New Symbol – “beta”

How “expandable” a material is.

Depends on the material. Look it up in the book, or wikipedia.

usually

Volume Expansion

Problem for aluminum is .0000643 A block of Al is 1m by 2m by 3m at 20 C. It is heated to 80 C. What is the new volume?

Applications of Thermal Expansion – Bimetallic Strip

Thermostats Use a bimetallic strip Two metals expand differently

Since they have different coefficients of expansion

Question (New subject) If two objects (touching) are not the same temperature, what will happen?

Heat Transfer Heat flows naturally from high temperature to low temperature.

There are 3 ways this happens.

KNOW THE 3 WAYS!!! of heat transfer.

New Symbol HEAT: Q Q is for “heat” Thermal energy. Joules. NOT THE SAME AS TEMPERATURE.

Which has more heat: 1.) A cup of very hot coffee 2.) An iceberg

New Symbol Which has more heat:

2.) An iceberg

The BIG iceberg has more heat even though it has a lower temperature.

New Symbol Teacher example:

2 parking lots. 1 with 10 Fast cars, 10 gal gas each. 1 with 1000 slow cars, 2 gal gas each. Who has higher speed or temp, Who has higher energy or heat?

Heat Flow Heat Flow: Q/t Q/t is for “heat flow” Movement of Thermal Energy. In Joules/Second or Watts.

Conduction Objects touching allows transfer of heat energy through collisions of the molecules.

Faster moving molecules beat on the slower moving ones and speed them up.

Heat flows from a warm object to a cool object that is in direct contact with it.

CONDUCTION = heat transfer by molecules vibrating and colliding.

Matter does not move places, but vibrates around a fixed position.

Conduction example Molecules vibrate in the heating element.

Molecules in the coil vibrate more than molecules in the pot.

They collide.

Conduction example Molecules vibrate. Molecules in the coil vibrate

more than molecules in the pot. They collide.

Molecules in the bottom of the pot begin vibrating more.

They collide with other molecules in the pot.

Conduction example

Molecules vibrate. Molecules in the coil vibrate more than

molecules in the pot. They collide. Molecules in the pot begin vibrating more. They collide with other molecules in the

pot.

Eventually, everything is vibrating the same amount.

The temp. has equalized.

All heat is transferred by vibrating molecules hitting each other.

Conductors What are some good conductors?

Conductors What are some good conductors?

Metals.(Note: Good heat conductors usually are also good electrical conductors. I wonder if the flow of electrons have anything to do with heat flow?)

What are some bad conductors?

Conductors What are some good conductors?

Metals. What are some bad conductors? (Also called insulators)

Air, wood, glass, rubber. Many insulators work by trapping pockets of stationary air.

Nothing is a bad conductor, LOL (means vacuums)

What will make heat flow faster?

What will make heat flow faster? Higher Temp difference. Better conductor. Wider area to go through.

What will make it go slower?

What will make heat flow faster? Higher Temp difference. Better conductor. Wider area to go through.

What will make it go slower? Longer distance to travel.

Heat Flow Equation Q/t = kAΔT/L

Q = Heat t= time Q/t = Heat Flow k = conductivity (unique for each material) in Joules/(sec*meter*Celsius)

A = cross sectional area ΔT = temp difference between areas. L = distance heat needs to travel. Note: A and T on top = faster L on bottom = slower

Problem A copper wire 18 cm long, and 1 mm in diameter passes through the wall of house.

It is 29 Celsius indoors,-17 outdoors.

How much heat per second will pass through the wire?

Q/t = kAΔT/L

Copper has a conductivity of 390 J/(s*m*C)

Convection 2nd method of heat transfer.

Heat is transferred by moving matter. Matter actually moves to a new location. Usually a liquid or a gas.

Think of weather clouds moving and bringing in “coldness”.

Convection Start with a heat source.

Air near the source heats up.

It expands, causing it to rise.

Convection The hot air rises and creates a vacuum. Cool air gets drawn in to take it’s place.

After hot air rises, it cools down and starts to sink.

So the air moves up and down, causing heat transfer.

(Sounds like a lava lamp.)

Convection By that time, the air that took its place has heated up and is rising.

We get a cycle.

Just like on the lava lamp.

Convection

Lawrence Kansas City10 cooler 10 deg hotterfarm land buildings and concrete

Convection

Demo now. Use a gravity mat to demonstrate convection and

weight of air.

Convection What are some examples of convection?

Convection What are some examples of convection?

Convection What are some examples of convection? Boiling water The earth’s mantle Heating and air conditioning systems.

Radiation Last type of heat transfer.

Transfer of heat through a vacuum by electromagnetic energy motion (energy moving as radiation).

Electromagnetic waves

Radiation Warm material gives off light.

Cool material absorbs light.

Energy flows from hot material to cool material.

Example Guess a famous example of a radiation source.

Example We get heat from the sun.

Even though there’s no air in space to conduct or convect it to us.

Radiation Equation Q/t = AeσT4

A = surface area of object e = “emissivity of the material”

a “table value” between 0 & 1

σ = The “Stephan-Boltzmann” Constant σ = 5.67 10-8 W/m2.K4.

T = Temp. (must be in Kelvin)

A common Radiator. A lightbulb gives off 60 Watts of heat. It is made of Tungsten (e = .15). If it is 2700 K, what is the surface area of the filament?