P11

P11

1. P11.1

   • If an object's surroundings are hotter than it, then energy will be transferred from the hotter object to the colder object until they are the same temperature.
     • This can happen in one of 3 way: Infrared radiation, conduction and convection.
       • Everything above absolute 0 (-273^oC) emits infrared radiation into all directions.
         • As infrared radiation moves from the hot body to the cool body, the hot body loses heat.
           • Infrared is an electromagnetic wave which travels at the speed of light and can pass though a vacuum. It is part of the Electromagnetic Spectrum, and is also how the Earth gets heat and light from the Sun.
             • Dark, black, matt, rough surfaces absorb radiation better than light, white, shiny, bright surfaces, which are better reflectors.
             • Dark, matt surfaces emit radiation and cool down quickly, but light, shiny surfaces emit radiation and cool down slowly.

2. P11.2

   • All particles vibrate. The particles in a solid are arranged in a pattern and are tightly packed. The particles in a liquid are slightly more spread out, but some still touch eachother. However, they do not have a regular pattern. In a gas, the particles are randomly arranged. They have no pattern to them and do not touch.
     • When heated, a solid will melt and become a liquid. Then the liquid will evaporate and become a gas. When a gas is cooled, it will condense and become a liquid, and when this liquid is cooled it will freeze become a solid.
       • Heating causes particles to gain more kinetic energy. As they vibrate more, they begin to move apart and the substance becomes less dense.
         • Placing a lid on a container prevents gas particles from escaping. The air above the liquid becomes saturated. Evaporation does not stop, but is balanced by condensation
       • Cooling causes the particles to lose energy and stick closer together, causing the substance to become denser.

3. P11.3

   • In conduction, the particles closer to the heat source will gain energy and vibrate, passing heat down the object to the other particles.
     • Conduction is the flow of thermal energy through a material. The particles that the material is made of pass the energy along the material
       • Because the energy is transferred by particles, it is not possible for conduction to occur in a vacuum.
         • Conduction works for solids, not liquids or gases, because the particles are close enough together to be able to allow heat to flow to the other particles.
           • Objects which contain trapped air aren't good conductors but are good insulators. Non-metals are also good insulators.
             • Although non-metals are better insulators, they can still conduct. However, because they don't have any free moving electrons they conduct slower than metals.

4. P11.4

   • Liquids and gases can both flow, so they are called fluids.
     • Although things expand when heated. they do no not gain mass. They seem to expand because the particles get further away from eachother.
       • Convection is the transfer of energy by moving particles through a fluid.
         • The movement is called a convection current.
           • Convection happens because the particles nearest to the heat source gather kinetic energy and become less dense, so they move upwards. When they are further away from the heat source they will become colder, lose energy and sink back to the bottom. The whole process will then happen again.

5. P11.5

   • In the winter, you try to reduce the heat loss from your house, and in the summer you try to increase it.
     • Energy is transferred from the hotter object to the cooler object. The greater the temperature difference, the the greater the rate at which heat is transferred.
       • Animals that live in colder climates tend to be larger, because they will have a smaller surface are in comparison to their mass, so there will be less energy transfer to their surroundings.

6. P11.6

   • You can reduce energy bills by installing alternative sources of energy.
     • Hot water pipes need to be insulated to prevent heat transfer by convection.
       • Money is wasted when energy escapes from the house, and damage is caused to the environment when the lost energy is regained,
         • Curtains reduce draughts leaving the house. They are opaque and so don’t allow much radiated heat to pass through them.
           • Most outside walls have a cavity that traps air which is a good insulator. This reduces heat transfer by conduction Foam is pumped into the cavity. This traps the air in pockets and reduces heat transfer by convection.
             • Convection currents carry warm air upwards Heat escapes through the roof Loft insulation is fibreglass that traps pockets of insulating air The loft may get very cold!
               • Two glass panes are twice as good as one glass pane. Trapped air between the panes also helps to reduce heat transfer by conduction. If a vacuum is used, it is even better because convection is now also reduced.
                 • Aluminium foil between the radiator and the wall reflects heat back into the room.
                   • Draughts are convection currents Draught excluders fill the gaps that allow the draughts.
                     • The payback time of an energy-saving solution is a measure of how cost-effective it is.

7. P11.7

   • All materials store heat energy when they are heated When we stop heating, the heat energy slowly leaks away.
     • The higher the final temperature the more heat energy is needed and the more energy is stored.
       • The greater the mass of material heated, the more heat energy is needed and the more energy is stored.
         • The amount of heat needed to heat an object depends on the material. Every material is different – each material has its own Specific Heat Capacity that tells us how much heat is needed or stored.
           • E = mcΔθ
             • E = thermal energy supplied to, or lost by, the body (J), m = the mass of the body (kg), c = specific heat capacity (J kg-1 C-1), Δθ = the temperature change of the body (C).