understanding heat transfer: conduction, convection and radiation

Understanding Heat Transfer:

Conduction, Convection and Radiation

What is Heat?

• HEAT is the TRANSFER of thermal energy• Heat always moves from a warmer place to

a cooler place.• Hot objects in a cooler room will cool to

room temperature.• Cold objects in a warmer room will heat up

to room temperature.

Question

• If a cup of coffee and a red popsickle were left on the table in this room what would happen to them? Why?

• The cup of coffee will cool until it reaches room temperature. The popsickle will melt and then the liquid will warm to room temperature.

Heat Transfer Methods• Heat transfers in three ways:

–Conduction–Convection–Radiation

ConductionHow it Happens: When you heat a material at one end, the heat travels to the other end.

• As you heat the material, the particles vibrate.

• These vibrations make the adjacent particles vibrate, and so on.

• The vibrations passes along the heat.

ConductionWhere it Happens: Conduction occurs in solids and liquids.

Gases do not conduct heat well, because the atoms/molecules are farther apart.

Why? Because the atoms/molecules in solids and liquidsare close together, so they can pass along the vibrations and heat.

Metals are good conductors

Remember metallic bonds?

Valence electrons in metals are free to move!

This movement lets metals conduct heat quickly (easier to pass along vibrations!)

Insulators = Poor conductors

Insulators (like wood and plastic) do not have free moving electrons. So they do not conduct heat as well as metals.

When it is cold out, why does metal feel colder than wood, if they are both at the same temperature?

Metal is a conductor. Metal conducts the heat away from your hands.

Examples of other good conductors: Stone, tile

Wood is an insulator (bad conductor). Wood does not conduct the heat away from your hands as well as the metal, so the wood feels warmer than the metal.

Example of an insulator: Styrofoam

Convection• How it happens: As a fluid (liquid or gas) heats up,

the particles in it spread out.

• This makes it less dense. And it begins to rise.

• When it cools, it becomes more dense and sinks. This creates a convection current.

Convection

Where it happens: Convection occurs in all fluids.

What is a fluid?

A liquid or a gas!

War

mer

Colder

Water movement

Hot water rises

Cooler water sinks

Convection current

Cools at the surface

Why is it windy at the seaside?

Cold air sinks

Where is the freezer

compartment put in a fridge?

Freezer compartmen

t

It is put at the top, because cool air sinks, so it cools the food on the way down.

It is warmer at the

bottom, so this warmer

air rises and a

convection current is

set up.

Radiation

How it happens: Radiation is the transfer of heat by electromagnetic waves (“EM”).

Radiation requires NO atoms/molecules. So it can happen in outer space, where there are almost no atoms.

All objects radiate energy. The hotter an object is, the more energy it radiates.Examples of “EM”:

• Light

• X-rays

• microwaves

• Ultraviolet (UV rays)

Radiation

Where it happens: It happens EVERYWHERE. All objects radiate heat.

Radiation requires NO atoms/molecules. So it is the only transfer that can happen in outer space, where there are almost no atoms.

Remember: Convection and Conduction DO need atoms/molecules to transfer heat.

How does heat energy get from the Sun to the Earth?

There are no particles between the Sun and the Earth so it CANNOT travel by conduction or by convection.

?RADIATION

Radiation

Radiation

Radiation travels in straight linesTrue/False

Radiation can travel through a vacuum (empty space)

True/FalseRadiation requires particles to travel

True/FalseRadiation travels at the speed of light

True/False

Convection questions

Why are the heaters in a hot water tank placed at the bottom of the tank?

Hot water rises.

So when the boiler heats the water, and the hot water rises, the water tank is filled with hot water.

Why does hot air rise and cold air sink?

Cool air is more dense than warm air, so the cool air ‘falls through’ the warm air.

Conduction questions

How does a frying pan cook food?

The metal pan conducts heat from the stove into the food

Why does a metal bar placed in a fire get hot at the end?

The atoms and electrons in the bar begin to pass along the heat as they vibrate.

Radiation questions

How can you feel a fire even though you stand far back from it?

The fire radiates the heat out into the room.

Why are shiny foil blankets wrapped around marathon runners at the end of a race?

The shiny metal reflects the heat radiation from the runner back in, this stops the runner getting cold.

1. Which of the following is not a method of heat transfer?

A. RadiationB. InsulationC. ConductionD. Convection

2. Which would be the BEST conductor of heat?

A. SolidB. LiquidC. GasD. Fluid

3. How does heat energy reach the Earth from the Sun?

A. RadiationB. ConductionC. ConvectionD. Insulation