The temperature of a substance is defined by the amount of internal energy it has. Generally, warmer substances will transfer thermal energy to cooler substances until they are the same temperature - this is known as thermal equilibrium.
In every day life, we use either °C or °F to measure temperature. The degree, °, however, shows that the temperature is relative to a substance-specific standard. Degrees centigrade are related to water, with 0 °C being its freezing point and 100 °C its boiling point. Degrees Fahrenheit is jut a mess with water being liquid between 32 °F and 212 °F.
In physics, using a relative temperature scale is not practical. Instead, absolute temperature is used, the units of which are Kelvin, K. Zero Kelvin is not relative to any particular substance - rather it is the absolute minimum possible internal energy (where particles have no kinetic energy at all, so do no vibrate)
T = θ + 273 Absolute temperature (in K) = Relative temperature (in °C) + 273
A change of 1 K is the same as a change of 1 °C
T is used for absolute temperature, θ for relative
Absolute zero is -273 °C
Solids, Liquids & Gasses
The kinetic model explains the three phases of matter:
Solids have their particles arranged in a regular pattern, all touching one another. They vibrate around fixed positions, and are held together by strong electrostatic attractive forces. The greater the thermal energy, the greater kinetic energy the particles have, so they vibrate faster.
The particles in a liquid are further spaced apart, but generally still touching. They are not fixed into a regular arrangement, but are free to move around - this allows liquids to flow. Due to this, liquids adapt to the shape of a container. The greater the thermal energy, the faster the particles move (as they have more KE).
In a gas, the particles are completely free to move, and do so at a high speed. They fill the space they are confined to, and collide with the walls of the container. This is why gasses exert pressure on a container. Again, the more thermal energy, the more KE the particles have.
Liquids and gasses are grouped together as fluids, because their particles are free to move. Their movement is random and in a zigzag (caused by collisions with other particles and boundaries). This is known as Brownian motion, and supports the kinetic model.
This can be demonstrated with pollen particles suspended in water, or smoke particles in air, viewed through a microscope.