# 5. Heating a substance -

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## Temperature and Particles

Before move deeper into thermal physics, it's really important that we clarify a few essential physics ideas and definitions.

Firstly, in a hot cup of tea the hot liquid in the cup has energy. We call this internal energy.

Internal energy describes the energy of the particles that the substance is made from - hence its name.

Internal energy is made up of Kinetic (thermal energy) and Potential energy

1) The kinetic energy of the particles is due to their movement energy, for example in the vibrations of solid particles about a fixed position. This kinetic energy of the individual particles is also called thermal energy.

The measurement of temperature gives us an indication of how much kinetic energy (thermal energy) the particles have on average. So a material which is very hot has particles which have a lot of kinetic energy.

2) Another part of internal energy is the potential energy of the particles. This energy is determined by how strongly the particles are bonded together.

A good example of how potential energy in a substance can change is when a substance changes state.

If you were to add thermal energy to ice at zero degrees (its melting point), then that energy will not raise the temperature of the ice. Instead, the thermal energy will be used to break the solid ice bonds and form liquid water.
Once we have broken all the solid bonds and have a puddle of water, we would say that the liquid particles have a higher potential energy than the solid particles, because we have had to put energy in. We have essentially stored energy in the liquid ('potential' energy is energy that is easily stored).
Note that even though the liquid bonds are weaker than the solid bonds, the liquid particles have a higher potential energy.

So:

Internal Energy = Kinetic Energy (thermal energy) + Potential Energy

And now for another essential thermal energy idea...

Take two blocks of metal - one is hot and the other is cold. We now know that the hot material has particles which are vibrating a lot more than the cold body.

If you then hold the two blocks together, then the vibrations of the hot body's particles will bump into the cooler body's particles making them vibrate more. Thermal energy has therefore transferred from hot to cold.

This process where energy moves from a hotter body to a cooler body is called heating. Heat is the flow or transfer of thermal energy.

So technically it's not correct to say there is heat energy in a material. Rather, we should say that there is thermal energy in a material.

However, we can say that there is a heat flow from a hotter body to a cooler body.

(Note that heat cannot spontaneously flow from a cooler body to a hotter body - this is known as the 2nd law of thermodynamics).

So to sum up this flashcard:
When a substance is heated, there is a transfer of thermal energy into the substance. This transfer of energy increases the substance's internal energy.

GCSE Physics Keywords: Temperature, Particles, Vibration, Internal Energy

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