Physics P1

Physics P1

1. Energy transfer by heating processes

   • Radiation
     • Radiation is the transfer of heat energy by infrared radiation
     • Infrared radiation can be emitted from solid, liquid or gas
     • Rate of infrared transfer can be affected by:
       • Temperature
         • An object hotter than surroundings released more radiation than it absorbs (cools down)
         • Object cooler than surroundings absorbs more radiation than it emits (warms up)
         • The hotter an object, the more radiation it gives off in a given time
       • Surface colour or texture
         • Dark, matte surfaces absorb infrared radiation much better than light, shiny surfaces
         • Dark, matte surfaces emit infrared radiation better than light, shiny surfaces too
         • Light, shiny surfaces reflect a lot of radiation. This makes them ideal to reduce energy transfer by radiation
   • Kinetic theory
     • Solids
       • Strong forces of attraction hold particles together in a fixed, regular arrangement
       • Particles Not much energy so they can only vibrate about their fixed positions
     • Liquids
       • Weaker forces of attraction between particles.
       • Particles can move past each other and form irregular arrangements
       • More energy than particles in a solid - can move in random directions at slow speed
     • Gases
       • Almost no forces of attraction
       • Particles have more energy than solids or liquids - can move and travel in random directions at high speeds
   • Conduction
     • Process of heat transfer through a solid
     • Particles gain energy from heat, vibrate faster and more violently, collide more frequently and with more force with the nearby particles and pass on energy
     • Rate of conduction is faster in denser solids as particles are closer together so will collide more often to pass on energy
     • Materials with relatively large spaces between particles transfer heat much more slowly. These are insulators
     • Metals are extra good at conduction as they have free electrons too that can move inside the metal and transfer energy
   • Convection
     • Transfer of heat in liquids or gases
     • Particles gain energy from heat, vibrate more, gaps become larger, air less dense, rise
   • Condensation
     • When a gas cools, particles slow down and lose energy. The attractive forces pull them closer together r
     • If it gets cold enough and gas particles become close enough together then the gas becomes a liquid - condensation
     • Gas to liquid
   • Evaporation
     • Liquid to gas
     • Particles escape from the liquid and become gas particles
       • If they're travelling in the right direction to escape
       • If they're travelling fast enough (have enough kinetic energy) to escape
     • Because the high energy particles escape, the average energy of the remaining particles is less, so the temperature falls - cooling effect
   • Rate of energy transfer
     • Larger temperature difference between object and surroundings = faster rate
     • Surface area - heat is radiated from the SA so larger means more radiated
     • Surface area to volume ratio - higher proportion SA means faster rate
     • Type of material - conductors better than insulators
   • Vacuum flask
     • Cap to stop rvaporation
     • Vacuum stops conduction and convection because they both need particles - vacuum has none
     • Silvered walls to minimise radiation - it's reflected back
     • Supported by insulating foam - reduces heat loss by conduction - trapped air can't make convection current
     • Plastic case - insulator - minimises conduction
   • Saving energy in home
     • Cavity wall insulation - traps air particles so they can't make convection current, also trapped air is an insulator so limits conduction too
     • Loft insulation - same as cavity wall
     • Draught-proofing - stop cold air blowing in, reduce convection
     • Double glazing - air gap reduces conduction as trapped air insulator
     • Hot water tank jacket - same as loft insulation
     • Effectiveness vs cost - payback time etc
     • U-Values - better insulator = lower U-Value
     • Solar hot water - heat water using sun - black surface absorbs heat and pipes are under it so the water is heated
   • Specific Heat Capacity
     • Amount of energy required to heat 1kg of a substance by 1 degree Celsius
     • Water takes 4200J
     • High specific heat capacity used to store energy

2. Energy and efficiency

   • Energy transfer
     • Types of energy: electric, light, sound, kinetic, nuclear, thermal, gravitational potential, elastic potential, chemical
     • Gravitational and elastic potential are forms of stored energy - it's waiting to happen
     • Conservation of energy - energy cannot be created or destroyed, just transferred from one form to another
     • Devices transfer energy, e.g. A TV transfers electrical into light and souns
     • Waste energy is energy that is transferred into other forms than the desired ones
   • Efficiency
     • Energy supplied to a device is the input energy. Energy is either transferred usefully or wasted
     • Efficiency = useful energy out / energy input
     • Cost effectiveness - newer things will be cheaper to run as they're more efficient, but the cost of buying the new thing could be lots
     • Payback tims
     • We can use waste energy sometimes so it isn't wasted - in car engines wasted heat is reused or used to heat the car on cold days
     • Sankey diagrams visualise efficiency