Thermal energy transfer
You can measure the amount of energy required to change the temperature of a substance using
the following formula:E=mc0
specific heat capacity (c)
The specific heat capacity (c) of a substance is the amount of energy required to increase the
temperature of 1 kg of a substance by 1 C/1 K, without changing its state.
You can measure the amount of energy required to change the state of a substance using the
following formula:E=Lm
specific latent heat (L)
The specific latent heat (L) of a substance is the amount of energy required to change the state
of 1 kg of material, without changing its temperature. There are two types of specific latent heat:
the specific latent heat of fusion (when solid changes to liquid) and specific latent heat of
vaporisation (when liquid changes to gas).
Internal energy
The internal energy of a body is equal to the sum of all of the kinetic energies and potential
energies of all its particles. The kinetic and potential energies of a body are randomly distributed.
When the state of a substance is changed, its internal energy also changes, this is because the
potential energy of the system changes, while the kinetic energy of the system is kept
constant.
Absolute zero
Absolute zero (- 273C ), also known as 0 K, is the lowest possible temperature, and is the
temperature at which particles have no kinetic energy and the volume and pressure of a gas are
zero.
The absolute scale of temperature is the kelvin scale. A change of 1 K is equal to a change of
1C, and to convert between the two
Kinetic theory model
The kinetic theory model equation relates several features of a fixed mass of gas, including its
pressure, volume and mean kinetic energy. There are several underlying assumptions, which
lead to the derivation of this equation, these assumptions and the derivation are outlined below.
Assumptions -
. No intermolecular forces act on the molecules
. The duration of collisions is negligible in comparison to time between collisions
. The motion of molecules in random, and they experience perfectly elastic collisions
. The motion of the molecules follows Newton’s laws
. The molecules move in straight lines between collisions
Ideal gases
In an ideal gas there is no other interaction other than perfectly elastic collisions between the
gas molecules, which shows that no intermolecular forces act between molecules. As potential
energy is associated with intermolecular forces, an ideal gas has no potential energy, therefore
its internal energy is equal to the sum of the kinetic energies of all of its particles.
The following equation describes the relation between the pressure (p), volume (V), number of
molecules (N) and absolute temperature (T) of an ideal gas:
pV = NkT
Black body radiators and their radiation curves
A black body radiator is a perfect emitter and absorber of all possible wavelengths of radiation.
Radiation curves are graphs of intensity against wavelength of radiation emitted by objects at
different temperatures. Below is an example of a radiation curve for a black body radiator.
The Stefan-Boltzmann law
Stefan’s law states that the power output (also known as luminosity (L)) of a black body radiator
is directly proportional to its surface area (A) and its (absolute temperature).
This law can be used to compare the power output, temperature and size of stars.
Wien’s law
Wien’s displacement law states that the peak wavelength (Amax) of emitted radiation is inversely
proportional to the absolute temperature (T) of the object.
The peak wavelength is the wavelength of light released at maximum intensity.
Wien’s law 2
Wein’s law shows that the peak wavelength of a black body decreases as it gets hotter, meaning
the frequency increases so the energy of the wave increases (as expected).
This law can be used to estimate the temperature of black-body sources.
You can see Wien’s law being followed in the black-body curve below - as the temperature of the
body increases, the peak wavelength decreases.
