P1 Energy and Heat Transfer

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  • Energy and Heat transfer
    • Heat radiation
      • Heat energy flows away from a hotter object to its cooler surroundings
        • Heat is transferred in three different ways - radiation, conduction or convection
          • Heat radiation is the transfer of heat energy by infrared radiation
            • Infrared radiation can be emitted by solids, liquids and gases
              • Any object can both emit and absorb infrared radiation, whether or not conduction and convection are also taking place
                • Heat radiation is the transfer of heat energy by infrared radiation
                  • Infrared radiation can be emitted by solids, liquids and gases
                    • Any object can both emit and absorb infrared radiation, whether or not conduction and convection are also taking place
                    • All objects are continually emitting and absorbing infrared radiation
                      • Infrared radiation is emitted from the surface of an object
                        • An object that's hotter than its surroundings emits more radiation than it absorbs (as it cools down)
                        • An object that's cooler than its surroundings absorbs more radiation than it emits (as it warms up)
                        • Radiation depends on surface colour and texture
                          • Dark, matt surfaces absorb infrared radiation falling on them much better than light, shiny surfaces, such as gloss white or silver
                            • They also emit much more infrared radiation (at any given temperature)
                          • Light, shiny surfaces reflect a lot of the infrared radiation falling on them
                          • Solar hot water panels contain water pipes under a black surface
                            • Radiation from the Sun is absorbed by the black surface to heat the water in the pipes
                              • This water can be used for washing or pumped to radiators to heat the building
                      • The hotter an object is, the more radiation it radiates in a given time
              • All objects are continually emitting and absorbing infrared radiation
                • Infrared radiation is emitted from the surface of an object
                  • An object that's hotter than its surroundings emits more radiation than it absorbs (as it cools down)
                  • An object that's cooler than its surroundings absorbs more radiation than it emits (as it warms up)
                  • Radiation depends on surface colour and texture
                    • Dark, matt surfaces absorb infrared radiation falling on them much better than light, shiny surfaces, such as gloss white or silver
                      • They also emit much more infrared radiation (at any given temperature)
                    • Light, shiny surfaces reflect a lot of the infrared radiation falling on them
                    • Solar hot water panels contain water pipes under a black surface
                      • Radiation from the Sun is absorbed by the black surface to heat the water in the pipes
                        • This water can be used for washing or pumped to radiators to heat the building
                • The hotter an object is, the more radiation it radiates in a given time
            • Conduction and convection involve the transfer of energy by particles
              • Conduction is the main form of heat transfer in solids
                • Conduction of heat energy is the process where vibrating particles pass on their extra kinetic energy to neighbouring particles
                  • This process continues throughout the solid and gradually some of the extra kinetic energy (or heat) is passed all the way through the solid, causing a rise in temperature at the other side of the solid
                  • Usually conduction is faster in denser solids, because the particles are closer together and so will collide more often and pass energy between them
                  • Materials that have larger spaces between their particles conduct heat energy much more slowly - these materials are insulators
                  • Metals are good conductors because of their free electrons
                    • At the hot end the electrons move faster and collide with other free electrons, transferring energy. These other electrons then pass on their extra energy to other electrons, etc.
                    • Because the electrons can more freely, this is obviously a much faster way of transferring the energy through the metal than slowly passing it between jostling neighbouring atoms
                  • Conduction is more efficient through a short, fat rod than through a long, thin rod. It all comes down to how far the electrons have to transfer the energy
              • Convection is the main form of heat transfer in liquids and gases
                • Convection occurs when the more energetic particles move from the hotter region to the cooler region - and take their heat energy with them
                  • This is how immersion heaters in kettles and hot water tanks and convection heaters work
                    • Immersion heaters
                      • Heat energy is transferred from the heater coils to the water by conduction
                        • The particles near the coils get more energy, so they start moving around faster
                          • This means there's more distance between them, i.e. the water expands and becomes less dense
                            • This reduction in density means that the hotter water tends to rise above the denser, cooler water
                              • As the hot water rises it displaces the colder water out of the way, making it sink towards the heater coils
                                • This cold water is then heated by the coils and rises - and so it goes on
                                  • You end up with convection currents going up, round and down, circulating the heat energy through the water
                  • Convection currents are all about changes in density
                • Convection can't happen in solids because the particles can't move
                  • Convection occurs when the more energetic particles move from the hotter region to the cooler region - and take their heat energy with them
                    • This is how immersion heaters in kettles and hot water tanks and convection heaters work
                      • Immersion heaters
                        • Heat energy is transferred from the heater coils to the water by conduction
                          • The particles near the coils get more energy, so they start moving around faster
                            • This means there's more distance between them, i.e. the water expands and becomes less dense
                              • This reduction in density means that the hotter water tends to rise above the denser, cooler water
                                • As the hot water rises it displaces the colder water out of the way, making it sink towards the heater coils
                                  • This cold water is then heated by the coils and rises - and so it goes on
                                    • You end up with convection currents going up, round and down, circulating the heat energy through the water
                    • Convection currents are all about changes in density
      • Condensation and Evaporation
        • Condensation is when gas turns to liquid
          • When a gas cools, the particles in the gas slow down and lose kinetic energy
            • The attractive forces between the particles pull them closer together
              • If the temperature gets cold enough and the gas particles get close enough together that condensation can take place, the gas becomes a liquid
          • Water vapour in the air condenses when it comes into contact with cold surfaces, e.g. drinks glasses
          • The steam you see rising from a boiling kettle is actually invisible water vapour condensing to form tiny water droplets as it spreads into cooler air
          • The rate of condensation will be faster if the:
            • Temperature of the gas is lower
              • The average particle energy in the gas is lower - so more particles will slow down enough to clump together and form liquid droplets
            • Airflow is less
              • The concentration of the substance in the air will be higher, and so the rate of condensation will be greater
            • Density is higher
              • The forces between the particles will be stronger
                • Fewer particles will have enough energy to overcome these forces and will instead clump together and form a liquid
            • Temperature of the surface the gas touches is lower
        • Evaporation is when liquid turns to gas
          • Evaporation is when particles escape from a liquid
            • Particles can evaporate from a liquid at temperatures that are much lower than the liquid's boiling point
            • Particles near the surface of a liquid can escape and become gas particles if:
              • The particles are travelling in the right direction to escape the liquid
              • The particles are travelling fast enough (they have enough kinetic energy) to overcome the attractive forces of the other particles in the liquid
          • The rate of evaporation will be faster if the:
            • Temperature is higher
              • At higher temperatures, the average particle energy will be higher, so more particles will have enough energy to escape
            • Density is lower
              • The forces between the particles will usually be weaker, so more particles will have enough energy to overcome these forces and escape the liquid
            • Surface area is larger
              • More particles will be near enough to the surface to escape the liquid
            • Airflow over the liquid is greater
              • The lower the concentration of an evaporating substance in the air it's evaporating into, the higher the rate of evaporation
                • A greater airflow means air above the liquid is replaced more quickly, so the concentration in the air will be lower
                  • Airflow over the liquid is greater
                    • The lower the concentration of an evaporating substance in the air it's evaporating into, the higher the rate of evaporation
                      • A greater airflow means air above the liquid is replaced more quickly, so the concentration in the air will be lower
        • Factors that affect the rate of heat transfer
          • Type of material
            • Objects made from good conductors transfer heat away more quickly than insulating materials
            • If an object is in contact with a conductor, the heat will be conducted away much faster than if it is in contact with an insulator
          • Surface area and volume
            • Heat energy is radiated from the surface of an object
              • The bigger the surface area, the more infrared waves that can be emitted from (or absorbed by) the surface - so the quicker the transfer of heat
                • For example, radiators have large surface areas to maximise the amount of heat they transfer
                • This is why car and motorbike engines often have 'fins' - they increase the surface area so heat is radiated away quicker, meaning the engine cools quicker
                • Heat sinks are devices designed to transfer heat away from objects they're in contact with, for example computer components
                  • They have fins and a large surface area so they can emit heat as quickly as possible
            • If two objects at the same temperature have the same surface area but different volumes, the object with the smaller volume will cool more quickly
              • This is because a higher proportion of the object will be in contact with its surroundings

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