Heat Transfer Heat Transfer Heat always moves from

Heat Transfer

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 v. Conduction v. Convection v. Radiation

CONDUCTION v. Heat travels along a substance from molecule to molecule (between two materials that touch each other) v. Good conductors (silver, copper, gold) v. 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, the vibrations are passed along the metal and so is the heat. We call this?

Metals are different The outer e_______ lectrons for metal atoms drift, and are free to move. When the metal is heated, this ‘Sea of inetic electrons’ gain k_____ energy and transfer it throughout the metal. Insulators, such as w___ ood and p____, lastic do not have this ‘Sea of electrons’ which is why they do not conduct heat as well as metals.

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.

CONDUCTION EXAMPLES • 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 vtransfer of energy as it is carried through a liquid or gas vheat transfer by a circulation of rising warm air (less dense) and sinking cooler air (denser). v“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 A liquid is aorfluid? gas.

Fluid movement Cooler, more d____, ense fluids sink through w_____, armer less dense fluids. In effect warmer liquids and gases r___ ise up. Cooler liquids and gases s___. ink

Water movement Cools at the surface Cooler water sinks Convection current Hot water rises

Why is it windy at the seaside?

Cold air sinks Where is the freezer compartment put in a fridge? It is put at the top, because cool air sinks, so it cools the food on the way down. Freezer compartment It is warmer at the bottom, so this warmer air rises and a convection current is set up.

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 v. Carries energy away from the hot object and causes it to cool down v. 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 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

CONDUCTION a spoon in a cup of hot soup becomes warmer

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

CONVECTION Heated air rises, cools, then falls. Air near heater is replaced by cooler air, and the cycle repeats.

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

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

ANSWER KEY • • • • • 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