THERMAL ENERGY

By definition, thermal energy is the sum of all internal energy.

Kinetic Theory

In chemistry, you learned that all particles are moving as long as the temperature is above absolute zero.

Even solids particles vibrate. This is the internal energy that is called thermal energy.

Three types of thermal energy.

Conduction Temperature difference exists between

substances. Particles collide and transfer energy. Typically between solids. Demonstration of dominoes.

Three types of thermal energy.

Convection Different density fluids at different

temperatures. Typically liquids and gases. Weather is a great example, hot air

rises.

Three types of thermal energy.

Radiation Electromagnetic Radiation waves (EMR) This includes light, ultraviolet, infrared,

gamma rays, microwaves, radio waves, Xrays and more.

All travel at the speed of light, which is c=3.00 x 108m/s

Temperature or Heat?

Heat Amount of energy that flows due to a

difference in temperature. Measured in joules or calories.

Temperature Average speed of the particles. Measured in Celsius or Kelvin. K=C+273

SPECIFIC HEAT

The heat required to raise 1g of a substance 1oC.

In chemistry, we only used water and used c=1calorie/gram oC

In physics, we will use many materials and use the units joules.

SPECIFIC HEAT CONSTANTS

Water c=4180joules/kg oC

Others from page 279 in your text:Aluminum c=903joules/kg oC

Copper c=385joules/kg oCIron c=450joules/kg oC

H = mcT

H is the change in heat, measured in joules

m is the mass measured in kilograms. C is specific heat constant. T is the change in temperature, can

be measure in Celsius or Kelvin.

1st Law of Thermodynamics

Thermal Energy = work done on a system + heat added to a system.

Example: An engine converts heat into mechanical energy.

2nd Law of Thermodynamics

Entropy = The disorder of system.This is a natural process, all things tend to move away from order to a state of disorder.

Example: The energy of a hot system will move to a cold system.

More on Entropy

If you reach into the freezer and pull out some ice, your hand feels cold because your hand is transferring heat to the ice.

If you touch a hot pan on the stove your hands feels hot because the pan is transferring heat to your hand.

Practice Problem

You place a 200g piece of copper in the oven until it is 234oC. You place the hot copper into 300g of water that is at room temperature (22oC).

What is the final temperature of the water?

Problem Solving Hints

Convert to kilograms Assume the heat lost by the hot

metal is equal to the heat gained by the cold water.

Set variable T equal to the final temperature. Solve for T.

Water c = 4180joules/kg oC Copper c=385joules/kg oC

Answer

H (hot Cu) = H (cold H2O)

mcT = mcT .2kg*385*(234-T) = .3kg*4180*(T-22)

18,018 – 77T = 1254T – 27,588

+27,588 +77T +77T +27,588 45,606 = 1331T

T = 34.3oC

Look at your Discovery

#1 – The water in the cup absorbed the heat and didn’t allow the cup to burn.

# 2 – Rubber bands are a strange material, they release heat when they expand. Most materials expand when the gain heat.

More about the Discovery

#3 - The fish is a multi-layered material and bends due to the heat of your hand. One layer contracts while the other layer does not.

#4 – The black side absorbs the light and the white side reflects the light.

More about the Discovery

#5 – Same as #1, water absorbs the heat and doesn’t allow the balloon to bust.

#6 and #7 – The fluid inside has a low boiling point. The heat causes the fluid to expand, hence the center of gravity tips the bird.

More about the Discovery

#8 – Hot air rises and hot air has a lower density. Therefore the teabag floats in the column of hot air.

#9 – Styrofoam is a material that does not conduct heat, therefore hot liquids stay hot OR cold liquids stay cold.

More about the Discovery

#12 – Fat is a great insulator, that’s one of the reasons why polar bears can stay so warm in the arctic.

Another problem

I want to use a 500g piece of iron to raise the temperature of 100g of water from 20oC to 45oC. How hot does the piece of iron have to be before it is put into the water?

Hint: c=450joules/kg oC for iron

FINAL ANSWERThe iron will have to change 46.4oC.

Therefore, it will have to start at 91.4oC.