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:

A

B

C

D

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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:

A

B

C

D

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Practice Questions; Kinematics

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

v start

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+

0

-

Vel

ocity

time

+

0

-time

#1

#1

+

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Vel

ocity

time

Acc

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-time

#1

#1

+

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ocity

time

+

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-time

+

0

-

Vel

ocity

time

Acc

eler

atio

n +

0

-time

A

DC

B

Acc

eler

atio

nSketch yourpredictions for thevelocity andacceleration of thecart moving towardthe motion detector,slowing down at asteady rate, andthen reversingdirection andspeeding up.+ is to the right

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+

0

-

Vel

ocity

time

+

0

-time

#1

#1

+

0

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Vel

ocity

time

Acc

eler

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-time

#1

#1

+

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ocity

time

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+

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Vel

ocity

time

Acc

eler

atio

n +

0

-time

A

DC

B

Acc

eler

atio

nAnswer is DAcceleration isconstant, andpositive

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A skier goes down a frictionless slope,decending 5m. She hits a level stretchwith friction, and the friction force is

50N. How far will she travel before shestops. The mass of the skier is 50kg.

• A 4.9m• B 49m• C 98m• D 9.8 m

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A skier goes down a frictionless slope,decending 5m. She hits a level stretchwith friction, and the friction force is

50N. How far will she travel before shestops. 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