unit 24: thermal energy

itp 150 - prof. djassemi - cal poly

thermal energy transfer

heat moving from a warmer to a cooler object

fundamental principle of heat transfer

heat always moves from warm → cold

objects in rooms thermally

• hot objects in cooler rooms: cool down to room temp.

• cold object in warm rooms: heat up to room temp.

methods of heat transfer

1. conduction

2. convection

3. radiation

conduction

• transfer of heat thru direct contact btwn objects

• key rule: objects must be touching

• heat moves thru electron vibration & collisions btwn atoms

conduction ctd.

• metals conduct heat well b/c of free-moving electrons transferring energy quickly

• metal feels colder than wood b/c it conducts heat away from your hand faster

  • wood is an insulator

• conduction ex: a metal spoon in boiling water becoming hot

convection

• transfer of heat by movement of liquids/gases

• key concept: heat moves thru circulation currents in fluids

• when particles are heated in fluid: they spread apart & become less dense

hot air vs. cold air

• hot air: less dense → rises

• cold air: more dense → sinks

convection examples

• heating water in a pot

• warm air rising in a room

• ocean currents

radiation

• transfer of heat thru electromagnetic waves

• doesn’t require matter; can travel thru vacuum

• travels at speed of light

radiation examples

• sun heating Earth

• heat lamps

• campfire heat

heat transfer examples

• how heat reaches Earth from Sun: radiation (no particles in space)

• heat transfer in BBQ grill

  • conduction thru grill

  • convection from hot air

• heat from fireplace when standing nearby: radiation & convection

thermal power systems

• tech for:

  • producing

  • storing

  • controlling

  • transmitting

  • using

• heat energy

heat use vs. waste

• useful heat: cooking, space heating, hot water

• wasteful heat: heat from friction in bearings/engines

heat engines

• machines that convert heat energy → mechanical work

• ex:

  • IC engines

  • steam turbines

heat pumps

• device that moves heat from a low-temp to higher-temp location using work

• ex: refrigerator & AC

refrigeration cycle

• basically a heat engine running backwards

• extract heat from inside & releases it outside

refrigeration system components

• evaporator: absorbs heat from inside

• compressor: pumps refrigerant & increases pressure

• condenser: releases heat to outside environment

refrigeration cycle: step 1 - compression

gas is compressed → pressure & temp increase

refrigeration cycle: step 2 - condensation

hot gas releases heat & becomes liquid

refrigeration cycle: step 3 - expansion

liquid moves to low-pressure area thru expansion valve

refrigeration cycle: step 4 - evaporation

refrigerant absorbs heat & turns back into gas

refrigeration key gas law

• when pressure of gas decreases: temp. decreases

• when temp. increases: pressure of gas increases