Energy transfer by heating

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Infrared Radiation

Infrared Radiation is an EM wave and = thermal radiation

All objects emit and absorb thermal radiation

The hotter an object is, the more IR energy it emits + faster the energy transfer to its surroundings

Best              Material                          Examples

Absorbers      Black, Matt surfaces        Solar furnaces, pans

Emitters         Black, Matt surfaces        Wires

Reflectors       Light, Shiny surface         Inside of a vaccum flask, tin foil

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Solids, Liquids and Gases

              Shape         Volume    Bonds     Density     Arrangement   Energy  Movement

Solids     Fixed          Fixed       Strong     High             Regular         Low        Vibrate    

Liquids    Fills the      Fixed      Quite        High             Random       Some      Slide                        bottom of                   strong                                                          randomly                    container

Gases   Fills up the   Changeable   Weak  Low              Random        High      Fast,                            whole                                                                                         random                        container

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Conduction

Positive metal ions are held in a sea of electrons. When thermal energy is applied, the free electrons attain the energy and diffuse at high speeds, colliding with metal particles and other electrons and tranferring the energy through the metal. 

The particles also vibrate more violently and collide with neighbouring particles to transfer the energy as well, like a non metal, but the transfer of energy through metals is faster and more effective because of the free electrons that are free to move and transfer the energy through the metal.

Free electrons = metal = good conductor & No free electrons = non metal = poor conductor

It's all about the TRANSFER of ENERGY to other, cooler particles in the solid.

Conduction happens best in solids because the particles are tightly packed > more collisions > more energy tranferred due to more frequent collisions.

Gases are AWFUL conductors, if they just gain energy then they just whizz around faster, the liklihood of a collision is much smaller > little energy tranferred > little conduction.

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Convection

Convection only happens in FLUIDS (liquids or gases)

Water/gases conduct heat. 

When this happens, the move around more and become less dense

Less dense fluids always rise on top of denser fluids, like oil on water (so oil is less dense than water). Therefore, the less dense, heated part of the fliud rises above the cooler part of the fluid which fill up the space at the bottom where the warmer fluid left.

The warmer fluid cools and falls as it becomes more dense and fills up the space of the once-cooler fluid that got heates at the bottoms and became less dnese and rose.

A viciously confusing cycle at first, but easy to get your head around with a good diagram :)

Examples: Sea breezes, hot water boilers.

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Evaporation

Evaporation is a natural process of when a liquid turns into a gas much below its boiling point.

To evapoate, the particle must:

  • Be near enough the surface to escape
  • Be moving fast enough (have enough energy) to overcome the bonds it has between it and the liquid particles
  • Be moving in the right direction, preferrably towards the surface and not towards the bottom of the container

Factors effecting the rate of evaporation:

  • Surface area of the liquid (more water in contact with the air > more particles near enough to the surface)
  • Temperture (higher average energy > more particles have enough energy to overcome bonds)
  • Density (lower density = weaker forces to overcome between liquid particles)
  • Air flow (draught of air replaces air above surface> lower air concentration> faster rate)
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The Cooling Effect

When a particle leaves a liquid from evaporation, it takes it the high kinetic energy it used to escape the liquid with it. This decreases the average energy of the liquid. So evaporation causes this cooling effect.

If something that easily evaporates is sprayed on your skin, it attains the energy in your skin and uses it to evaporate, taking the heat energy with it. This leaves your skin cold. These kinds of sprays are often used before an injection, to numb the skin and reduce pain.

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Condensation

When gas particles lose energy through transferring it to a cooler surface, causing bonds to strengthen between the particles and a liquid to be formed

Factors effecting the rate of condensation:

  • Temperture of the gas. Lower temperature = lower energy > likilhood of forces of attraction between the particles strengthening to form a liquid is greater
  • Temperature of the surface. Lower temperature = More energy from the particle is transferred to the surface > lower energy in particle > likilhood of forces of attraction between the particles strengthening to form a liquid is greater
  • Airflow. Lower airflow = Concentration of air will be greater > more particles to clump together to form a liquid due to greater forces of attraction
  • Density. Higher density = More forces of attraction between particles> more particles clump together to form a liquid/ greater liklihood
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Heat transfer

The transfer of thermal energy from one object to another

The larger the difference between the temperatures of an object and its surroundings, the faster the rate of energy transfer

E.g. a cup of REALLY hot coffee is going to lose its heat faster than a mildly warm cup of coffee

Heat transfer is affected by three factors:

Surface area and volume: larger surface area/ small volume = more of the object is in contact with its surroundings > faster rate of energy transfer.

The surface the object is in contact with: e.g. patato chips and tin foil. Tin foil reflects heat back into the patato but can also conduct the heat

The material of which the object is made from: e.g. special bricks, once heated, lose their heat very slowly (reference to SHC). A normal brick may lose or attain heat faster or slower depending on the material it's made from, assuming size, shape and mass are the same.

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Specific Heat Capacity (SHC)

The measure of the amount of energy (J) needed to heat 1kg of a substance by 1°C. So how easily it heats up once energy is applied to it, assuming that the mass is controlled.

The mass has to be 'controlled' as the greater the mass of the substance, the slower it takes for the object to heat up.

So factors that affect the rate of temperature change are:

  • Energy supplied
  • Mass
  • SHC

This can be put into an equation:

E = mcƟ

Energy (J) = Mass (kg) x SHC (J/Kg°C) x Temperature CHANGE (°C) 

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Heating and Insulating Buildings

Ways to reduce energy transfer:

  • Loft insluation: conduction
  • Cavity wall insulation: convection
  • Alunium Foil: radiation and conduction
  • Double Glazing: conduction and convection

U Values are the measure energy per second that passes through 1m² of a material (when the tempertaure difference across it is 1°C) . Kind of like SHC.

The lower the U Value, the more effective the material is as an insulator.

Payback time is the time it takes for you to save up as much money as it took you to pay for an energy saving measure (e.g. double glazing). = cost of measure /  annual savings

An energy saving measure can be:

  • Effective = good at insulating/ keeping the heat in
  • Cost effective = Low pay back time
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