How ‘Heat’ Moves

Define “Heat”: Heat is the movement of thermal energy from a substance at a higher temperature to another substance at a lower temperature.

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The Nature of Heat

Heat moves in only one direction: Under normal conditions and in nature,

heat energy will ALWAYS flow the warmer object to the cooler object.

Heat energy will flow from one substance to another until the two substances have the same temperature.

2

3How ‘Heat’ Moves

Thermal energy in the form of heat can move in three ways.ConductionConvection Radiation

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CONDUCTION: The transfer of heat from

one object to another through direct contact.

5Conduction

Example: A metal rod being heated by a flame.

a. A Flame heats one end of the metal rod.

b. The heat travels down the rod toward the cooler parts of the metal (away from the fire).

c. The temperature increases along the length of the rod, even in parts not touching the flame.

6Conduction

Application: Describe the process of conduction when you place a hot spoon into a bowl of ice cream.

7Convection

Convection: the transfer of thermal energy (heat) through the movement of a liquid or gas.

Thermal Energy heat is carried by the liquid or gas, as it moves from one location to another.

8Convection

Example: Heating water:

a. When the water at the bottom of the beaker (nearest the burner) is heated, the water absorbs energy by conduction as it touches the bottom.

b. The water expands and becomes less dense.

c. This causes the heated (hot) water to rise.

d. As the hot water rises it loses its energy into the surrounding water and begins to circulate back to the bottom.

9Convection

Application: How do convection currents form in a room when the heater is turned on?

10Radiation

Radiation: the transfer of (thermal) energy by electromagnetic waves.

Radiation does not require matter to transfer thermal energy. All the sun’s energy that reaches

Earth travels through millions of kilometers of empty space (a vacuum).

All matter can radiate energy. You feel the radiation of thermal

energy from a bonfire, a heat lamp and a light bulb.

11Radiation

Other examples of the transfer of heat by Radiation:

a. Charcoal grill.

b. Hot tin roof.

c. Burner on a stove top.

d. ?

e. ?

12Radiation

Key Point: For radiation to be felt as heat it must first be absorbed by a material.

Example: Why does a black car feel hotter than a white car on a sunny day?

13The Nature of Heat

What happens when you put ice in a warm soft drink?

The heat energy moves from the soft drink into the ice by conduction (particle to particle contact) causing the ice to melt.

14Review

Describe the three kinds of heat transfer.

a. Conduction – transfer of heat energy from one particle to another by direct contact. (Primarily in solids)

b. Convection – transfer of heat energy in fluids-gases and liquids) through the bulk movement of matter from one place to another. (Produces currents)

c. Radiation – transfer of energy through electromagnetic waves. (Matter is not required!) (Radiant & infrared radiation from the sun)