Thermal Energy
Form 4 Physics (SPM) – Chapter 4
Thermodynamics
1st Law: Energy is conserved. i.e. It can’t be created or destroyed, only transferred from one form to another
Definitions Thermal Energy
Total mechanical energy contained in a body Temperature
Degree of hotness or coldness of a body Heat
The transfer of energy from one system to another
Thermal energy depends on the temperature, number of particles and arrangement of particles in a body
Heat on the other hand is thermal energy moving from one place to another
Temperature depends on kinetic energy in an object
Heat and Temperature Similarities
Both are quantitative (measureable) Both are scalar quantities (no direction)
Differences Temperature is measured in Kelvin (SI unit) with a
thermometer Heat is measured in Joule (Derived unit) with a
joulemeter or calorimeter
Thermal Equilibrium State where there is no net heat transfer
between two or more systems, resulting in constant temperature
0th Law of Thermodynamics
Heat exchange between System A and System B occurs through thermal conduction
Time taken for both systems to reach thermal equilibrium depends on the rate of heat transfer
Thermometer
A good thermometer has Suitable thermometric liquid Thin bulb to allow quicker response to heat Thin capillary tube to increase sensitivity Thick glass bore to allow magnification of scale for
easier reading and for increased durability
Capillary tube
Glass bore with scale
Thermometric properties Properties that change with changing temperature Example
When temperature , object expands (volume ) When temperature , pressure When temperature , electrical resistance
Thermometric fluid Properties:
Should be easily seen Able to expand and contract uniformly with
temperature Does not stick to wall of capillary tube Good heat conductor
Types: Mercury
Opaque and suitable for measuring high temperatures due to high boiling point and non-volatility
Alcohol Volatile and very low melting point makes it suitable for
measuring low temperatures
Thermometer - Calibration Thermometer placed in melting ice has a
column length of l0 When placed in boiling water, the length is l100
Thermometer placed in a solution of unknown temperature has a length of lϴ
Based on the recordings, 100˚C = (l100 – l0)
and Unknown temperature, ϴ = (lϴ – l0)
Proportionally, ϴ = (lϴ – l0)
100 ˚C (l100 – l0)
Hence, ϴ = (lϴ – l0) X 100 ˚C
(l100 – l0)
Heat Capacity The amount of heat change required to
change the temperature of an object by 1˚C Heat capacity, C = ∆Q/ ∆T , where ∆Q = Heat
change and ∆T = Temperature change Unit = J˚C-1
Specific Heat Capacity Amount of heat change required to change
the temperature of a 1kg object by 1˚C Specific means a unit quantity of a physical
property (in this case, mass) Specific heat capacity, c = ∆Q/(m∆T) , where
m = mass. Unit = Jkg-1˚C-1
Observations of SHC Sea breeze
During the day, temperature of air above land rises quicker than air above sea (land has a lower SHC than the sea)
This warmer air moves upwards and toward the sea, creating a convection current
The cooler sea acts as a heat sink for this warm air, causing air above the sea level to blow inland to replace risen air
Land breeze During the night, the sea is warmer than the land
due to accumulated heat gained during the day becomes enough to raise its temperature.
Air above the sea is now warmer causing the air above the sea to rise upwards, flow toward and sink at the land.
The convection current created causes the air above the land to blow towards the sea
Sea Breeze
Ocean is cooler than land (cold source, a.k.a. heat sink)
Ocean is warmer than land (heat source)
Land Breeze
This means… A body with high SHC will heat or cool
slower (i.e. poor conductor)
A body with low SHC will heat or cool faster (i.e. good conductor)
Water has a very high SHC value (4200 Jkg-1˚C-
1). It’s suitable as a ‘coolant’ in engines and machines to sink heat away from hot components
Water is used as coolant in cooling systems, radiators and the mammalian body
Change in physical state
Heating At gradients:
Heat absorbed Kinetic energy (Temperature rises)
At plateaus: Heat absorbed is used to overcome bonds Kinetic energy (and temperature) is constant
(melting and boiling point)
Cooling At gradients:
Kinetic energy Heat released (Temperature drops)
At plateaus: Rebonding releases heat energy Kinetic energy (and temperature) is constant
(condensation and freezing point)
Techniques Insulation
Prevents heat loss or gain from the surroundings Stirring with the thermometer
To ensure even heating and cooling. If stirring is uneven during cooling, supercooling
(liquid state below freezing point) occurs
At gradients of both curves The heat change is causing a change in
temperature. This heat is the heat capacity At the plateaus of both curves:
The heat change occurs at constant temperature. This is latent heat
Latent Heat Heat change that occurs when a substance
changes its physical state at constant temperature
Latent heat, L = ∆H, where ∆H = Heat change Unit = Joule (J)
Specific Latent Heat Heat change that occurs when 1kg of
substance changes its physical state at constant temperature
Specific latent heat, L = ∆H/m , where ∆H = Heat change and m = mass
Unit = Jkg-1
Two types of specific latent heat
Specific latent heat of fusion (Lf) Heat change that occurs when 1kg of substance
changes between the solid and liquid phases with no change in temperature
Specific latent heat of vapourisation (Lv) Heat change that occurs when 1kg of substance
changes between the liquid and gas phases with no change in temperature
Applications of Latent Heat Steam cooking
Steam has a high latent heat and when it condenses on food, the heat released is used to cook the food.
Sweating Evaporation of sweat makes us feel cold because
when water evaporates, the latent heat of vapourisation is absorbed from the surface of the skin, thus cooling it down.
Ideal Gas An idealistic paradigm of gases in real life The absolute zero is the temperature where
all motion of ideal gas particles ceases (Kinetic energy = 0)
The absolute zero is -273 ˚C The absolute zero scale is Kelvin (K) 0K = -273 ˚C
Ideal Gas Laws Boyle’s Law
Pressure of a gas is inversely proportional to its volume at constant temperature
P1V1 = P2V2
Charles’ Law Volume of a gas is directly proportional to its temperature
in the absolute zero scale at constant pressure V1/T1 = V2/T2
Pressure Law Pressure of a gas is directly proportional to its
temperature in the absolute zero scale at constant volume
P1/T1 = P2/T2
Boyle’s Law Charles’ Law
Pressure Law
Universal Gas Law
P1V1 / T1 = P2V2 / T2