Electricity

Combined Science

?
View mindmap
  • Electricity
    • Circuits
      • Parallel Circuits
        • each component is separately connected to the battery - except the ammeter as its always connected in a series
        • if you remove one of them it will hardly affect the others
        • how most things are connected - in houses and cars you have to be able to switch things off separately
        • potential difference is the same across all components - V1 = V2 = ....
        • the total current is the total amount of current through different components - total I = I1 + I2 + ...
        • adding a resistor reduces the total resistance
      • Series Circuit
        • the different components are connected in a line end to end
        • if you remove one component the circuit is broken
        • the potential difference is shared - total V = V1 + V2 + ...
        • current is the same everywhere - I1 = I2 = ......
        • the total resistance is the sum of the components resistances - total R = R1 + R2
      • Resistance
        • anything that slows the flow down
        • for some components, as the current through them is changed the resistance changes aswell
          • the resistance of ohmic conductors doesnt change with the current - at a constant temp the current flowing through  is directly proportional to the potential difference
          • eg. diodes, a filament lamp
          • filament lamp - transfers some energy to the thermal energy store of the filament - as the current increases the lamp heats up and resistance increases
          • diode - resistance depends on the direction of the current - will happily let current flow in one direction but have a very high resistance if reversed
        • resistance increases temperature
      • Current
        • electric current is a flow of electrical charge
        • electrical charge will only flow round a complete (closed) circuit if theres a potential difference
          • potential difference is the driving force that pushes the charge around - unit is the volt
        • measured in amps
        • in a single, closed loop the current has the same value everywhere in the circuit
        • the greater the resistance across a component, the smaller the current that flows
      • Symbols table*
      • circuit devices
        • LDR
          • Light Dependent Resistor
          • dependent on the light intensity
          • bright light = resistance falls
          • darkness = resistance is high
          • lots of applications - automatic night nights, outdoor lighting and burglar detectors
        • thermistor
          • temperature dependent resistor
          • hot conditions - resistance drops
          • cool conditions - resistance goes up
          • make useful temperature detectors
        • you can use them in sensing circuits
          • sensing circuits can be used to turn on or increase the power to components depending on the conditions
      • everyday circuits often include a mixture of series and parallel circuits
    • Electricity in the Home
      • electricity supplies
        • alternating current (ac)
          • current constantly changing direction
          • produced by alternating voltages in which the positive and negative ends keep alternating
          • UK mains supply is ac at around 230V
          • frequency = 50 cycles per second or 50 Hz
        • direct current (dc)
          • cells and batteries
          • current always flowing in the same direction
          • created by direct voltage
      • cable wires
        • LIVE WIRE
          • brown
          • provides alternating potential difference from the mains supply
          • can give you an electric showck
        • EARTH WIRE
          • green and yellow
          • protects the wiring and for safety
          • stops appliance casting from becoming live
          • usually holds no current
        • NEUTRAL WIRE
          • blue
          • completes the circuit
          • when the appliance is operating the current flows through the live and neutral wires
          • around 0 V
        • any connection between the live and earth wire can be dangerous
    • Power
      • the power of an appliance is the energy that it transfers per second
      • power ratings
        • appliances are labelled with the maximum safe power they can operate at
        • tells you the maximum amount of energy transferred between stores per second when in use
        • helps customers choose between models
        • the lower the rating, the less electricity an appliance uses in a given time - cheaper to run
        • a higher power doesnt mean that it transfers more energy usefully - less effiecient
    • The National Grid
      • a giant web of wires that cover the whole of Britain - gets electricity from power stations to homes
      • demand
        • energy demands changes at different times in day
          • power stations can predict when the most electricity will be used - when people get up, come home and when it starts to get cold and dark
        • power stations often run at well below their maximum power output - spare capacity to cope with high demand
        • small power stations are kept in standby just incase
      • to transmit huge amounts of power you need high potential difference or high current
        • high current - lose loads of energy as wires heat up - energy is transferred to thermal energy store of surroundings
        • cheaper to boost potential difference high and keep current low
        • increasing the potential difference decreases the current - makes the national grid efficient
      • transformers
        • changes the potential difference
        • have two coils - a primary and secondary coil joined with an iron core
        • potential difference is increased using a step-up transformer
          • they have more turns on the secondary coil
          • the potential difference is then reduced at the local consumer end using a step-down transformer
            • they have more turns on the primary coil
        • power of primary coil = potential difference x current

Comments

No comments have yet been made

Similar Physics resources:

See all Physics resources »See all Electricity resources »