Heat Transfer

Heat always transfers from hot objects to cold object.

Metals conduct heat very well partially because valence electrons are delocalized and can flow freely and transfer

energy.

Heat vs Temperature

• Temperature is the average energy of the molecules in a substance.

• Heat is the transfer of that energy between objects

• Question: A sparkler can get up to 3000°F, why doesn’t it burn you?

• Answer: Heat transfer is dependent upon mass!

Heat is transferred between objects (hot to

cold) until they are all the same temperature.

If we had two objects next to one another, one hot and one cold, what would happen over time?

100°C 100°C

50°C 50°C

Thermal equilibrium: objects are the same temperature and no

more heat will be transferred.

Conduction, Convection and Radiation

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Convection: heat transfered through a moving fluid (e.g. air or water). Hotter fluids rise up and colder fluids sink. Ex. Boiling water, hot air rising, hot air balloons.

Conduction: heat transfer through touching via molecular collisions. Ex. Touching a hot cup of coffee, a metal pan getting hot on a burner, stepping on hot sand.

Radiation: transfer of thermal energy through space via electromagnetic radiation without the need of conduction or convection. Space is empty and this is how the sun’s heat travels 93 million miles through a vacuum to reach us.

hyperphysics

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Wind Chill Effect1. Our bodies generates a natural shield of heat which surrounds us.2. Air is a very poor conductor of heat so this layer keeps up warm.3. When wind blows it removes this natural layer of heat allowing our body

to cool more quickly.

Warm Cold

Heat Shield No Heat Shield

WIND

WIND

WIND

1. Its 30°F outside and wind is 20mi/hr, what is the apparent temperature?2. Why is it important to cover your skin when its cold and windy out?3. It is forecasted to be 10°F outside with 20mi/hr wind what temperature would a car be?