U NIT 2.3 – T HE T RANSFER OF H EAT (R ADIATION, C ONDUCTION, AND C ONVECTION )

F LOW C HART Thermal energy Heat Convection Conduction Radiation is the transfer of by

H OW IS H EAT T RANSFERRED ? Heat can be transferred (moved) in three ways: conduction, convection, and radiation. They can occur at the same time or at different times. They all need a starting point: a heat source.

CONDUCTION Conduction - heat is transferred from one particle of matter to another without the movement of the matter. For example, if you put an ice cube on a metal spoon and you are holding the handle (stem), the thermal energy from your hand will be transferred (conducted) to the ice.

RADIATION Radiation – the transfer of energy by electromagnetic waves (usually as infrared radiation) Heat can radiate through empty space that contains no matter at all. Example: The sun’s energy travels through millions of kilometers of empty space and radiates the Earth with its energy.

C ONVECTION You have heard that heat rises. This is true in terms of liquids and gases. Remember that when something is heated, it becomes less dense. When a liquid or gas is heated, the particles on the bottom will become heated (and less dense). Those particles will want to rise to the top of the object holding them. The cooler particles that were previously above the heated particles will move and take place of the heated particles. This continues to happen repeatedly as long as the matter is heated. Convection Current – current caused by the rising of heated liquid or gas and the sinking of cooled liquid or gas.

C ONVECTION One way that liquids and gases differ from solids is that they can flow. (What does flow mean?) This makes them fluids. Convection – heat transferred by the movement of currents within a fluid Examples: convection in a convection oven, convection currents in the mantle, boiling water, and lava lamps

Examples of convection can be seen in the Earth’s atmosphere and in the ocean currents. Where is the cooler water? Where is the warmer water?

CAN YOU TELL? WHICH IS IT?

CAN YOU TELL? WHICH IS IT?

CAN YOU TELL? WHICH IS IT?

C AN YOU T ELL ?

H EAT M OVES O NE W AY ONLY RECAP: Heat – when thermal energy is transferred from a warmer object to a cooler object. If two objects have different temperatures, heat will flow from the warmer object to the colder one. Heat will flow from one object to the other until the two objects have the same temperature. This is called a state of equilibrium.

C ONDUCTORS VS. I NSULATORS Conductors – a material that conducts heat well Ex: A metal spoon conducts heat better than a wooden spoon A good conductor like a tile floor feels cold to the touch because it is quickly and easily taking heat from your skin Insulators – a material that does not conduct heat well Ex: Wood, wool, straws, and paper. Clothes, jackets, and blankets are insulators that slow the transfer of body heat out of your body Windows also good insulators. Shown by the diagram next.

C ONDUCTORS VS. I NSULATORS Air between the panes of this window acts as an insulator to slow the transfer of heat.