Thermal Equilibrium

1
Heat Transfer

• Thermal Equilibrium
• Conduction
• Convection
• Radiation

2
Thermal Equilibrium

• Heat is defined as a transfer of thermal energy
  because of a difference in temperature.
• Thermal energy always transfers from areas of
  higher temperature to areas of lower temperature.
• Energy transfer stops when objects are at the
  same temperature.
• When objects are at the same temperature as their
  surroundings, they are in thermal equilibrium.

3
Thermal Energy Transfer
If two objects of different temperatures are in a
room at 20 C (comfortable room temperature),
what will happen?
B
A
20 C
20 C
20 C
70 C
5 C
Thermal energy will transfer from container A to
the room. Thermal energy will transfer from the
room to container B. Why? Because they are all
at different temperatures.
4
Thermal Energy Transfer

• Thermal energy can only be transferred in three
  possible ways.
• Three processes
• Conduction
• Convection
• Radiation

5
Conduction

• Conduction
• - transfer of thermal energy when molecules
  collide
• - transfer of energy through an object or
  between objects in contact
• Examples
• - at home conduction from a stove top to the
  copper bottom of a pan
• - on Earth conduction from the ground to the
  air just above the ground

6
Conduction
Why are the bottoms of some pots and pans coated
with copper? Why are pot handles long and
sometimes made of wood or plastic? Why is the
bottom of the troposphere (near ground) so much
warmer than higher up in the troposphere?
7
Convection

• Convection
• - transfer of thermal energy by a moving fluid
• - moving fluids can include liquids and gases,
  because they can flow
• - occurs because of differences in density
• Examples
• - at home boiling water churning in a pot
  stirring a hot drink or while preparing food
• - on Earth warm air rising and cool air
  sinking because of their different densities

8
Convection
Look familiar? Warm water in a pot rises because
it is less dense, and cooler water sinks because
it is more dense, and so the water
circulates. Warm air and cool air in the
atmosphere circulate in much the same way,
generating sea and land breezes as well as the
surface winds.
9
Radiation

• Radiation
• - transfer of thermal energy across space
• - any object at a higher temperature than its
  surroundings will radiate heat
• - radiation can be absorbed or reflected
• Examples
• - at home heat radiating from different
  sources
• fireplaces, radiators, stoves, light bulbs
• - on Earth radiation from the Sun warms the
  surface of the Earth (differently in different
  places)
• Question Why is radiation the only process
  that can transfer thermal energy from the sun
  to the Earth?

10
Radiation

• Why wear light colored clothes in summer?
• Why does snow melt quicker on a dark roof?
• Why does the sand warm up more quickly than the
  water?
• Why carry a space blanket when you go camping or
  hiking?

11
What does this all mean?
Solar radiation warms the Earths surface, and
the ground absorbs more of the suns
heat. Conduction from the ground warms the air
close to the ground. Warm air is less dense and
rises (Archimedes Principle). Warm air rising and
cool air sinking cause convection. We have a sea
breeze!