Heat

Heat

1. Measurement

   • Instruments
     • Temperature Senor
       • Connected to computer and measures the temperature and display the results on the computer screen
     • Thermometer
   • Units
     • Common unit: Degrees Celcius
     • S.I unit: Kelvin (K)
     • Conversion: 1 Kelvin = 273 Degrees Celcius

2. Conduction, Convection and Radiation

   • Conduction
     • Movement of heat through a material without the obvious movement of the material itself
     • The metal items closest to the heat source gain heat energy and vibrate faster. These atoms hit neighbouring atoms and transfer some energy to them causing them to vibrate faster too. This process the repeated from atom to atom along the (metal) from the hot end to the cool end
     • Thermal Conductivity is the ability of a material to _______ heat. Heat transfer by conduction will occur at a higher rate (become faster) across materials of high thermal conductivity.
     • Difference between metals and non-metals
       • In metals, electrons are free to move. During heating, other than atoms, heat electrons carry the energy and vibrate. Heat electrons travel very short distances but are very fast-moving. Thus, conduction happens very quickly in metals.
       • Liquids and gases are poor conductors of heat because the atoms are further apart from one another. Substances which have air within are poor conductors of heat.
     • Conduction VS Convection
       • -conduction mainly occurs in solids and convection mainly occurs in gases and liquids
         • Conduction- heat tranfers in all directions, does not involve solid moving Convection- heat flows upwards, involves liquid/gas moving freely and carries energy from one place to another
   • Convection
     • Comvection is the transfer of heat from one place to another by the movement of the medium itself.
       • The moving water forms a current called a convection which carries heat around the beaker.
         • Involves the movement of water from hot regions to cooler regions.
           • A convection current (caused by differences in density) is set up
   • Radiation
     • The transfer of heat energy denies the need for a medium (eg solid, liquid,gas)
     • Can take place in space
     • Travels in the form of infrared waves
     • Variables
       • Surface Area
         • The greater the surface area, the faster the rate of heat gained or lost.
       • Temperature of the body
         • The higher the temperature difference between body and its surrounding, the faster the rate of heat gained or lost.
       • Colour and Texture of Surface
         • Dull, black surfaces radiate more heat than shiny, white ones.
     • Radiation and Absorption of Heat
       • All objects can absorb radiation. When radiation is absorbed it changes into heat
       • Black, dull or rough surfaces are a good emitter/radiation of heat. White, shiny or smooth surfaces are por radiators of heat.
         • e.g. White and shiny inner surfaces reduce the amount of heat lost from a container by reflecting heat from the inner walls back. Thus, the energy remains in the container.
       • Black, dull or rough surfaces are good absorbers of heat. Whte, shiny or smooth surfaces are poor absorbers of heat.

3. A form of energy

   • Same temperature but difference in volume, the larger volume on has more heat energy

4. Expansion and Contraction

   • Expansion
     • When solids, gases and liquids gain heat, they expand and is less dense
       • Thus warm water, is less dense than cooler water and rises. As warm water rises, it is replaced by cooler, denser water from above.
   • Contraction
     • When solids, gases and liquids lose heat, they contract and is denser

5. Global Warmiing

   • Reefers to the increase in temperature of the atmosphere globally
   • Caused mainly by the greenhouse gases which trap heat and re-emits it towards the Earth's surface

6. Greenhouse effect

   • When solar radiation passes through the clear atmosphere, some solar radiation is reflected by the earth and he atmosphere. However, most radiation is absorbed by the earth's surface and warms it.
   • When infrared radiation is emitted from the earth's surface, some of the infrared radiation passes through the atmosphere, and some is absorbed and re-emitted in all directions by greenhouse gas molecules. The effect of this is to warm the earths surface and the lower atmosphere.