Internal Energy and Temperature

Temperature is the measure of the degree of heat of an object.

It has two scales:

Celcius (Degrees Celcius [DC])
Absolute (Kelvin [k])

Both scales are defined by set points of temperature.

Celcius Scale is defined by;

0DC - Melting Point of Pure Ice
100DC - Steam Point

Absolute Scale is defined by

273.16K - Water Triple Point (Point at which it can exist in three major states) 

-273DC = 0K

Note: When working in AQA Physics A, ALL thermal energy questions must be answered using the Absolute scale, though values of temperature will often be given in the Celcius scale. 

Energy Transfer only occurs when;

One object does work on another
There is a temperature difference between the two objects. This happens by;
Conductions
Convection
Radiation

Internal Energy is the measure of the measure of the molecular energy with relation to movement and position. It can change when;

Work is done on or by the object
During Energy Transfer

Thermal Energy is a measure of molecular energy due to temperature.

Temperature is not the only source for internal energy. A magnatised iron core has a higher internal energy than a non magnatised iron core. The internal energy of an object is the sum of the random distribution of the kinetic energy and potential energy of its molecules.

-273DC = 0K