# Today’s Topic: Heat Transfer. What is heat, anyway? Heat is the flow of energy from a hotter object to a colder object.

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Today’s Topic: Heat Transfer

What is heat, anyway? Heat is the flow of energy from a hotter object to a colder object.

There are 3 ways in which heat can be transferred from one object to another:

CONDUCTION: the transfer of energy through matter CONDUCTION: the transfer of energy through matter by direct contact of particles. by direct contact of particles. This can happen in solids, liquids and gases. CONVECTION: the transfer of energy because of the CONVECTION: the transfer of energy because of the movement of warm fluids. movement of warm fluids. (Fluids can be liquids or gases) (Fluids can be liquids or gases) This can happen only in liquids and gases - not in solids. RADIATION: the transfer of energy by RADIATION: the transfer of energy by electromagnetic waves. Energy can move by radiation in air like the heat from your electric stove top, or in the vacuum of space the way the Sun heats the Earth. In radiation, the energy does not have to transfer through mass (particles). Heat Transfer

Conduction: the “d” means “direct contact” the teapot gets hot because it’s touching the hot burner.

Conduction Conduction is the transfer of heat within a substance, atom by atom. If you put one end of a metal rod over a fire, that end will absorb the energy from the flame. The atoms at this end of the rod will gain energy and begin to vibrate faster. As they do, their temperature increases and they begin to bump into the atoms next to them. The heat is being transferred from the warm end to the cold end.

Conductors The young woman in the pink dress is using a straightened out coat hanger wire to cook her marshmallow. Metals are good conductors of heat. The heat from the fire travelled along the thick metal wire. The wire got so hot that she had to let go of it. The young woman in the blue dress is using a wooden rod to hold her marshmallow. Wood does not conduct heat well. The wood in her hand is still cool and the marshmallows are almost done

Conduction A typical example of conduction would be the heat transferred from hot coffee, through the cup, to the hand holding the cup. A typical example of conduction would be the heat transferred from hot coffee, through the cup, to the hand holding the cup.

Interesting question: Which feels colder, the top of your desk or the leg of your desk? Which is colder?

Convection: the v represents the “v” in movement No fluid moving means no convection!

Convection Gases and liquids expand as they get warm, so warm fluids are less dense than cold fluids. The warm fluid will rise, creating the convection current. Warm air is less dense than cold air, making cold air heavier than warm air.

Warm air rises in your house through convection:

Radiation Heat transfer by electromagnetic radiation Examples: -anything being warmed by the sun - the heat radiating from a lightbulb, or a person who is warm from exercising

Radiation Radiation is the only form of heat transfer that doesn’t require matter. The heat from the sun travels through the vacuum of empty space, but it still gets here.

All three types of heat transfer:

Practice questions: Heat convection occurs in gases and liquids. Heat convection does not occur in solids because solids are unable to — A absorb heat by vibrating B transfer heat by fluid motion C emit radiation by reflecting light D exchange heat by direct contact

Practice Questions: In which container is the substance unable to transfer heat by convection? 

The moon’s surface becomes hot during the long lunar day because the sun transfers heat to the moon. This heat transfer is accomplished almost entirely through the process of — F convection G refraction H conduction J radiation

A man who was sleeping wakes up because he hears the smoke alarm go off in his house. Before opening the bedroom door, the man feels the door to see whether it is warm. He is assuming that heat would be transferred through the door by — A conduction B convection C radiation D compression

The transfer of heat by the movement of air currents in Earth’s atmosphere is an example of — A conduction B convection C radiation D fusion

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