HEAT TRANSFER
Heat can be transferred in three ways:
(1) Conduction
(2) Convection
(3) Radiation
CONDUCTION
When a hot object touches a cooler object, energy moves from the hot object to the cooler object.
The cooler object therefore gets warmer and the hotter object gets cooler. For examples when boiling water in a pan. The bottom of the pan touches the hot burner. Heat moves from the burner to the bottom of the pan. The water inside the pan touches the bottom of the pan. Heat then moves from the
pan to the water inside the pan.
Conductors are materials that allow heat to pass through them. Metals are good heat conductors.
Insulators are materials that do not allow heat to pass through them. Examples include plastic, rubber,
wood, liquid and air.
CONVECTION
Heat moves through a fluid. A fluid is something that flows such as liquid or air. The heat is carried from one place to the next. For example:
(1) Air conditioning systems. The air conditioning unit is placed high up in a room. Cool air flows down from the unit. It picks up the warm air. This warm air is then carried back up into the unit. This warm air is cooled. The cool air flows down from the unit. The cycle continues.
(2) Land and sea breezes. During the day warm air above the land rises. Cool air moves from the sea and comes over the land. The cool air picks up the heat from the land. This air now rises and moves back to the sea. At night, warm air over the sea rises. Cool air from the land comes over the sea. The cool air picks up heat from the sea. This air now rises and moves back to the land.
Convection takes places in the some appliances such as hot water systems, refrigerators, heaters, radiators, and kettles.
RADIATION
A hot object gives off heat rays. These rays hit a cooler object. The cooler object absorbs the heat rays and becomes warmer. The two objects do not touch. There is an empty space/ vacuum between the two objects. For example:
(1) A microwave is used at home to heat or cook food. The microwave gives off rays of heat. The rays of heat are absorbed by the food and the food cooks.
(2) Walking in the sun. The rays of heat from the Sun hit the skin. The skin absorbs the heat and gets hot.
Dull, black, and rough surfaces absorb heat energy and radiate heat energy.
Shiny, silvery, and smooth surfaces do NOT absorb heat energy and do NOT radiate energy. Instead, they reflect heat energy.