P2-Electicity

• current is the flow of electrical charge
• the greater a resistance across a component the smaller the current that flows
• total charge through a circuit depends on current and time and the more charge passes around the circuit when a larger current flows
• circuit symbols

  A) cell

  B) battery

  C) oepn switch

  D) closed switch

  E) filament lamp

  F) fuse

  G) LED

  H) resistor

  I) variable resistor

  J) ammeter

  K) voltmeter

  L) diode

  M) LDR

  N) thermistor

  
• you can investigate the factors affecting resistance. the resistance of a circuit can depend on a number of factors like whether components are in series or parallel or the length of wire used in a circuit.
• required practical : attach clip to the wire level with 0cm on the ruler, attach the second crocodile clip to the wire e.g 10cm away from the first clip and write that length between the clips down, close the switch then record the current through the wire and the PD across it, open the switch then move the crocodile clip e.g another 10cm away then close the switch and write down the new length. finally use measurements of current and PD to calculate the resistance for each length of wire using R = V / I
• ohmic conductors have a constant resistance
• diode , filament lamp and ohmic conductor graphs

  A) ohmic conductors

  B) filament lamp

  C) diode

  
• LDR is short for light dependant resistor. in the bright light the resistance falls and in the darkness the resistance is its highest
• the resistance of a thermistor depends on the resistor. in hot conditions the resistance drops and in cool conditions the resistance goes up
• sensing circuits can be used to turn on or increase the power to components depending on the conditions that they are in
• in series circuits the different component are all connected in a line end to end
• in series circuits the total PD of the supply is shared between the various components so the potential difference round a series circuit always adds up to equal the source PD
• current is the same all the way round in series circuits
• in series circuits the total resistance of two components is the sum of their resistance
• in parallel circuits each components is separately connected, if you remove one it will hardly affect the others
• in parallel circuits the PD is the same across all components
• adding a resistor in a parallel circuit reduces the total resistance
• investigating resistance required practical

  A) resistors

  B) potential difference

  C) calculate resistance

  D) resistor

  E) total resistance

  
• main supply is AC
• battery supply is DC
• neutral wire - blue. the neutral wire completes the circuit when the appliance is operating normally current flows through the LIVE and neutral wires and has 0v
• live wire is brown. the live wire provides the alternating potential difference at about 230v from the mains supply
• earth wire - green and yellow. it is for protecting the wiring and for safety and it stops the appliance casing from becoming live. it doesn`t usually carry a current and only when there's a fault and it also has 0v
• the live wire can give you an electric shock
• electrical appliances are designed to transfer energy to components in the circuit when a current flows
• kettles transfer energy electrically from the mains AC supply to thermal energy store of the heating inside the kettle
• energy is transferred electrically from the battery of a handheld fan to the kinetic energy store of the fans motor
• the total energy transferred by the appliance depends on how long the appliance is on for its power. the power of an appliance is the energy that it transfers per second so the more energy it transfer in a given time the higher its power energy transferred = power x time
• potential difference is energy transferred per charge passed. when an electrical charge goes through a change in potential difference then energy is transferred. energy is supplied to the charge at the power source to raise it through a potential difference. the charge gives up this energy when it falls through any potential drop in components elsewhere in the circuit energy transferred = charge flow x potential difference
• power also depends on current and potential difference
• electricity is disputed by the national grid
• the national grid is a giant system of cables and transformers that covers the UK and connects power stations to consumers and transfers electrical power from power stations anywhere on the grid to anywhere else on the grid where its needed e.g homes
• electricity has to meet the demand
• the national grid uses high potential difference and low current and the problem with high current is that to much energy is lost due to the wires heating up. PD is much cheaper
• to get the potential difference to 400000 v for efficient transmissions we use transformers. potential difference is increased using step up transformers they have more turns on the secondary coil than the primary coil and as the PD is increased by transformer the current decreased. then the PD is then reduced again at the step down transformer they have more turns on the primary coil than the secondary

  A) power station

  B) step up transformers

  C) consumers

  D) step down transformer