Physics Study Notes (3.1)
Internal energy
Internal energy is the sum of total kinetic energy (total thermal energy) and total
potential energy. Kinetic energy is energy associated with the random/translational
rotational motions of molecules. Potential energy is associated with forces between
molecules.
Specific heat capacity
The specific heat capacity of a substance is given by
and is defined by the amount of heat needed to raise the temperature of 1kg of the
substance by 1K. Different substances have different specific heat capacities because
of different densities and physical properties. The thermal capacity of an object is given
by Q=cθ (or Q=cΔT) and is defined by the amount of heat needed to raise an object’s
temperature by 1K.
Phase Change
Change of Phase
Process
Potential Energy
Solid to Liquid
Melting
Increases
Liquid to Solid
Freezing
Decreases
Liquid to Gas
Boiling
Increases
Gas to Liquid
Condensation
Decreases
During a phase change, temperature and kinetic energy remain constant and potential
energy changes (which increases as molecules spread out and vice versa).
Temperature and Absolute Temperature
Temperatures describe how hot or cold an object is and determines the direction of heat
flow between two bodies. Thermal energy transfers from an object with higher
temperature to another object with lower temperature. The energy transfer is called heat
(the flow of energy due to temperature difference). Objects which are in thermal
equilibrium have the same temperature.
Temperatures in Kelvin can be calculated by deducting the temperatures in Celsius by
273.15.
The absolute temperature of a body in the Kelvin scale is directly proportional to the
average kinetic energy per molecule inside the body.
Absolute zero is 0K or -273 degrees Celsius.
Temperatures cannot be lower than absolute zero. It is the temperature where particles
have zero average kinetic energy (no random motion).
Worked Examples