1.1 Heat

1.1 Heat

1. Infrared Radiation

   • Transfers energy by infrared waves (which are electromagnetic).
     • Particles aren't needed
       • Can travel through a vacuum
   • If two objects are at different temps, heat energy will be transferred from the hotter one to the colder one.
     • This also means that objects hotter than their environment transmit energy into it.
   • Surfaces reflect some of the energy that reaches it, and absorb the rest.
     • Some surfaces are better at absorbing and emitting this radiation than others.
       • Matt black>shiny black>dull silver>shiny silver
   • Uses
     • Infrared radiation from the Sun can be used to heat water.
     • Heaters and lamps emitting IR can be used as to heat buildings, and for physiotherapy.
     • Police and fire services use thermal imaging cameras to detect people.
       • Some burglar alarms do the same, called PIRs (Passive Infrared Devices). This is because they just absorb IR, instead of emitting it too.

2. Kinetic Theory

   • The Theory that everything is made up of tiny particles that vibrate, and it is the arrangement of these that determines the properties of the substance.
   • Strength between bonds = solids>liquids>gases
   • Energy per particle = gases>liquids>solids
   • When a solid is heated, some of the particles get more energy, so can break away from it. The solid takes in energy when heated but it's being used to break bonds between particles, so temp stays the same.
     • When evaporation happens, the temperature of the remaining liquid goes down. This is because temperature is an average of the energy of all the particles, and it's the particles with the most energy that can escape the liquid. Therefore, the average energy, and so temperature is less.
   • Scalds- they hurt so much from steam because it's hot. Also, more heat is released when the gas condenses- as the particles come closer together and form stronger bonds.
   • Evaporation happens faster when...
     • temp is higher.
     • there's more surface area from which particles can escape.
     • air is moving over the surface of the liquid
       • This carries the particles away, so they can't condense back into the liquid.
         • If air isn't moving, it becomes saturated- the number of particles evaporating is the same as the number condensing, so the amount of gas in the air doesn't change.

3. Conduction

   • When a solid is heated, the particles gain more energy.
     • They cannot move around, but they vibrate more.
       • These particles bump into nearby particles, which transfers energy to them and so on.
       • If there's a higher temperature, they vibrate faster.
   • Metals conduct heat better because they have free electrons.
     • These electrons are free to move throughout the material, and so transfer energy more quickly.
     • Gases and liquids tend to be poor conductors, because they move around too much and don't pass the energy from one particle to another.
       • Since air is a poor conductor, good insulators have pockets of trapped air inside. For example, polystyrene and bubble wrap.

4. Convection

   • When a fluid is heated, the particles get more energy and bounce off each other more. This makes them spread out, becoming less dense. Therefore, the cooler particles, which are more dense, sink down underneath them. They get to the heater, whilst the other particles cool, so they circulate back again.

5. Rate of Heat Transfer

   • Increase
     • Surface Area- the bigger it is, the more infrared waves can be emitted
       • e.g. "fins" in engines
     • High surface area : volume- more of the material in contact with surroundings
     • Conductor/insulator
   • Decrease
   • Animal and human stuff too

6. Heating Buildings

   • Ways to reduce energy bills.
     • Solar panels to heat water
     • Cavity wall insulation
     • Double/triple glazed windows
     • Loft insulation
     • Draught-proofing
     • Hot water tank jacket
     • Effective methods give you bigger annual savings on heating bills.
     • Cost-effective methods have a shorter pay-back time.
       • Payback time = initial cost / annual saving
   • The lower the U-value, the better the insulator, because it transfers less energy.

7. Specific Heat Capacity

   • E = m * c * temp change
   • Amount of energy needed to increase the temperature of 1kg of a material by 1 degree.
     • Water is used in radiators because it has a high specific heat capacity, and can be pumped around the house.
     • Electric heaters often "store" heat in concrete.

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