Thermal Energy Defined
Thermal energy – total energy of
the particles in a material
Heat defined
Heat - is the thermal
energy that flows
from something of
a higher temperature
to something of a
lower temperature
Temperature Defined
Temperature – measure of the
average kinetic energy of the
particles in a sample of matter.
Electronic (Digital) Thermometers
The most common sensor
is a thermoresistor (or
thermistor).
This device changes its
resistance with changes in
temperature.
A computer or other
circuit measures the
resistance and converts it
to a temperature.
Temperature units depend of scales
• The temperature scales most widely
used today are the Fahrenheit,
Celsius and Kelvin Scales.
1700s Gabriel Daniel Fahrenheit
Daniel Gabriel Fahrenheit
(1686-1736)
He was the German physicist who invented the
alcohol thermometer in 1709, and the mercury
thermometer in 1714. In 1724,he introduced the
temperature scale that bears his name
- Fahrenheit Scale.
Born in the Polish city of Gdansk
in 1686, he was only fifteen when
his parents both died from eating
poisonous mushrooms.
• He discovered, among other things, that water can
remain liquid below its freezing point and that the
boiling point of liquids varies with atmospheric
pressure.
• Fahrenheit temperature scale, scale based on 32
degree for the freezing point of water and 212 degree
for the boiling point of water, the interval between the
two being divided into 180 parts.
• Fahrenheit originally took as the zero of his scale the
temperature of an equal ice-salt mixture and selected
the values of 30 degree for the freezing point of water
and 90 degree for normal body temperature.
After Fahrenheit died in
1736, scientists calibrated
his model of thermometer
using 212 degrees, the
temperature at which water
boils, as the upper fixed point,
and 32 degree for water
freezing.
When the Fahrenheit
thermometer was recalibrated,
normal human body
temperature
registered 98.6 rather than 96.
The Celsius temperature scale
was invented by Anders Celsius
(1701-1744) a Swedish Astronomer
• For his meteorological observations
he constructed his world famous Celsius
thermometer, with 0 for the boiling point
of water and 100 for the freezing point.
• After his death in 1744 the scale was
reversed to its present form with zero
as water’s freezing point.
Celsius Temperature Scale
• The Celsius scale often
used by countries
using the metric system.
It is based on water’s
freezing
point – set at 0o and
its boiling point –
set at 100o.
Kelvin temperature scale
The temperature scale is
named after the British
mathematician and physicist
William Thomson Kelvin
(1824 – 1907) who proposed
it in 1848.
Kelvin temperature
scale, has an absolute
zero below which
temperatures
do not exist.
Absolute zero or 0 K, is the temperature at which
molecular energy is a minimum, and it is equal to a
temperature of -273.15° on the Celsius scale.
The Kelvin degree is the same size as the Celsius degree.
(When writing temperatures in the Kelvin scale, convention the
degree symbol is left out and we merely use the letter K.)
Temperature Conversion Formulas
• Fahrenheit to Celsius
• Celsius to Fahrenheit
• Kelvin to Celsius
Thermal equilibrium
• The state in which two bodies in physical contact
with each other have identical temperatures. (Not
net transfer of thermal energy or heat)
• Thermal equilibrium is the basis for measuring
temperature with thermometers in that the
thermometer must be in thermal equilibrium with an
object before a temperature reading can be taken.
Serway/Faughn Physics
State of matter is related to temperature
Solids, liquids, gases- states of matter determined by
the rate at which atoms or molecules move.
• Solids- tightly packed with fixed
volume & shape
• Liquids – not as tightly linked
with fixed volume, but not shape.
• Gases- particle with little or no
attraction, no fixed volume or shape.
• For gases with one type of atom (monoatomic)
temperature is understood in terms of kinetic
energy.
• For other kinds of substances, molecules can
rotate or vibrate, so other types of energy is
present. Serway/Faughn Physics
Thermal Expansion
• Increasing the temperature of a gas at constant
pressure causes the volume of the gas to
increase. (Also occurs in solids and liquids)
• In general, as the temperature of a substance
increases, the volume also increases. This is
known as thermal expansion.
Plasma – Fourth State of matter
• It comprises the major component of the Sun
and other stars. Heating a gas may ionize its
molecules or atoms (reducing or increasing
the number of electrons in them), thus
turning it into plasma, which contains charged
particles: positive ions and negative electrons
or ions.
Units of Heat
Thermal conduction
• Thermal conduction is the flow of heat by
transfer of energy between particles. It is due
to the collision or diffusion of particles along
a gradient (higher kinetic energy to lower
Example: Heating food in a
kinetic energy).
pot or pan on the stove.
• We usually think
heating by conduction as
a direct heat transfer
between materials.
of
Thermal conductors
Thermal conductors are
substances that rapidly transfer
energy as heat.
Metals are very good thermal
conductors
Thermal insulators
Substances that slowly
transfer heat.
ces
R values of Insulation
• The R-value indicates resistance to heat flow. The Rvalue of a material is a measure of the resistance of that
a 1 m x 1 m square slab of material has , to heat flow per
centimeter of the material’s thickness.
To have a well-insulated home, materials with a
total R-value of at least 19 should be used in the
outer walls and those with total R-values of 30 –
44 should be used in the roof and ceiling
because the warmest air inside the house is
carried upward by convection currents.
– Glencoe Physical Science
• Bird feathers can insulate the bird by trapping
air (dead air spaces).
A puffy winter coat insulates with
pockets of air trapped within the
fill sewn into the coat.
Dead-Air Spaces as Insulation
Even though glass is a good insulator, windows transfer
thermal energy.
A single pane of glass has an R-value of 1.
A double-pane window traps
air between the panes to
produce greater insulating
properties.
Some double pane windows
use Argon gas between the
panes to increase the
R-value to an even
greater number.
Transfer of heat through Convection
• Convection heat transfer involves
the movement of molecules within
fluids, such as gases and liquids.
• Convection heat and mass transfer
happens as the result of diffusion.
Convection Currents
• Movement of cold or hot matter, such as
heated air rising upward from a flame. Colder
air is denser than warmer air.
• As it moves downward it caused convection
currents which heat the air faster than by
conduction alone. (Similar with water or other
liquids)
Convection Examples
convection in pot
Convection & radiation
• A) The heating element of
the stove heats the water
which transfers heat to the
spoon.
• The water circulates
through convection
currents in the water.
• B) The metal spoon in
direct contact with the
person’s hand directly
conducts heat.
heat
Transfer of Heat Through Radiation
• Transfer of heat in the form of waves. It
does not require matter to be present
for transfer.
• Heat from the sun that warms the Earth
is an example of radiant energy transfer.
• Shiny materials reflect more radiant
energy than dull.
• Lighter materials absorb less radiant
energy than darker materials.
Sources of radiant heat energy
electric space heater
Home Heating Systems
• Forced air (gas or oil furnace)
convection–
(uses fan in air handler)
• Hot Water Radiators – radiation
• Fireplace – radiation &
convection (with fan)
• Electric heater – radiation &
convection (with fan – air handler)
• Coal or wood stoves – radiant
Specific Heat Capacity
• The specific heat of a substance is defined as
the energy required to change the
temperature of 1 kg of a substance by 1oC.
• Every substance has a unique specific heat
capacity. This value tells you how much the
temperature of a given mass of that substance
will increase or decrease, based on how much
heat energy is added or removed.
Specific Heat of Water
• A relatively high specific heat capacity
of water means that it resists changes in
temperature. This makes it great for cell
cytoplasm since it provides more
stability in temperature.
Calculation of Specific Heat
• C = specific heat capacity
• m = mass
Units [J/(kg.K)]
• T = change in temperature Tfinal – Tinitial
• Q = heat energy transferred
Sample Calculation
A 32.0 g silver spoon cools from 60o C to 20oC.
What is the change in thermal energy?
m = 32.0g = 0.0320 kg
C = 235 J/(kg.K) –from table of values for silver
Tinitial =60oC
Tfinal =20oC Q = (m)( T)(C)
= (0.0320 kg)(-40.0oC)(235J/kg . K)
= -301 J
The spoon loses 301 J of thermal energy as it cools.