GCSE Physics - Heating & Cooling (GCSE AQA P1.1)

Nota:

• http://www.bbc.co.uk/schools/gcsebitesize/science/aqa/heatingandcooling/

1. Infrared Radiation
   1. All objects emit and absorb infrared radiation. The hotter the object is, the more IR it emits
   2. The hotter an object is the more infrared radiation it radiates in a given time
   3. Dark, matt surfaces are good absorbers and good emitters of infrared radiation
   4. Light, shiny surfaces are poor absorbers and poor emitters of infrared radiation
   5. Light, shiny surfaces are good reflectors of infrared radiation
2. Kinetic Theory
   1. The particles of solids, liquids and gases have different amounts of energy.
   2. SOLIDS
      1. PARTICLE ARRANGEMENT: close together + regular pattern
      2. PARTICLE MOVEMENT: vibrate about a fixed position
      3. They have a fixed shape and cannot flow because the particles cannot move from place to place
      4. They cannot be compressed or squashed because the particles are close together and have no space to move into
   3. LIQUIDS
      1. PARTICLE ARRANGEMENT: close together + random
      2. PARTICLE MOVEMENT: move around each other
      3. They flow and take the shape of their container because the particles can move around each other
      4. They cannot be compressed or squashed because the particles are close together and have no space to move into
   4. GASES
      1. PARTICLE ARRANGEMENT: far apart + random
      2. PARTICLE MOVEMENT: move quickly in any direction
      3. They flow and completely fill their container because the particles can move quickly in all directions
      4. They can be compressed or squashed because the particles are far apart and have space to move into
3. Energy Transfer by Heating
   1. U-values
      1. U-values measure how effective a material is as an insulator
      2. The lower the U-value, the better the material is as an insulator
   2. Solar Panels
      1. Solar panels may contain water that is heated by radiation from the Sun. This water may then be used to heat buildings or provide domestic hot water
      2. Solar panels do not generate electricity, but rather they heat up water
      3. They are often located on the roofs of buildings where they can receive heat energy from the sun
      4. How they work
         1. 1. cold water is pumped up to the solar panel where it heats up and is transferred to a storage tank
         2. 2. a pump pushes cold water from the storage tank through pipes in the solar panel
         3. 3. the water is heated by heat energy from the sun and returns to the tank
         4. 4. in some systems, a conventional boiler may be used to increase the temperature of the water
      5. Advantages
         1. solar energy is a renewable energy resource
         2. no harmful polluting gases are produced
      6. Disadvantages
         1. solar panels may only produce very hot water in very sunny climates, and in cooler areas may need to be supplemented with a conventional boiler
         2. although warm water can be produced even on cloudy days, solar panels do not work at night
   3. Specific Heat Capacity
      1. The specific heat capacity of a substance is the amount of energy required to change the temperature of one kilogram of the substance by one degree Celsius
         1. E = m × c × θ
            1. E is energy transferred in joules, J
            2. m is mass in kilograms, kg
            3. c is specific heat capacity in J / kg °C
            4. θ is temperature change in degrees Celsius, °C
      2. Temperature and heat are not the same thing:
         1. temperature is a measure of how hot something is
         2. heat is a measure of the thermal energy contained in an object
      3. Temperature is measured in °C, and heat is measured in J. When heat energy is transferred to an object, its temperature increase depends upon the:
         1. the mass of the object
         2. the substance the object is made from
         3. the amount energy transferred to the object
      4. For a particular object, the more heat energy transferred to it, the greater its temperature increase
      5. The specific heat capacity of a substance is the amount of energy needed to change the temperature of 1 kg of the substance by 1°C
      6. Different substances have different specific heat capacities
         1. WATER = 4181 J/kg °C
            1. water has a particularly high specific heat capacity. This makes water useful for storing heat energy, and for transporting it around the home using central heating pipes
         2. OXYGEN = 918 J/kg °C
         3. LEAD = 128 J/kg °C
4. Heating & Insulating Buildings
   1. The transfer of energy by conduction, convection, evaporation and condensation involves particles, and how this transfer takes place
   2. The factors that affect the rate of evaporation and condensation
   3. The rate at which an object transfers energy by heating depends on:
      1. surface area and volume
      2. the material from which the object is made
      3. the nature of the surface with which the object is in contact
   4. The bigger the temperature difference between an object and its surroundings, the faster the rate at which energy is transferred by heating
   5. Convection
      1. Liquids and gases are fluids
         1. The particles in these fluids can move from place to place
      2. CONVECTION: the transfer of heat energy through a moving liquid or gas
      3. Convection occurs when particles with a lot of heat energy in a liquid or gas move and take the place of particles with less heat energy
      4. Heat energy is transferred from hot places to cooler places by convection
      5. Liquids and gases expand when they are heated. This is because the particles in liquids and gases move faster when they are heated than they do when they are cold. As a result, the particles take up more volume. This is because the gap between particles widens, while the particles themselves stay the same size
      6. The liquid or gas in hot areas is less dense than the liquid or gas in cold areas, so it rises into the cold areas. The denser cold liquid or gas falls into the warm areas. In this way, convection currents that transfer heat from place to place are set up
   6. Conduction
      1. CONDUCTION: the transfer of heat energy through a material - without the material itself moving
      2. Metals are good conductors of heat
      3. Non-metals and gases are poor conductors of heat
      4. Heat energy is conducted from the hot end of an object to the cold end
      5. Poor conductor = Good insulator
      6. Heat conduction in metals
         1. The electrons in a piece of metal can leave their atoms and move about in the metal as free electrons. The left over parts of the metal atoms become charged metal ions. The ions are packed closely together and they vibrate continually. The hotter the metal, the more kinetic energy these vibrations have. This kinetic energy is transferred from hot parts of the metal to cooler parts by the free electrons. These move through the structure of the metal, colliding with ions as they go
   7. Evaporation & Condensation
      1. Evaporation
         1. The particles in a liquid have different energies. Some will have enough energy to escape from the liquid and become a gas. The remaining particles in the liquid have a lower average kinetic energy than before, so the liquid cools down as evaporation happens.
            1. This is why sweating cools you down. The sweat absorbs energy from your skin so that it can continue to evaporate
      2. Condensation
         1. The particles in a gas have different energies. Some may not have enough energy to remain as separate particles, particularly if the gas is cooled down. They come close together and bonds form between them. Energy is released when this happens
            1. This is why steam touching your skin can cause scalds: not only is the steam hot, but energy is released into your skin as the steam condenses
      3. Factors affecting the rate of condensation and evaporation
         1. The rate of condensation increases if the temperature of the gas is decreased. On the other hand, the rate of evaporation increases if the temperature of the liquid is increased. It is also increased if
            1. the surface area of the liquid is increased
            2. air is moving over the surface of the liquid
      4. changes of state
         1. evaporation = liquid --> gas
            1. the reason why damp clothes dry on a washing line
         2. condensation = gas --> liquid
            1. the reason why windows become foggy on a cold day
5. Keeping Warm & Cool
   1. The bigger the difference in temperature between an object and its surroundings, the greater the rate at which heat energy is transferred. Other factors also affect the rate at which an object transfers energy by heating. These include the:
      1. surface area and volume of the object
      2. material used to make the object
      3. nature of the surface that the object is touching
   2. Animal adaptations
      1. Small animals like mice have a large surface area compared to their volume. They lose heat to their surroundings very quickly and must eat a lot of food to replace the energy lost. Large animals like elephants have a different problem. They have a small surface area compared to their volume. They lose heat to their surroundings more slowly and may even have difficulty avoiding overheating
      2. Elephants have large ears with a large surface area compared to their volume. These allow heat to be transferred from the elephant to its surroundings, helping to keep the animal cool
      3. In general, similar animals have different ear sizes depending on the climate in which they live. The arctic fox has much smaller ears than the fennec fox, which lives in the desert. The arctic fox must conserve its heat energy in the cold climate, while the fennec fox must avoid overheating in the hot climate
   3. Engineering design
      1. Engineers design heat transfer devices so that they gain or lose heat energy efficiently. For example, car radiators are flat, with many small fins to provide a large surface area. Similarly, household radiators are thin and flat, and may have fins so that heat energy is transferred to the room quickly