Chapter 6
Thermal Energy
6 – 1 Temperature and
Thermal Energy
Do not use “HOT” or “COLD” to
describe temperature.
Temperature – a measure of the
average value of the kinetic energy of
the molecules.
The Mountain Dew in Mr. Gill’s bottle is not
moving, but the atoms that make it up are
moving. We are measuring the kinetic
energy of the atoms.
The greater the kinetic energy, the
greater the temperature.
Measuring Temperature
We cannot measure the kinetic energy
of each and every atom in a material.
We use a thermometer to measure the
expansion and contraction of a material.
Used to use mercury, now we use alcohol.
Temperature Scales
Fahrenheit, Celsius, Kelvin
Water :
Freezing Point -
32 F
Boiling Point -
212 F
100C 373K
Kelvin = 273 + C
Kelvin Scale
0 K is the lowest possible temperature.
It is called Absolute Zero.
Thermal Expansion
When an object warms up :
The atoms move faster
The atoms tend to spread out
This causes the object to expand.
When an object cools off :
The atoms move slower
The atoms tend to move closer together
This causes the object to contract ( shrink )
The amount of expansion or contraction
depends upon the material and the
change in temperature.
Liquids tend to expand more than solids
Thermal Energy
Thermal Energy – the sum of the
potential and kinetic energy of the
atoms in an object.
Kinetic Energy comes from the movement
of the atoms.
Potential Energy comes from the attractive
force between the atoms.
Thermal Energy vs. Temperature
Thermal Energy and Temperature are
One does not affect the other.
If you have a glass of water and a
pitcher of water, both at room
temperature, which has more Thermal
Energy ?
Pitcher because it has more atoms.
6 – 2 Heat
Heat is not
Heat – The thermal energy that flows from
something with a higher temperature to
something with a lower temperature.
Place your hand on your desk.
 Your hand has a higher temperature.
 Thermal Energy flows from your hand to
your desk.
If you were to place an ice cube into a glass
of water…
 Does the ice cube cool the drink ?
 Does the drink warm the ice cube ?
 By warming the ice cube, the drink loses
thermal energy and is cooler.
Transferring Thermal Energy
3 Ways Thermal Energy is Transferred :
Conduction – the transfer of thermal energy
through matter by direct contact of particles.
 Object # 1 to Object # 2
 Throughout an entire object
The particles collide with one another.
Momentum is transferred from one particle
to the next.
Solids work best because their particles are
packed the closest together.
The ability to easily transfer thermal energy.
 Metals
 Loosely held electrons make metals work
 Silver, Copper, Aluminum, Gold
 Do not easily allow the transfer of thermal
energy between particles.
 Wood, Plastic, Glass, Fiberglass
 Convection – the transfer of thermal energy by
the bulk movement of matter.
 Requires a Fluid
 Fluid – any material that flows.
 Liquids and Gases
Convection Currents
 The rotation caused by warmed fluids rising
and cooled fluids sinking.
 Winds ( weather )
 Ocean Currents
Radiation – the transfer of thermal energy in
the form of waves.
Radiant Energy – energy that travels in the
form of radiation.
Heat Absorption
Why is it that the pavement is hotter
than grass ?
The change in temperature of a
material as it absorbs heat depends
upon the material it is made of.
Specific Heat
Specific Heat – the amount of thermal
energy needed to raise the temperature
of 1 kg of a substance 1 °C.
The greater the specific heat, the more
energy is needed to change the
temperature of a substance.
Water has a very high specific heat.
Thermal Pollution
Thermal Pollution – the increase in the
temperature of a body of water caused by
adding warmer water.
Some electric power plants and factories
that use water to cool machinery produce
hot water as a by-product.
If the hot water is placed back into its
source, it will raise the temperature of the
water nearby.
Effects of Thermal Pollution
Higher water temperature requires fish
to use more oxygen.
Warmer water contains less dissolved
Fish and other organisms can die.
Reducing Thermal Pollution
Cooling towers are used to bring water
temperatures back down closer to the
temperature of the main body of water.
6 – 3 Engines and Refrigerators
Heat Engine
Heat Engine – A device that converts
thermal energy into mechanical energy.
Engines in cars, trucks, motorcycles, tractors.
Mechanical Energy – The sum of the
kinetic and potential energy of an object.
Differs from thermal energy because this is
the energy of the object, not the energy of
the atoms that make it up as in thermal
Internal Combustion Engine
The fuel is burned on the inside of the
engine in a combustion chamber.
Most cars have an engine that has 4 or
more cylinders.
A mixture of fuel and air is injected into
the cylinder and ignited by a spark.
The burning fuel produces a gas.
The expansion of this gas pushes the
piston down.
Each cylinder contains a piston that
moves up and down.
The up and down motion of the piston
turns a crankshaft that turns the
Heat Mover
Heat Mover – A device that moves
thermal energy from one location,
having a lower temperature, to another
location having a higher temperature.
A refrigerator, air conditioner, freezer.
A refrigerator absorbs thermal energy from
food and carries it to outside the
Air conditioners work under the same
principles as a refrigerator.
Thermal energy is absorbed from inside
the building and transferred to outside the
A heat pump can move thermal energy
either into or out of a building.