Thermal Expansion

1
Heat and Heat Transfer
2
Thermal Expansion

• When the temperature of a substance is increased,
  the particles jiggle faster and move further
  apart on average.
• The result is called thermal expansion.
• Cooling a material has the opposite effect.
• For the same temperature change, some materials
  expand more than others.

Most long bridges have expansion joints to allow
for expansion and contraction caused by
temperature changes.
3
Heat Flow

• Heat flows from hot to cold
• Three major heat transfer mechanisms
• Conduction
• Convection
• Radiation
• To minimize heat flow, minimize all three

4
Conduction

• Energy is transferred by collisions of particles.
  (Fast moving particles bump into slow moving
  particles and make them speed up)
• Works best in solids, particularly metals
  (highest density).
• Substances that are good conductors conduct heat
  faster and better than other substances.
• Insulators conduct heat poorly.

5
Convection

• Takes place in liquids and gases.
• When the liquid or gas is heated, the particles
  move faster and spread further apart so they
  become less dense and rise, and the cooler,
  denser molecules sink setting up a convection
  current.
• Convection currents occur in the Earths
  atmosphere, oceans and interior.

6
Radiation

• Heat energy is transferred through space in the
  form of invisible light called infrared
  radiation.
• Earth is heated by the Sun by radiation.

7
Insulation

• Prevents heat loss by reducing the transfer of
  heat by conduction and convection. e.g. wood,
  Styrofoam, fiberglass
• Good conductors of heat at room temperature feel
  cool to the touch because they draw heat from our
  bodies, which are warmer.

8
Heat Capacity and Specific Heat

• Some substances require more heat to change their
  temperature than do others.
• The specific heat capacity of a substance is the
  quantity of heat required to raise the
  temperature of a unit mass of that substance by 1
  degree.
• Water has the highest specific heat capacity of
  almost any known substance.

9
Heat Units

• Units Joules or calories. (1 calorie 4.184 J )
• A calorie is the amount of heat required to
  change the temp. 1 oC for 1 g water.

10
Calories

• Energy rating of foods- burn them and measure the
  energy released.
• 1000 calorie 1 kc 1 Calorie

11
Phase Changes

• Temp is constant during melting/boiling
• Why?
• Melting Energy needed to break molecules loose
  from ice
• Vaporization Energy needed to break attractions
  holding liquid water molecules together

12
Liquid to gas

• Vaporization liquid changes to gas
• Evaporation particles escape from the surface of
  the liquid
• Boiling liquid changes to bubbles of water vapor
  that appear throughout the liquid

13
Heating Curve