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