THERMAL ENERGY AND HEAT SPH4C April 2010

THERMAL ENERGY James Prescott Joule ( ) spent much of his honeymoon studying waterfalls. He noticed that the water at the bottom of 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 CONDUCTION Molecular vibrationFree 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 PROCESS Taking 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 Law of Conservation of Energy When energy changes from one form to another, no energy is created or destroyed. In ideal situations, no energy is lost to friction. In real situations, some energy is needed to overcome friction. This results in the production of waste thermal energy and, sometimes, sound energy.

ENERGY TRANSFORMATION