Chapter 6: Thermal Energy
RECALL: Temperature is a measure of the average kinetic
energy of the particles in the object.
As temperature increases, the average speed (and thus the
average kinetic energy) of the particles increases.
Thermal energy is the sum of the kinetic energy and the
potential energy of the particles in the object.
Note: potential energy is made up of chemical, electrical and
nuclear potential energy and any other potential energy due to
interactions between the particles.
Thermal energy, Temperature and Mass
T = 25 °C
T = 25 °C
More thermal E!
50 g
100 g
Q: Is the thermal energy of both beakers the same?
A: No. While both beakers have the same average kinetic
energy (same T), there are more particles in the 100 g
sample of water so it must have more thermal energy.
Heat: thermal energy that flows FROM a region of higher
temperature TO a region of lower temperature.
As the temperature difference increases, the amount of heat
transferred increases.
HEAT
Hot
Very
Hot
DT1
Cool
DT2 > DT1
MORE
HEAT
DT2
DT = difference in temperature
Cold
Specific Heat: The amount of heat needed to raise 1 kg of a
substance by 1°C
Substance
Specific Heat
[J/(kg°C)]
Substance
Specific Heat
[J/(kg°C)]
Water
4184
Iron
450
Wood
1760
Copper
385
Carbon
710
Silver
235
Glass
664
Lead
129
Water is often used as a coolant because of its high specific heat.
Substance
Specific Heat
[J/(kg°C)]
Substance
Specific Heat
[J/(kg°C)]
Water
4184
Iron
450
Wood
1760
Copper
385
Carbon
710
Silver
235
Glass
664
Lead
129
Q: What will heat up faster, water or iron?
Iron: it has a lower specific heat
Q: What will stay hot longer, copper or silver?
Copper: it has a higher specific heat
Calculating changes in thermal energy
Q = mCDT
Q = heat energy, units are Joules (J)
m = mass, units are kg
C = specific heat, units are J/(kg°C)
DT = change in temperature (Tf - Ti)
EX: A room containing 72 kg of air with a specific heat of
1010 J/kg°C is heated from 20°C to 25°C. How much
thermal energy was used?
m = 72 kg
C = 1010 J/kg°C
DT = 25°C - 20°C = 5°C
Q = mCDT = (72 kg)(1010 J/kg°C)(5°C) = 363,600 J
Calorimetry: A technique used to measure the amount of
heat transferred between substances.
Thermometer
Air
Water
Metal
pieces
Stirrer
0.025 kg of metal is heated to
100°C and then put into 0.1 kg
of water that is at 20°C.
The excess heat in the metal
is transferred to the water
warming it up to 23°C.
1. How much heat was transferred?
QH2O = mCDT
QH2O =(0.1 kg)(4184 J/kg°C)(3°C)
QH2O = 1255 J
2. What is the specific heat of the 0.025 kg of metal?
• If 1255 J of heat was gained by the water, that means that
1255 J of heat was lost by the metal (Conservation of Energy).
What is DT?
Qmetal = 1255 J
• Metal started out at 100°C and ended up at the same
temperature as the water, 23°C.
DT = 100°C - 23°C = 77°C
• Substitute the values into Qmetal = mCDT and solve for C.
Qmetal = mCDT
1255 J = (0.025 kg) C (77°C)
1255 J = (1.93) C
C = 650 J/kg°C
Thermodynamics: describe the inter-relationship between
heat, thermal energy and work.
First law of thermodynamics: The increase in thermal energy
(DU) of a system is the sum of the work done on the system
and the heat transferred to the system
DU = Q + W
In closed systems, heat cannot enter or leave the system.
DU = 0
In open systems, heat can be exchanged with the environment
DU  0
Second law of thermodynamics: It is impossible for heat to
flow from a cool object to a warm object unless work is done
on the system.
HEAT
HOT
COLD
NO WORK = NO HEAT TRANSFER
Section 2: Transferring Thermal Energy
Heat
Conduction: heat is transferred through
a material (glass, wood, metal etc.)
Rate of heat transfer depends upon the material.
Metals: FAST, good conductors
Gases: SLOW, good insulators
Convection: transfer of thermal energy in liquids and gases
by movement of warmer and cooler regions from place to
place.
• The warm water is less dense than
the cooler water around it so it
rises.
• The warm water cools (through
conduction) as it rises.
• The cooled water sinks back to the
bottom.
Radiation: transfer of energy by electromagnetic radiation.
No matter is needed for transfer to occur.
• When solar radiation strikes a light
colored object, most of the radiation
is reflected. The small amount that
is absorbed causes a small
temperature increase.
• When solar radiation strikes a dark
colored object, most of the radiation
is absorbed. This causes a larger
temperature increase.
• Because the molecules are far apart, gases do not absorb
much solar radiation.
Heating systems
Forced-Air
Steam/hot water Radiators
Electric Heating
Passive Solar Heating
Limitations for both types:
• Doesn’t work at night
• Doesn’t work well on cloudy days
Active Solar Heating
Heat Engine: a device that converts heat into work
Example: An internal combustion engine
• Chemical energy of gasoline is converted into heat,
which causes gases to expand and push (a force)
against the piston heads. When the pistons move
under the force of the expanding gases, work is being
done.
• Only about 25% of the heat generated by the engine
is converted into work. The rest is transferred out to
the surroundings.
Q: Is an internal combustion engine a closed or open system?
A: Heat is transferred to the surroundings, so it is an OPEN
system.
Heat movers: do work in order to move
heat from a COLD area to a WARMER
area.
Example: A refrigerator
• Electrical energy is used to compress
the coolant into a liquid. B
• The liquid coolant absorbs some of
the the thermal energy from the inside
of the refrigerator A so it can
expand into a gas. This causes the
inside of the refrigerator to cool.
Air conditioners and heat pumps are
also heat movers.