Week 1

Heat PropagationLiceo Alfano

Concept Question

What is heat?

A) A measure of how hot or cold something is.

B) A basketball team from Boston.

C) Energy transferred due to a temperature difference.

D) A quantity measured in degrees Kelvin.

Answer

What is heat?

A) A measure of how hot or cold something is.

B) A basketball team from Boston.

C) Energy transferred due to a temperature difference.

D) A quantity measured in degrees Kelvin.

How does heat travel?

• Heat energy always travels from HOT to COLD

• When the sun shines on the Earth, does energy travel from the sun to the Earth or from the Earth to the sun?

• If I burn my hand by touching the stove, does energy travel from the stove to my hand or from my hand to the stove?

• If I take a cold shower to cool down, does energy travel from the water to my body or from my body to the water?

Kinds of Heat Propagation

• 3 types

• Heat Propagation Rap

• Conduction

• Convection

• Radiation

Conduction• What happens to the roof of your

mouth if you eat a piece of pizza before it cools down?

• If you have coffee in a mug does the outside of the mug feel hot or cold?

• Two objects touching = CONDUCTION

• Two objects = pizza and your mouth, two molecules in mug

Conduction

• Different materials conduct heat differently

• Which will get hotter, a metal spoon or a wooden spoon in a pot of boiling water?

• Thermal conductivity – How well a material can conduct heat

vs.

Conduction

• Thinking about if something is a good thermal conductor

• Ask, “if I put one end of this in a boiling pot, how long would it take for the other end to get hot?”

• Short time = good conductor

• Long time = poor conductor (also called insulator)

Concept Question

Which of the following are good thermal conductors?

1.Copper

2.Plastic

3.Cloth

4.Wood

5.Gold

Yes (thermal conductor)

No (insulator)

No (insulator)

No (insulator)

Yes (thermal conductor)

Concept Question

• Would you use a material with high or low thermal conductivity in the walls of your house to keep heat in during the winter?

A)High thermal conductivity

B)Low thermal conductivity

Concept Question

• Would you use a material with high or low thermal conductivity in the walls of your house to keep heat in during the winter?

A)High thermal conductivity

B)Low thermal conductivity

Conduction

• The thermal conductivity of a material is measured in W/m*C (ex: for aluminum, k = 250 W/m*C)

• We can measure how much heat has gone through a bar based on the following equation:

• Q = k*A*ΔT*Δt/L

• A = cross-sectional area, T = temperature, t = time, L = length

Conduction

• Q = k*A*ΔT*Δt/L

• ΔT = T1 – T2

• Δt = elapsed time

• Q = amount of heat transferred

Concept Question

• Would aluminum or wood have a higher value for k, thermal conductivity?

A)Aluminum

B)Wood

Concept Question

• Would aluminum or wood have a higher value for k, thermal conductivity?

A)Aluminum

B)Wood

Concept Question

• If I increase the cross-sectional area of a bar, the amount of heat transferred will

A)Increase

B)Decrease

Concept Question

• If I increase the cross-sectional area of a bar, the amount of heat transferred will

A)Increase

B)Decrease

Concept Question

• If I increase the length of a bar, the amount of heat transferred will

A)Increase

B)Decrease

Concept Question

• If I increase the length of a bar, the amount of heat transferred will

A)Increase

B)Decrease

Concept Question

• If I increase the change in temperature across a bar, the amount of heat transferred will

A)Increase

B)Decrease

Concept Question

• If I increase the change in temperature across a bar, the amount of heat transferred will

A)Increase

B)Decrease

Concept Question

• If I increase the amount of time elapsed, the amount of heat transferred will

A)Increase

B)Decrease

Concept Question

• If I increase the amount of time elapsed, the amount of heat transferred will

A)Increase

B)Decrease

Practice Problems

• A 2 m copper rod with a cross-sectional area of 0.1 m2 has one end at 80C and the other at 50C. The thermal conductivity of copper is 370 W/m*C. How much heat is transferred in 10 seconds?

• How much heat is transferred in an hour through a 5 m long copper rod with a square cross-sectional area, where each side measures 0.1 m? One end of the rod is at 300K and the other is at 320K

Convection

• Which way does a flame point on a candle?

• Right after you burn yourself, what should you do?

• Fluids flowing = CONVECTION

• Fluid = liquid (i.e. water), air

Convection

• Two parts to convection:

1)Hot air always travels UP (ex: hot air balloon)

2)Flowing liquids transfer heat (ex: run something frozen under warm water to heat it up)

• Cool convection video!

Convection

• Remember, FLUIDS MOVING (air, liquid) = CONVECTION!

Radiation

• Where do you normally hear the word “radiation?”

• Cell phones, X-rays, nuclear power plants, etc.

• What do all these have in common?

• Two objects that DON’T touch

• Energy transferred through electromagnetic waves

Radiation

• If you light a campfire and sit a few feet from it, what do you feel?

• If you put something in a microwave, does it come out hotter or colder?

• In heat propagation, radiation means heat transfer between two objects that DON’T touch

• Heat energy is transferred through electromagnetic waves

• Radiation experiment!

Radiation

• We can predict how much heat energy an object will give off based on its temperature!

• The Stefan-Boltzmann Law gives us the amount of heat energy radiated per unit surface area per second (flux)

• Think: would a higher temperature mean an object gives off more or less heat energy?

Stefan-Boltzmann Law

• THE EQUATION: F = σT4

• F = heat propagation flux (energy radiated per unit surface area per second)

• σ = Stefan-Boltzmann constant (5.67 * 10-8 J/s*m2*K4)

• T = temperature (but be careful! Temperature must be measured in Kelvin, not degrees Celsius or Fahrenheit)

• Does higher temperature mean more or less heat propagation?

Practice Problems

• Remember: F = σT4

• A star’s temperature is 5000K. According to the Stefan-Boltzmann law, how much energy is radiated into space per second by each square meter of the star’s surface?

• If the star is a perfect sphere with radius 640000000 m, what is the total power output of the star?

Examples from earlier

• Conduction, Convention, Radiation

• Sun shining on Earth

• Radiation

• Burning my hand on the stove

• Conduction

• Taking a cold shower

• Convection

Heat transfer applet

• http://www.teachersdomain.org/asset/lsps07_int_heattransfer/

More Examples

• For each of the following examples, what kind of heat transfer is it (and why)?

• Hot water rises in a boiling pot

• An incandescent light bulb feels warm when your hand is a few centimeters away

• A chocolate bar melts in my hand on a hot day

• A firefighter uses a hose of water to put out a fire

• A crazy man walks across burning coals

More Examples

• I ride on a waterslide to cool down on a hot day

• You feel the coolness of a breeze on a warm day

• The cold outside makes the inside of the windows feel cold

• Venus is much hotter than Saturn

• You stir fry vegetables in a pan

• You put your finger in a cup of hot coffee