electricity (topic 2)

• Series circuits

  • the different components are all connected in a line, end to end, between the +ve and -ve of the power supply (except for voltmetres but they dont count as part of the circuit)
  • if you remove or disconnect one component the circuit is broken and they all stop
  • not very usefull in practice and very little things are connected in series
• P.D in series circuits

  • potential difference is shared
  • the total p.d is shared between the components
  • the p.d of all components always add up to equal to the source of the p.d
• Current in series circuits

  • the same current flows through all components
  • the size of the current is determined by the total resistance and and the total P.D - I= V/R
• Resistance in series circuits

  • the total resistance of 2 components is just the sum of their resistances
  • this is because by adding a resistor in series the 2 resistors have to share the P.D
  • the P.D across each resistor is lower, so the current through each resistor is also lower. this means the total resistance of the circuit increases.
  • the bigger a components resistance, the bigger its share of the total P.D
• Parallel circuits

  • each component is separately connected to the +ve and -ve of the power supply (except the ammeters)
  • if you remove or disconnect one of them it will hardly affect the others at all
  • Used commonly in practicle circuits (e.g. cars and household electrics), as you are able to switch everything on and off seperately
  • everyday circuits often include a mixture of parallel and series circuit parts
• P.D in parallel circuits

  • the same across all components
  • this means identical bulbs connected in parallel will be at the same brightness
• Current in parallel circuits

  • the total current flowing around the circuit is equal to the total of all the currents flowing through the separate components
  • in a parallel circuit, there are junctions where the current either slits or rejoins. the total current going into a junction has to equal the total current leaving
  • if 2 identical components are connected i parallel then the same component will flow through each component
• resistance in parallel circuits

  • adding a resistor in parallel circuits reduces the total resistance
  • in parallel both resistors have the same potential difference across them as the source
  • this means the 'pushing force' making the current flow is the same as the source P.D for each resistor that you add
  • but by adding another loop the current has more than 1 direction to go in
  • this increases the total current that can flow around the circuit, using R = V/I , an increase in current means a decrease in the total resistance
• what is the national grid?


  • a giant system of cables and transformers that covers the UK and connects power stations to consumers
  • the national grid transfers power from power stations anywhere on the grid (the supply) to anywhere else on the grid where its needed (the demand)
• How power stations cope with changes in demand:


  • they can (try) and predict when the most electricity will be used (e.g. when people get up in the morining, when they get back from work and when it starts to get dark or cold)
  • power stations often run at well below their maxiumum power output so theres spare capacity to cope with high demand, even if theres an unexpected shut down of another station
• what does the step up transformer do in the national grid?


  • increases the potential difference of the power (this decreases the current and the wasted thermal energy) making it more efficient
  • increases it to 400,000 V
• what does the step down transformer do in the national grid ?


  • decreases p.d (increasing current) to make it safe for the consumers
  • decreases p.d down to 230 V
• What is an electric field?


  • an electric field is created around any electrically charged object
  • the closer to the object you get the stronger the electric field is
  • you can show an electric field around an object by using field lines
• Electric field lines 


  • always go from positive to negative
  • always at a right angle to the surface
  • the closer the lines are the stronger the field is, you can see that the further from a charge you go the further apart the lines are so the weaker the field is

  
• Charged objects in an electric field :


  • when a charges object enters an electric field it feels a force
  • this force causes the attraction or repulsion seen in static electricity
  • this force is caused by the electric field of each charged object interacting with each other
  • the force of an object is linked to the strength of th electric field it is in
  • as you increase the distance between the charged objects the strength of the field decreases and the force between them gets smaller
• Sparks and how they link to electric fields:


  • sparks are caused when there is a high enough potential difference between a charges object and the earth (or an earthed object)
  • a high P.D causes a strong electric field between the charged object and the earthed object
  • this strong electric field causes electrons in the air particles tto e removed (known as ionisation)
  • air is normally an insulation but when it is ionised it is much more conductive so current can flow through it. this is the spark
• ammeter


  • measures the current (in amps)
  • must be placed in series (with the component)
• voltmeter


  • measures the potential difference (in volts)
  • must be placed in parralel (around component)
• the 2 types of electricity supplys:

  • alternating current (AC) - mains supply
  • direct current (DC) - battery supply
• what is voltage is the uk mains supply 


  230 V (AC)
• what is the frequency of the uk mains 


  50 hz
• what is AC


  current that is constantly changing direction, produced by alternating voltages in which the positive and negative ends keep alternating
• what is DC


  current that is always flowing in the same direction
• What 3 colour wires do you find inside plugs and cables ?

  • brown - neutral wire
  • blue - live wire
  • green and yellow - earth wire
• what is the neutral wire ?


  it completes the circuit - when the appliance is operating normally current flows through the live and neutral wires. it is around 0.7 V
• what is the live wire 


  it provides the alternating P.D (at about 230V) from the mains supply
• what is the earth wire ?


  it is for protecting the wiriing and for saftey - stops the appliance casing from becoming live. it doesnt usually carry a current- only when there is a fault
• Live wire can cause electric shocks :


  • your body (like earth) is at 0V, this means if you touch the live wire a large P.D is produced across your body and current flows through you causing an electric shock
• electrical appliances are designed to transfer energy to components in the circuit when a current flows
• the total energy transferred by an appliance depends on how long an appliance is on for and its power
• the power of an appliance is the energy that its transfers per second so more energy it transfers in a given time the higher its power
• appliances are often given a power rating, they are labeled with the maximum safe power that they can operate at
• LDR (light dependant resistor) 


  • a resistor that is dependant on the intensity of light
  • in bright light resistance falls
  • in darkness the resistance is highest
• Thermistor :


  • a temperature dependant resistor
  • in hot conditions resistance drops
  • in cool conditions resistance goes up
• what is current :


  the flow of electric charge

  • electrical charge will only flow round a complete circuit if there is a potential difference so current can only flow if theres a source of potential difference
• Potential difference is the driving force that pushes the charge round. its unit is the volt (V)
• in a single closed loop the current has the same value everwhere in the circuit
• resistance is the force that slows the flow down, unit: ohm
• the current flowing through a component depends on the potential difference across it and the resistance of the component
• the build up of static (electricity)

  • when certain insulating materials are rubbed together, negativley charged electrons will be scraped off one and transferred to the other
  • this will leave the materials electrically charged, with a positive static charge on one and an equal negative static charge on the other