THERMAL ENERGY AND HEATSPH4C

April 2010

THERMAL ENERGY• James Prescott Joule (1818-1889) spent much of his

honeymoon studying waterfalls.

• He noticed that the water at the bottomof a waterfall had a higher temperature than at the top.

• How might this happen?

THERMAL ENERGY, HEAT AND TEMPERATURE• Thermal Energy– As atoms and molecules move more rapidly, they

possess greater thermal energy.

• Heat– A measure of the energy transferred from a warm body

to a cooler body because of a difference in temperature.

• Temperature– A measure of the average kinetic energy of the atoms or

molecules of a substance.

HOW HEAT SPREADS FROM ONE REGION TO ANOTHER• All things are made up of molecules• When objects are heated, they absorb thermal

energy.• This means that the molecules are absorbing the

thermal energy.• With more energy, the molecules are able to move

faster.• When the molecules move faster, the temperature

of the object increases.• Temperature increase means the object gets hotter.

THREE METHODS OF HEAT TRANSFER• Conduction– Process by which the collision of atoms and electrons

transfers heat through a material or between two materials in contact.

• Convection– Process of transferring heat by a circulating path of fluid

particles.

• Radiation– Process in which energy is transferred by means of

electromagnetic waves.

CONDUCTION• Collision of atoms and electrons transfers heat

• Occurs mainly in solids

• Two types of conduction– Molecular vibration

– Free electron diffusion

Note: Conduction is not the main form of heat transfer in liquids and gases because their molecules are spaced further apart.

MOLECULAR VIBRATION

• When heat is supplied to one end, the molecules at the hot end start to vibrate more vigorously.

• In the process, they ‘bump’ into their neighboring molecules. In doing so, some energy is transferred to the neighbour.

• The neighbour molecule gains energy and starts to vibrate more vigorously. The cycle continues.

FREE ELECTRON DIFFUSION

• This form of conduction takes place only in metals. As only metals have free electrons.

• The electrons are freed from the molecule when heated and they travel towards the cold end.

• At the cold end they collide into a molecule therefore passing all their energy to the molecule.

METHODS OF CONDUCTIONMolecular vibration Free electron diffusion

Occurs in all solids

Slow process

Occurs in metals only

Fast process

This explains why metals heat up faster:

1. Metals have 2 mechanisms of conduction occuring at the same time.

2. In metals, free electron diffusion is the main mechanism, which is faster.

CONDUCTORS AND INSULATORS• Materials that can conduct heat easily

and readily (eg. Metals) are known as conductors.

• Materials that do not conduct heat easily (eg. Water, air, plastic) are known as insulators.

CONVECTION• Transferring heat by a circulating path of fluid

particles.

• Occurs in liquids and gases

• Does not occur in solids because the molecules are not free to move around

CONVECTION PROCESSTaking the example of heating water• Water at the bottom is heated first• Heated water expands• When water expands density decreases• Heated water of lower density starts to rise• Cooler water of higher density rushes in from sides

to take its place• The cooler water gets heated and the cycle repeats.• Convection currents are set up.

RADIATION• Energy is transferred by means of electromagnetic

waves.

• Radiation does not require a medium to transfer heat. (can occur in a vacuum)

• Sun releases electromagnetic waves (heat is contained in the waves as infra-red)

• Hotter objects radiates more heat.

EMITTERS AND ABSORBERS• The Sun gives out the heat.–It is known as an emitter / radiator

• The Earth takes in the heat.–It is known as an absorber.

CONSERVATION OF ENERGY

ENERGY TRANSFORMATION