P1 - Energy for the Home

Heat
1. Melting and boiling
  1. You need to put in energy to break intermolecular bonds
    1. 1. When you heat a liquid, the heat energy makes the particles move faster. Eventually, when enough of the particles have enough energy to overcome their attraction to each other, big bubbles of gas form in the liquid. this is Boiling.
    2. 2. It's similar when you're heating a solid. Heat energy makes the particles vibrate faster until eventually the forces between them are overcome and the particles start to be moved around. This is melting.
    3. 3. When a substance condenses or freezes, bonds are being formed between the particles.
  2. Specific Latent Heat is the energy needed to change state
    1. 1. The specific latent heat of melting or boiling is the amount of energy needed to melt/boil 1 Kg of material without changing its temperature (the material's got to be at melting temperature already).
    2. 2. Specific Latent heat is different for different materials, and it's different for boiling and melting.
    3. Energy = Mass × Specific Latent Heat
2. Heat Radiation
  1. Radiation is how we get Heat from the sun
    1. Radiation is different from conduction and convection in several ways:
      1. 1. It doesn't need a median (material) to travel through, so it can occur in a vacuum, like space.
      2. 2. It can only occur through transparent substances, like air glass and water.
      3. 3. The amount of radiation emitted or absorbed by and object depends to a large extent on its surface colour and texture. This definitely isn't the case for Conduction and convection.
  2. All objects emit and Absorb Heat Radiation
    1. 1. All objects are continually emitting and absorbing heat radiation. The hotter an object gets, the more radiation it emits.
    2. 2. Cooler objects will absorb the heat radiation emitted by hotter things, do their temperature increases. You can feel heat radiation, for example if you're indoors and the sun shines on you through a window.
    3. 3. Matt black surfaces are very good absorbers and emitters of radiation. Radiators should really be painted black to help emit radiation.
    4. 4. Light coloured, smooth and shiny objects are very poor absorbers and emitters of radiation. They effectively reflect heat radiation.
  3. Heat Radiation is important in cooking
    1. 1. Grills and toasters heat food by infrared radiation. The heat radiated by a grill is absorbed by the surfece particles of the food, increasing their kinetic energy.
    2. 2. People often line their grill pan with foil. This reflects the heat radiation back onto the bottom of the food being grilled, so the food gets cooked more evenly.
    3. 3. Microwave ovens also use radiation to cook food. Microwaves are electromagnetic waves that have a different wavelength to infrared.
    4. 4. Microwave penetrate about 1 cm into the outer layer of food where they're absorbed by water or fat molecules, increasing their kinetic . The energy is then conducted or convected to other parts.
    5. 5. You don't cover food with foil in a microwave oven though, the microwaves will be reflected away so they won't cook the food, and it causes dangerous sparks inside the oven.
3. Moving and storing heat
  1. Heat is a measure of energy
    1. When a substance is heated, its particles gain kinetic energy. This energy makes the particles in a gas or liquid move around faster. in a solid the particles vibrate more rabidly. This eventually causes solids to melt and liquids to boil.
    2. This energy is measured on an absolute scale, this means it can't go lower than 0°C, because there's a limit to how the particles can move. Heat energy is measured in Joules (J).
  2. Temperature is a measure of Hotness
    1. 1. Temperature is a measure of the average kinetic energy of the particles in a substance. The hotter something is, the higher its temperature, and the average KE of its particles.
    2. 2. Temperature is usually measured in °C, but there are other temperature scales, like °F. These aren't absolute scales as they can go below zero.
    3. Energy tends to flow from hot objects to cooler ones. E.g. Warm radiator heat up the cold air in the room.
    4. If there's a Difference in Temperature between two places, then Energy will flow between them.
    5. The greater the difference in temperature, the faster the rate of cooling will be.
  3. Specific heat capacity tells you how much energy something can store
    1. 1. Materials which need to gain a large amount of energy to warm up also release a lot of energy when they cool doen, so they can 'store' a lot of heat.
    2. 2. The measure of how much energy a substance can store is called its Specific heat capacity.
    3. 3. Specific heat capacity is the amount of energy needed to raise the temperature of 1 Kg of substance by 1 °C. Water has a specific heat capacity of 4200 J/Kg/°C.
    4. 4. The specific heat capacity of water is high. Once water's heated, it stores a lot of energy, which makes it good for central heating systems, also as water is a liquid it can be easily pumped around pipes.
    5. Energy = Mass × Specific Heat capacity × Temperature Change
4. Conduction and Convection in the home
  1. Conduction occurs mainly in solids
    1. 1. In a solid, the particles are held tightly together. So when one particle vibrates, it collides with other particles nearby and quickly passes the vibrations on.
    2. 2. Particles which vibrate faster than others pass on their extra Kinetic energy to neighbouring particles. These particles then vibrate faster themselves.
    3. 3. This process continues throughout the solid and gradually the extra kinetic energy (or heat) is spread all the way through the solid. This causes a rise in temperature at the other side.
    4. Conduction of Heat is the process where Vibrating Particles pass on Extra Kinetic Energy to Neighbouring Particles.
    5. 4. Metals conduct heat really well because some of their electrons are free to move inside the metal. Heating makes the electrons faster and collide with other free electrons, trensferring energy.
    6. 5. Most non-metals don't have free electrons, so warm up more slowly, making them good for insulating things. Thats why metals are used for saucepans, but non-metals are used for the handles.
    7. 6. Liquids and gasses conduct heat more slowly than solids. The particles aren't held so tightly together, which prevents them bumping into each other so often. So air is a good insulator.
  2. Convection occurs in Liquids and Gases
    1. 1. When you heat up a liquid or gas, the particles move faster, and the fluid (liquid or gas) expands, becoming less dense.
    2. 2. The warmer, less dense fluid rises above its colder, denser surroundings, like a hot air balloon does.
    3. 3. As the warm fluid rises, cooler fluid takes its place. As this process continues, you end up with a circulation of fluid (convection currents).
    4. Convection occurs when the more energetic particles move from the Hotter region to the Cooler region, and take their heat energy with them.
    5. 4. Radiators in the home rely on convection to make the warm air circulate round the room.
    6. 5. Convection can't happen in solids because the particles can't move, they just vibrate.
    7. 6. To reduce convection, you need to stop the fluid moving. Clothes, blankets and cavity wall foam insulation all work by trapping pockets of air. The air can't move so the heat has to conduct very slowly through the pockets of air, as well as the material in between.
Waves
1. Wave basics
  1. Waves have amplitude, Wavelength and Frequency
    1. 1. The amplitude is the displacement from the rest position to the crest (NOT from peak to trough)
    2. 2. The wavelength is the length of a full cycle of a wave, E.g. from trough to trough.
    3. 3. Frequency is the number of complete cycles or oscillations passing a certain point per second. Frequency is measured in hertz (Hz). 1 Hz is one wave per second.
  2. Wave Speed = Frequency × Wavelength
    1. Wave speed (m/s)
    2. Frequency (Hz)
    3. Wavelength (m)
  3. 1 MHz (1 megahertz) = 1,000,000 Hz
2. Wave properties
  1. All waves can be Reflected, Refracted and Diffrected
    1. 1. Waves travel in a straight line through whatever substance they're travelling in.
    2. 2. When waves arrive at an obstacle, their direction of travel can be changed. This can happen by reflection or by refraction, or diffraction.
  2. Reflection of Light lets us see objects
    1. 1. Reflection of light is what allows us to see objects. Light bounces off them into our eyes.
    2. 2. When a beam of light reflects from an uneven surface such as a piece of paper, the light reflects of at different angles.
    3. 3. When it reflects from an even surface (smooth and shiny like a plain mirror) then its all reflected at the same angle and you get a clear reflection.
    4. 4. The Law of Reflection applies to every reflected ray:
      1. Angle of Incidence = Angle of Reflection
Saving Energy and Efficiency
1. Insulating your house saves Energy and Money
  1. 1. Energy in the home is emitted and transferred (or wasted) in different areas.
  2. 2. Things that emit energy are called sources, E.g. Radiators. Things that transfer and waste or lose energy are called sinks, E.g. Windows and Computers.
  3. 3. To save energy, you can insulate your house so the sinks 'drain' less energy, E.g. Using curtains to reduce energy loss. You can also make sources and sinks more efficient, so they waste less energy. E.g. using Energy saving light bulbs.
  4. 4. It costs money to buy and install insulation, or buy more efficient appliances, but it also saves you money as your energy bills are lower.
  5. 5. Eventually, the money you've saved on energy bills will equal the initial cost, the time it takes is called Payback Time.
  6. 6. If you subtract the annual saving from the initial cost repeatedly then eventually the one with the biggest annual saving must always must always come out as the winner. However this might not always be the case if you sell the house or die.
2. Efficiency
  1. Machines always waste some energy
    1. 1. Useful Machines are only useful because they convert energy from one form to another. For example, in cars, you put in chemical energy (petrol or diesel) and the engine and the engine converts into kinetic energy.
    2. 2. The total energy output is always the same as the energy input, but only some of the output energy is useful. So for every Joule of chemical energy you put into your car, you'll only get a fraction of it converted into useful Kinetic Energy.
    3. 3. This is because some of the input energy is always lost or wasted, often as heat. In a car for example, the rest of the chemical energy is converted (mostly) into heat and sound energy, this is wasted energy.
    4. 4. The less energy that is wasted, the more efficient the device is said to be.
  2. More Efficient Machines waste less Energy
    1. You multiply the answer by 100 to get the percentage value

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P1 - Energy for the Home

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	Created by Rattan Bhorjee about 10 years ago