CHAPTER 8 : TRANSFER OF THERMAL ENERGY

thermal energy

- a form of energy - it is the total amount of kinetic energy of all particles in the substance. - it flow from higher temperature to lower temperature. - si unit is joule (J)

transfer of thermal energy

Heat is always transferred from a place of high temperature to low temperature. The greater the temperature difference between 2 objects, the greater the rate of heat flow.

mechanism of convection

- when the fluid is heated, the fluid expands and its density decreases. The hot fluid rises up and displaced by the colder (and denser) fluid. - the colder fluid is then heated up. - this fluid movement is called convection current. application : placing the air conditioner near the ceiling.

factors affecting the rate of heat transfer by radiation

1. colour and texture of surface - black, matt surface are good absorber and radiator and emitter of heat. - white, shiny surface are poor absorber and emitter of heat as most radiation is reflected. 2. surface temperature - the hotter the object, the more energy it radiates. 3. surface area - the greater the surface area, the more energy it radiates and absorbs.

temperature

A measure of how hot (or cold) something is; specifically, a measure of the average kinetic energy of the particles in an object. si unit : kelvin (K) other units : degree celsius, degree fahrenheit

convection

in convection, heat is transferred by a fluid due to movement of the fluid itself. The fluid expands on heating so its density falls. The warmer and lighter fluid rises to the cooler region, transferring heat in the process.

radiation

in radiation, heat is transferred by infra-red radiation, which is a part of the electromagnetic spectrum. Most solids and liquids absorb infra-red radiation, including water which is transparent to light.

vacuum flask

reduces heat transfer in and out through conduction, convection, and radiation. - conduction : the vacuum between double glass walls prevent any heat transfer by conduction (no particles). It also traps air which is a poor conductor of heat. - convection : the stopper prevents heat lost due to convection. - radiation : the silvery glass walls minimise heat transfer by radiation by reflecting radiation.

conduction

thermal conduction is the transfer of thermal energy from one place to another without the movement of the medium itself.

thermal equilibrium

when two objects are in thermal contact are in the same temperature, they are in thermal equilibrium. the rate of heat flow between 2 objects are equal.