Heat Transfer and Kinetic Theory

Heat Transfer and Kinetic Theory

1. Infrared Radiation

   • Infrared radiation transfers heat energy
   • Dark, matt surfaces are best at absorbing and emitting infrared radiation
   • Light, shiny surfaces are worst at absorbing and emitting infrared radiation and reflect a lot of the radiation
   • If an object is cooler than its surroundings it will absorb more infrared radiation than it emits
   • If an object is warmer than its surroundings it will emit more infrared radiation than it absorbs
   • Everything absorbs and emits infrared radiation

2. Kinetic Theory

   • Gases
     • Almost no forces of attraction
     • Particles move in a random direction at high speeds
     • High energy levels
   • Solids
     • Strong forces of attraction keep it in a fixed pattern
     • Low amount of energy so it can only vibrate in fixed positions
   • Liquids
     • Weak forces of attraction
     • The particles can move in random directions at low speeds

3. Conduction

   • A type of heat transfer that occurs mainly in solids but also in liquids and gases
   • The particles vibrate and transfer their heat energy to neighbouring particles
   • More dense materials are better conductors because their particles are closer together
   • Materials that have particles that are far apart transfer heat via conduction slower and are known as insulators
   • Metals are good conductors because they contain free electrons which allows them to transfer energy faster

4. Convection

   • Occurs mainly in liquids and gases
   • Particles vibrate more as they heat up and so take up less space and become less dense
   • Less dense particles rise taking the energy with them
   • Convection currents occur when the less dense particles rise and are replaced by the denser particles

5. Condensation

   • Condensation is the process of turning a gas into a liquid
   • If particles lose energy they slow down and the attractive forces between them grow stronger and if the forces are strong enough the particles join and become a liquid
   • Factors affecting the rate of condensation
     • Temperature of the surface the gas touches
     • Airflow
     • Density of gas
     • Temperature of the gas

6. Evaporation

   • Evaporation is the process of a liquid turning into a gas
   • If the particles have enough energy and are travelling in the right direction they can escape the attractive forces of the other particles and evaporate
   • Factors affecting the rate of evaporation
     • Airflow over the liquid
     • Surface area of the liquid
     • Temperature of the liquid
     • Density of the liquid

7. Rate of Heat Transfer

   • Factors affecting rate of heat transfer
     • Surface Area
     • Type of material
     • Colour
     • Surface area to volume ratio
       • The smaller this is, the faster the transfer
   • Controlling Heat Transfer
     • Vacuum Flask
       • The flask uses two walls to create a vacuum to stop heat loss through conduction and convection
       • The flask walls are silver and shiny to stop heat loss through radiation
       • The insulated support stops the flask walls from cracking under the pressure and minimises heat loss from conduction
       • The stopper is made of plastic and filled with cork and foam to reduce heat loss through conduction
     • Humans and Animals
       • Hairs on your skin stand up to trap a thicker layer of insulating air to limit heat transfer by convection
       • Blood is diverted to the surface where heat can be lost as radiation resulting in pinker skin tone
       • Arctic foxes have small ears to minimise heat loss via radiation
       • Desert foxes have large ears to maximise heat loss via radiation

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