# Understanding Heat Transfer: Conduction, Convection and Radiation

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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

Conduction How 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.

Conduction Where 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! Warmer Colder

Fluid movement Cooler, more dense, fluids sink through warmer less dense fluids. In effect, warmer liquids and gases rise up. Cooler liquids and gases sink. Creates a CONVECTION CURRENT.

Water movement Cools at the surface Convection current Hot water rises
Cooler water sinks

Why is it windy at the seaside?

Where is the freezer compartment put in a fridge?
Cold air sinks Where is the freezer compartment put in a fridge? Freezer compartment It is warmer at the bottom, so this warmer air rises and a convection current is set up. It is put at the top, because cool air sinks, so it cools the food on the way down.

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.

Radiation 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 can travel through a vacuum (empty space) Radiation requires particles to travel Radiation travels at the speed of light

Convection questions 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. 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.

Conduction questions 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. How does a frying pan cook food? The metal pan conducts heat from the stove into the food

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. Radiation B. Insulation C. Conduction D. Convection

2. Which would be the BEST conductor of heat?
A. Solid B. Liquid C. Gas D. Fluid

3. How does heat energy reach the Earth from the Sun?
A. Radiation B. Conduction C. Convection D. Insulation