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
Conduction How it Happens: When you heat a material at one end, the heat travels to the other end through vibrations. 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. Why? The atoms/molecules in solids and liquids are close together, so they can pass along the vibrations and heat. Gases do not conduct heat well, because the atoms/molecules are farther apart.
Good conductors Metals are very good conductors of heat. Free valence electrons in metallic bonds lets metals conduct heat quickly (easier to pass along vibrations!) Other good heat conductors include: Stone Tile Glass Water
Gases are bad conductors Bad conductors are called Insulators Insulators do not have free moving electrons OR the atoms are too far apart. So they do not conduct heat as well as metals. Examples include: Air (and all other gases) Wood Plastic Carpet Styrofoam
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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.
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, liquids and gases sink While… Warmer, less dense, liquids and gases rise up. This 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?
Examples of Convection and Convection Currents Convection Currents can be found in: A boiling pot of water Blowing Wind Heating a home In the ocean Inside the earth (plate tectonics)
Example Pictures
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. Examples of “EM”: Light X-rays microwaves Ultraviolet (UV rays) Infrared (IR rays)
Radiation Where it happens: It happens EVERYWHERE. All objects radiate heat. Radiation requires NO atoms/molecules. So… It is the only way to transfer heat 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 ?
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Radiation Radiation travels in straight lines True/False 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