CHAPTER 6: THERMAL Miss Luzma Fabre ENERGY AND HEAT
SECTION 2: THE TRANSFER OF HEAT ConductionHeat is transferred by Convection Radiation
CONDUCTION Heat is transferred from one particle of matter to another through direct contact. Ex: when a metal spoon warms up because the spoon is in a hot soup
HOW DOES CONDUCTION WORK? When objects touch each other, their particles collide. Thermal energy is transferred from the higher-temperature substance to the lower-temperature substance
CONDUCTORS AND INSULATORS A conductor is a substance that conducts thermal energy very well. Ex: metal such as silver or stainless steel
An insulator is a substance that does not conduct thermal energy very well Ex: wood, wool straw, paper Hats, coats, gloves and scarves are all insulators. They make it dif ficult for heat energy to be transferred from your body to the cold air, so they keep you warm.
CONVECTION The transfer of thermal energy by the movement of a fluid (liquid or gas). Liquids and gases expand when they are heated. As a result, the particles take up more volume. The liquid or gas in hot areas is less dense than the liquid or gas in cold areas, so it rises into the cold areas Ex: water being heated inside a pot
Convection works due to convection currents: the circular motion of liquids or gases due to density dif ferences that result from temperature dif ferences.
Ex: When boiling water in a pot, the water moves in circular patterns. The water at the bottom gets hot, and as it heats, it becomes less dense and rises to the top. On the surface, it begins to cool, making the water denser, so it goes back to the bottom. And the cycle begins again.
Why convection only happens in liquids or gases? What type of heat transfer occurs when you burn your feet on hot sand at the beach?
RADIATION The transfer of energy by electromagnetic waves, such as visible light and infrared waves. It does not require matter, so it can travel through empty space. This is why we can still feel the heat of the Sun even though it travels through a vacuum for 150 million km to reach the Earth! Ex: the sun´s radiation, the radiation from a fire in a fireplace
HEAT MOVES ONE WAY If two objects have dif ferent 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. Ex: a bowl of soup cooling to room temperature
WELL-INSULATED BUILDINGS Insulation prevents heat from entering the building in hot weather and from escaping in cold weather.
Ways to reduce heat in a house: the roof - fit loft insulation windows - fit double glazing and curtains gaps around the door - fit draught excluders the walls - fit cavity wall insulation the floor - fit a carpet
Conductor or insulator? Curling iron Oven mitt Plastic spatula Iron skillet
WHICH ONE IS A BETTER CONDUCTOR?
CW: The transfer of heat: Review and reinforce p 379