Heat Transfer

Physics 202Professor Lee

CarknerLecture 12

PAL #11 First Law Final temperature of 20 g, 0 C ice cube dropped into 300 g of hot tea

at 90 C. Add up all heats (Q = cm T and Q = Lm) Heat 1: melt ice

Q1 = (333000)(0.02) = 6660 J Heat 2: warm up now melted ice cube

Q2 = (4190)(0.02)(Tf-0) Heat 3: cool down tea

Q3 = (4190)(0.3)(Tf-90) Step 4: add up heat

Q1 + Q2 + Q3 = 0

6660 + 83.8Tf + 1257Tf –113130 = 0

1340.6Tf = 106470

Tf = 79.4 C

Heat Transfer

What is moving? In mechanics energy can be transferred

through a particle (e.g. a bullet) or a wave (e.g. a sound wave)

Heat can also be transferred by radiation both a particle and a wave

Conduction

The end in the fire experiences a large vibration of the molecules of the metal

This is called conduction The movement of heat from a high

temperature region to a low temperature region through another material

Conductive Heat Transfer The rate at which heat is transferred by

conduction is given byH = Q/t = kA (TH - TC)/L

Where: Q is heat and t is time A is the cross sectional area of the material (in

the direction of heat transfer) T is the temperature (hot or cold)

Thermal Conductivities

For Al, k=235 for Cu, k=428 (W/ m K)

Materials with low k are good thermal insulators

We use foam to insulate our houses Down filled winter coats trap air for

insulation

Composite Slabs

H = Q/t = A (TH - TC)/ (L/k) Where (L/k) is the sum of the ratios

of the thickness and thermal conductivity of each layer of the slab

Radiation

This is how you are warmed by sunlight The power (in Watts) that is emitted by

an object depends on its temperature (T), its area (A) and it emissivity ()

Pr = AT4

Where is the Stefan-Boltzmann constant = 5.6703 X 10-8 W/m2 K4

T must be in absolute units (Kelvin)

Absorption of Radiation

Pa =ATenv4

Where Tenv is the temperature of the environment

Pn = Pa -Pr = A(Tenv4 - T4)

Emissivity What is the value of emissivity?

Objects with an emissivity of 1 are called blackbody emitters or absorbers

Every object whose temperature is above 0 K emits thermal radiation People emit thermal radiation at infrared

wavelengths and thus can be detected at night with IR goggles

Convection

The hot air is thus lighter and rises If the hot air cools as it rises it will

eventually fall back down to be re-heated and rise again

Examples: baseboard heating, boiling water, Earth’s atmosphere

Convection Rate Factors

Fluidity

Energy exchange with environment

How rapidly will the material lose heat?

A small temperature difference may result

in not enough density difference to move

Next Time

Test #2 For Monday January 9:

Read: 19.1-19.7 Do Webassign homework

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

Ten joules of heat are added to a cylinder of gas causing the piston at the top to rise. How much work does the piston do?

A) 0 JoulesB) 5 JoulesC) 10 JoulesD) -10 joulesE) You cannot tell from the

information given

Which of the processes in the diagram produces the least work?

A) 1B) 2C) 3D) 4E) All are the same

Which of the processes in the diagram involves the least change in internal energy?

A) 1B) 2C) 3D) 4E) All are the same

Which of the processes in the diagram must have net heat output?

A) 1B) 2C) 3D) 4E) 3 and 4

Which of the processes in the diagram must have net heat input

A) 1B) 2C) 3D) 4E) 1 and 2