Heat Transfer • 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 an ice cube were left on the table in this room what would happen to them? Why? Answer The cup of coffee will cool until it reaches room temperature. The ice will melt and then the liquid will cool to room temperature.
HEAT TRANSFER METHODS • Conduction • Convection • Radiation
CONDUCTION • Heat travels along a substance from molecule to molecule (between two materials that touch each other) • Good conductors (silver, copper, gold) • Poor conductors ( glass, paper, Styrofoam)
Conduction When you heat a metal strip at one end, the heat travels to the other end. As you heat the metal, the particles vibrate, these vibrations make the adjacent particles vibrate, and so on and so on, the vibrations are passed along the metal and so is the heat. We call this?
Metals are different The outer e_______ for metal atoms drift, and are free to move. lectrons When the metal is heated, this ‘Sea of electrons’ gain k_____ energy and transfer it throughout the metal. inetic Insulators, such as w___ and p____, do not have this ‘Sea of electrons’ which is why they do not conduct heat as well as metals. ood lastic
Why does metal feel colder than wood, if they are both at the same temperature? Metal is a conductor, wood is an insulator. The metal conducts the heat away from your hands, the wood does not conduct the heat away from your hands as well as the metal, so the wood feels warmer than the metal.
CONDUCTIONEXAMPLES • water heating on an electric stove • hot sand touching your feet • touching a stove and being burned • ice cooling down your hand • boiling water by thrusting a red-hot piece of iron into it
CONVECTION • transfer of energy as it is carried through a liquid or gas • heat transfer by a circulation of rising warm air (less dense) and sinking cooler air (denser). • “Hot air rises” the more dense air sinks forcing the less dense air upward
Heating fluids What happens to the particles in a liquid or a gas when you heat them? The particles spread out and become less dense. What is a fluid? A liquid or gas.
Fluid movement Cooler, more d____, fluids sink through w_____, less dense fluids. ense armer In effect warmer liquids and gases r___ up. ise Cooler liquids and gases s___. ink
Water movement Cools at the surface Convection current Hot water rises Cooler water sinks
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.
CONVECTION EXAMPLES • macaroni rising and falling in a pot of heated water • heat rising from a chimney • an old-fashioned radiator (creates a convection cell in a room by emitting warm air at the top and drawing in cool air at the bottom)
RADIATION • Carries energy away from the hot object and causes it to cool down • The movement of heat in a wave-like motion through an empty space
The third method of heat transfer 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 Radiation travels in straight lines True/False Radiation requires particles to travel True/False Radiation travels at the speed of light True/False
RADIATION EXAMPLES • sunlight • heat from toaster • heat from a light bulb • heat from a fire • heat from anything else which is warmer than its surroundings
hot water rises and cold water sinks CONVECTION
stir frying vegetables CONDUCTION
a spoon in a cup of hot soup becomes warmer CONDUCTION
hot air balloon rises CONVECTION
you feel the heat from a campfire RADIATION
Heated air rises, cools, then falls. Air near heater is replaced by cooler air, and the cycle repeats. CONVECTION
boiling potatoes CONDUCTION
microwave oven RADIATION
Propane Heater RADIATION
basement is cooler than the attic CONVECTION
radiator warms your hands CONVECTION
a person takes a warm bath CONDUCTION
heat from a light bulb RADIATION
you get sunburned RADIATION
ironing a shirt CONDUCTION
wind currents CONVECTION
ANSWER KEY (kids may be able to justify other types of heat transfer) • hot water rises and cold water sinks - Convection • stir frying vegetables - Conduction • a spoon in a cup of hot soup becomes warmer - Conduction • grilling hamburgers over a charcoal flame - Radiation • hot air balloon rises - Convection • you feel the heat from a campfire - Radiation • a raw egg begins to fry as it hits a heated frying pan - Conduction • Heated air rises, cools, then falls. Air near heater is replaced by cooler air, and the cycle repeats. - Convection • boiling potatoes in water –Conduction (water to potato) • microwave oven - Radiation • propane heater - Radiation • basement is cooler than attic - Convection • radiator warms your hands - Convection • a person takes a warm bath - Conduction • heat from a light bulb - Radiation • hot pan is cooled by running it under cold water - Conduction • you get sunburned - Radiation • warm water at the surface of the swimming pool - Convection • Ironing a shirt - Conduction • wind currents - Convection