THERMAL ENERGY

 

Thermal energy, heat and temperature are not the same.

 

Thermal Energy:

Sum of the potential energy and the kinetic energy possessed by the atoms or molecules of the object.

 

Kinetic Energy of Molecules (EKinetic):

Kinetic energy possessed by the molecules of the object.  It depends on the speed of the molecules.  Faster the speed is the more kinetic energy.

 

Potential Energy Molecules (EPotential):

Potential energy possessed by the molecules of the object.  It depends on the distance between the molecules.  More potential energy if the molecules are further apart each other.

 

EThermal = EKinetic + EPotential

 

HEAT AND HEAT TRANSFER

 

Heat is the transfer of energy from a warmer body or region to a cooler one.  When heat is transferred from one body to another, the amount of heat lost by the hot body equals the amount of heat gained by the cold body.  Heat is transferred by conduction, convection and radiation.

 

Specific heat capacity:

Quantity of the heat needed to change the temperature of a unit mass of a substance through a unit change in temperature.  Substances with a small specific heat capacity warm and cool rapidly.

 

 
 

or    Q = m.∆t.c

 

 

Q:  Quantity of the heat (J)

m:  Mass (kg)

Dt:  Temperature change (Kelvin: K) or (Celsius:  oC)

c:  Specific heat capacity,  [J/(kg.K)] or [J/(kg. oC)]

 

0 oC (Celsius)= 273 K (Kelvin)

-273 oC = 0 K  (Absolute zero:  Lowest temperature possible)

 

Specific Heat Capacities of Some Substances

 

Substance Specific heat capacity
J/(kg.K)
Aluminum 900
Copper 390
Concrete 2900
Glass 600
Ice 2100
Sand 800
Water 4200
Air 990
Water Vapour 2000
Alcohol 2500
Iron 450
Oxygen 920

 

 

Change of States of Substance

 

Substances can be in three states:  Solids, liquids and gas.  If a substance is heated, molecules of the substance move faster.  It means that kinetic energy of the molecules increase .   The molecules of substance in solid state move slower than that of liquid state, it is slower in the liquid state than that of gas state.

 

Melting:  Change from solid to liquid state

Freezing or solidification:  Change from liquid to solid state

Evaporation: Change from liquid to gas state

Condensation: Change from gas to liquid state

 

When a substance receive heat, its state can change from solid to liquid or from liquid to gas, but the temperature of the substance does not change during the change of states.  The temperature stays constant.  It means the heat does not increase the kinetic energy of the molecules, but it increases the potential energy of the molecules.

 

Specific latent heat:

 

Specific latent heat is the quantity of heat required to change the state of a unit mass without changing its temperature 

 

l = Q/m

l:  Specific latent heat (J/kg)

Q:  heat energy (J)

m:  mass (kg)

 

Specific latent heat of Fusion (lf):

Quantity of heat required to melt 1 kg of substance without changing its temperature

Q = m. lf 

 

Specific latent heat of vaporization (lv):

Quantity of heat required to vaporize 1 kg of substance without changing its temperature

Q = m. lv

 

Melting Point, Boiling Point and Specific Latent Heats of Some Substances
 

Substance Melting Point(oC) Boiling Point
(oC)
Specific latent heat of Fusion
(lf) (kJ/kg)

Specific latent heat of vaporization

(lv) (kJ/kg)

Alcohol -114 78 104 853
Copper 1083 1187 134 1370
Gold 1063 2660 65 1578
Hydrogen -260 -253 56 450
Oxygen -220 -180 14 213
Water 0 100 333 2260