P2

  • Energy
    • When something happens it is because energy is transferred from one form → another
  • Work
    • Work is done when a force moved
      • Amount of work done depends on: size of force applied + distance it moves
    • Formula: work (J) = force (newton - N) x distance moved in direction of force (m)
      • Joules: work done when a force of 1 newton moves 1 metre
      • J = m x n
    • Work done = energy transferred
    • Ex. Pull crate with force of 50N for 3m in the direction of force
      • Work done = 50N x 3m
      • = 150
      • = 150J
    • Ex. Lift 3kg object into air through 2m. You have to exert a vertically upward force equal to the weight of the object.
      • Work done = 30N x 2m
      • = 60J
    • Force = acceleration x mass
      • Acceleration = always 10 → reason: gravity
  • Forms of energy
    • Chemical energy
      • Stored in food + fossil fuels
        • Energy of food = released by chemical in our bodies → other types of anergy → can do jobs
        • Energy transferred when fossil fuels are burnt in engine
        • EX. batteries = a lot of chemical energy → transferred → electrical energy
      • Potential energy (P.E)(Ep)
        • Def: energy something has because of its position or condition
        • Every body above Earth’s surface has potential energy as gravitational potential energy
        • Ex. stretch + compress spring → need work to be done → energy becomes potential energy in form strain energy
      • Kinetic energy (K.E)(Ek)
        • Any moving body has kinetic energy
        • The faster it moves → more it has
        • Ex. Kinetic energy of hammer transfers into other forms when it hits the nail
      • Electrical energy
        • Produced by energy transferred at power station + batteries
      • Heat energy/ thermal energy
        • Final fate of other forms of energy
        • Transferred by: conduction, convection or radiation
      • Other forms
        • Light energy
        • Electromagnetic radiation
        • Sound energy
        • Nuclear energy
  • Energy transfers
    • Demonstration
      • Battery changes chemical energy → electrical energy → kinetic energy (in electric motor)
      • Motor raises weight = potential energy
      • Weight drop, runing the generator which is connected to a lamp = kinetic energy → electrical energy
      • Light turns on = electrical energy → heat + light energy
    • Other examples
      • Microphone: sound → electrical
      • Loudspeaker: electrical → sound
      • etc.
  • Measuring energy transfers
    • Work (J) = force of 1N moving 1m
    • In energy transfer: work is done
    • Work done is a measure of the amount of energy transferred
    • Ex. Exert upward force of 10N to raise a ston 1.5M high → work done = 15J
      • 15J = also amount of chemical energy from muscles → potential energy of stone
    • Energy + work = J
  • Energy conversion
    • Energy can’t be created nor destroyed; always conserved
    • Electrical + chemical energy = easily transferred
    • Heat energy = hard transfer
    • All energy trasnfers cause heat in surroundings because of work against friction
      • Friction: wasted energy (spread out energy + hard to use)
    • Ex. Brick falls (potential → kinetic)
      • Hits ground: temperature rises → heat + sound are created
    • ‘Lost’ energy in an energy transfer = non-useful energy
    • Efficiency of energy transfers:
      • Def: the percentage of the energy supplied that is usefully transferred
      • Formula: efficiency = useful energy output/ total energy input x 100
      • Devise is useful when: energy input, mainly transfers to useful form + ‘lost’ energy is small
  • Kinetic energy (K.E) (Ek)
    • Def: energy a body has because of its motion
    • Formula: Ek(J) = 1/2 x mass of body(kg) x (velocity of body)^2
      • Ek = 1/2 x m x v^2
    • Ex. How much kinetic energy does a 0.4kg football have that is moving at 20m/s
      • Ek = 1/2mv^2
      • = 1/2 x 0.4 x (20)^2
      • = 1/2 x 0.4 x 400
      • = 80J
  • Potential energy (P.E)(Ep)
    • Def: energy a body has because of it sposition or condition
    • Every body above Earth’s surface has gravitational potential energy
      • Work has to be done against gravity by the force used to raise it
    • To lift a body with mass m through height of h, where Erath’s gravitational field strength is g, you need a force equal and oppsoite to mass x gravitational force
    • Formula: potential energy(J) = mass(kg) x gravitational field strength(N/kg) x height(m)
      • Ep = mgh
  • Power
    • the more power a car has, the faster it accelerates + does work
    • Power of device = work it does per second
    • Power = rate at which it transfers energy from one form to another
    • Power(W) = word done/ time taken = energy transferred/ time taken
    • Watt = word of 1J per second
      • 1W = 1J/s
      • 1000W = Kw
      • 100 000W = Mw
    • Ex. machine does 500J of work in 10s
      • Power = 500/10
      • = 50W
  • Non-renewable energy sources
    • Once used up they can’t be replaced
    • Fossil fuels: main energy source
      • Coal
      • Oil
      • Natural gas
      • Remains of plants + animals that dies milliones of years ago
      • Originally got energy from sun
    • Burning of fossil fuels polutes the atmosphere with CO2 + sulfur dioxide
    • Natural gases cause least CO2
    • Coal = most
    • Burnt coal and oil → sulfir dioxide → acid rain
      • Can be filtered out = expensive
    • Nuclear fuels
      • Energy released in nuclear reactor from uranium → energy
        • Uranium: found as ore in ground
      • Does not emit CO2 or sulfur sioxide, but produces raadioactive waste
        • Hard to store away safely
      • Accident = radiation risk
      • Advantages of non-renewable fuels:
        • High energy density
        • Small energy transfer devise, which releases energy
        • Availability: if demand suddeny increases
  • Renewable energy sources
    • Can’t be exhausted
    • Generally non-polluting
    • Solar:
      • Energy from sun → earth
      • Mostly sunlight
      • Low energy density → need large collecting devices
      • Availability varies
      • Best use: energy source for low-temperature water heating
        • Heat up swimming pools
        • Produce domestic hot water (70°C)
      • Energy transfer devise: solar panels → light → heat energy
      • Source of sun’s energy = nuclear fusion
        • A lot of energy so sun can stay hot
        • Temperature on sun so high that hydrogen → helium
        • Mass is lost + energy is released
    • Wind energy
      • Windmills/ wind turbines
        • 2-3 blades
          • 30m long
          • drive electrical generators
      • Wind farms
        • 20-30 turbines
        • 400m apart
        • supply 400MW in UK
      • Environmental objection to wind farms because they are loud + ugly
    • Wave energy
      • Rise + fall of waves = transferred by wave-energy converter → rotary motion required to drive generator
      • Not likely to be large-scale production, but works for small systems
    • Tidal + hydroelectric
      • Tides: gravitational field of moon exerts pull on oceans
        • Water closest + furthest from moon = water bulges out (high tide)
        • 2 high + 2 low tides a day
      • Movement of water during tides moves turbine = connected to generator
      • Disadvantage: destroys habitat + floods land
    • Geothermal energy
      • Cold water pumped into shaft with hot rocks below Earth’s surface → steam → forced up another shaft
      • Steam tunes turbine = connected to generator
      • Energy that heats rocks = radioactive elements in Earth
    • Biomass
      • Contains:
        • Cultivated crops
        • Crop residue
        • Natural vegetation
        • Trees
        • Animal dung
        • Sewage
      • Creates: biofuels
        • Alcohol (ethanol)
        • Methane gas
      • How:
        • Fermentation with enzymes
        • Decomposition by bacteria when there is no air
      • Liquid biofuels:
        • Replace petrol
        • 50% less energy per litre
        • Lead + sulfur free
        • Cleaner
      • Biogas
        • Methane + CO2
        • Energy: 1/2 of natural gas
        • From animal + human waste in ‘digesters’
  • Power stations
    • process at power station depends on energy source
    • Non-renewable sources
      • In thermal power stations: produce heat energy: water → steam
      • Steam drives turbines → drives generator → electricity
      • Fossil fuels: steam comes from boiler
      • Nuclear fuels (uranium/ plutonium): steam comes from heat exchange
    • Renewable sources
      • Usually: drive turbine directly
      • 85-90% more efficient than thermal power stations
        • Don’t have causes of loss in thermal power stations
        • Costs can be halved
    • Economic, environmental and social issues
      • Consider:
        • Cost
        • benefit
        • clean
        • cheap
        • pollution
      • Renewable energy:
        • No fuel cost
        • Dilute nergy
        • Setting it up is expensive
      • Nuclear
        • Low cost
        • Building station = very expensive + dismantling
      • Reliability
        • Natural gas: short start up time
        • Coal + oil: take very long
        • Nuclear: longest
          • Can produce electricity every season, day, night
        • Hydroelectric: short start up time
          • Very reliable
        • Tidal
          • Not reliable
          • Depends on height of tide
        • Wind + solar
          • Not reliable
          • Depend on wind + sun
      • Cost per unit of electricity
        • Coal + gas: 6-15
        • Nuclear: 8
        • Wind: 8-21
      • Average energy consumption per person per year: 69 x 10^9 J