Lesson 2

Heat PropagationLiceo Da Procida

Reminder

• Last time we learned about kinetic theory of matter

• Which state of matter has the fastest moving particles?

• How do we make the particles move even more quickly?

• Today, we will explore the different ways to transfer heat to a substance

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.

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

Practice Problems

• Q = k*A*ΔT*Δt/L

• 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