Heat Transfer

Heat Transfer

1. INFRARED RADATION

   • Emitted from the surface of an object
   • The hotter and object is the more radiation it radiates in a given time.
   • An object thats hotter than its surroundings emits more radiation than it absorbs(when cooling)
   • An object that's cooler than its surroundings absorbs more radiation then it emits(when it warms up)
   • Dark, matt surfaces absorb infrared radiation better than light, shiny surfaces.
   • Shiny surfaces reflect infrared radiation. e.g. vacuum flasks have a silver inner surface to keep heat in when storing hot liquid to keep it warm.

2. KINETIC THEORY

   • The three states of matter are solids, liquids and gases.
   • LIQUIDS
     • Weak forces of attraction between particles
     • Particles close together but can move past each other, form irregular arrangements
     • More energy than solid particles, move in random directions at a slow speed.
   • SOLIDS
     • Strong forces of attraction hold particles close together in a fixed, regular arrangement
     • Don't have much energy so can only vibrate in their fixed positions.
   • GASES
     • Almost no forces of attraction between particles
     • Particles have more energy than solids and liquids and are free to move
     • Travel in random direction at high speeds

3. CONDUCTION

   • Conduction of heat energy is the process where vibrating particles pass on their extra kinetic energy to neighbouring particles
   • Usually faster in denser solids because particles are closer together .
   • Materials with large spaces between their particles conduct heat energy much slower - these are called insulators.
   • METALS
     • Electrons are free to move inside metals
     • Electrons move faster at the hot end
     • The atoms collide into other atoms passing the energy on

4. CONVECTION

   • Convection occurs when the more energetic particles move from the hotter region to the cooler region - and ttake their heat energy with them
   • HOW IMMERSION HEATERS WORK:
     • 1. Heat energy is transferred from the heater coils to the water by conduction
       • 2. The particles near the coils get more energy, so they start moving around faster
         • 3. This means there's more distance between them,
           • 4. This reduction i density means that the hotter water tends to rise above the denser, cooler water
             • 5. As the hotter water rises it displaces (moves) the colder water out of the way, making it sink towards the heater coils
               • 6. The cold water is then heated by the coils and rises.
   • Convection currents are all about change in density

5. CONDENSATION

   • When a gas cools, the particles slow down and lose kinetic energy, the attractive forces then pull them close together. If the temperature gets cold enough the gas particles will get close enough and condensation will take place. Water vapour in the air condenses when it comes into contact with a cold surface.
   • The rate of condensation will increase if:
     • Temperature of the gas is low
     • Temperature of the surface the gas touches is lower
     • Density is higher
     • Airflow is less

6. EVAPORATION

   • When particles escape from liquid
   • Particles evaporate at temperatures much lower than the liquids boiling poit
   • The fastest particles are most likely to evaporate from the liquid
   • The rate of evaporation will be faster if:
     • Temperature is higher
     • Density is lower
     • Surface area is larger
     • Airflow over the liquid is greater

7. NINE TYPES OF ENERGY

   • Electrical - whenever a current flows
   • Light - from the sun, light bulbs
   • Sound - loudspeaker or anything noisty
   • Kinetic - anything that moves has it
   • Nuclear - released only from nuclear reactions
   • Thermal - flows from hot objects to cold ones
   • Gravitational Potential - possessed by anything which can fall
   • Elastic Potential - stretched springs, elastic
   • Chemical - possessed by fuels, foods