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