Electricity

• what is the relation of current and resistance?


  the greater the resistance across a component, the smaller the current that flows
• what is electrical current?

  • the flow of electric charge
  • will only flow round a complete circuit if there is a potential difference so a current can only flow if there is a source of potential difference
• How to investigate the factors affecting resistance

  1. Attach a crocodile clip to the wire level with 0 cm on the ruler

  2. Attach the second crocodile clip to the wire (e.g. 10 cm away) and write down the length of the wire between the clips

  3. Close the switch and record the current through the wire and the pd across it

  4. Open the switch and move the second crocodile clip further along (e.g. another 10 cm), close switch and record pd and current again

  5. Repeat this for a number of different lengths
• Calculating resistance


  Calculate the resistance for each length of wire using V=IR
• Graph

  • Plot a graph of resistance against wire length and draw a line of best fit
  • Graph should be a straight line through the origin means there is no systematic error
• Ohmic conductors

  • constant resistance - at a constant temperature, the current through the conductor is directly proportional to the pd
  • resistance of some resistors and components does change - diode and filament lamp
  • when electrical charge flows through a filament lamp it transfers some energy to thermal store - resistance increases with temp so as current increases the lamp heats more
• Diodes

  • resistance depends on the direction of current - lets current flow in one direction but has a very high resistance if they were reversed
• What does the graph of a ohmic conductor look like?


  The current through an ohmic conductor is directly proportional to potential difference so you get a straight line
• What does the graph of a filament lamp look like?

  • as current increases, the temp of the lamp increases so the resistance increases
  • this means less current can flow per unit pd so the graph gets shallower
  • hence the curve
• What does the graph of a diode look like?

  • current will only flow in one direction
  • diode has a very high resistance in the reverse direction
• How does an LDR work?

  • a resistor that is dependent on the intensity of light
  • in bright light the resistance falls
  • in darkness the resistance is higher
  • lots of applications - automatic night lights, outdoor lighting, burglar detectors
• How does a thermistor work?

  • a temperature dependent resistor
  • hot conditions - resistance falls
  • cool conditions - resistance goes up
  • make useful temperature detectors - electronic thermostats
• Series circuits

  • potential difference is shared between various components - pd's round the circuit always add up to the source pd
  • current is the same everywhere - same current flows through all components
  • resistance adds up
  • cell pd adds up - bigger pd when there are more cells in the series
• Parallel Circuits

  • pd is the same across all components - meaning identical light bulbs connected in parallel will all be at the same brightness
  • current is shared between branches - there are conjunctions where current either splits or rejoins
  • adding a resistor in parallel reduces the total resistance
• what is direct current?

  Current that always flows in the same direction.
• what is alternating current?


  Current that is constantly changing direction.
• UK mains supply

  • alternating current supply - 230 V and 50 Hz
• What is the neutral wire?

  • blue
  • completes the circuit - when the appliance is operating normally, current flows through the live and neutral wires
  • around 0 V
• What is the live wire?

  • Brown
  • provides the alternating potential difference - 230 V - from the mains supply
• What is the earth wire?

  • green and yellow
  • for protecting the wire and for safety - it stops the appliance casing from becoming live
  • doesn't usually carry a current - only when there is a fault
  • also at 0 V
• How can the live wire give an electric shock?

  • your body is 0 V, just like the earth, meaning that if you touch the live wire a large pd is produced across your body and a current flows through you
  • causes a large electric shock - could harm or kill you
• What is the national grid?

  a giant system of cables and transformers that covers the UK and connects power stations to the consumers
• How does the national grid meet demands?

  • power stations can predict when the most electricity will be used - demand increases in the mornings, when people get home from work/school and in the dark and cold seasons.
  • power stations often run well below their maximum power output to leave capacity to cope with a higher demand
  • smaller power stations can start up quickly so are kept in standby in case
• How does the national grid transmit energy?

  • need either a high potential difference or a high current - a high current loses a lot of energy as the wires heat up and energy is transferred to the thermal store in the surroundings
  • it's much cheaper to boost pd really high and keep current as low as possible
• How does static electricity build up?

  • when insulating materials are rubbed together, negatively charged electrons are scraped off one and put onto the other
  • leaves the materials electrically charged - positive static charge on one and a negative static charge on the other
  • examples - polythene being rubbed with a cloth duster
• What moves in static electricity?

  • POSITIVE CHARGES DO NOT MOVE
  • a positive charge is created by electrons moving away - the material that loses the electrons loses some negative charge and is left with a positive charge
• How does static electricity cause sparks?

  • electric charge builds on an object the pd between object and the earth increases
  • if the pd is big enough then electrons can jump across the gap between charged object and the earth - spark
  • they can also jump to any earthed conductor
• What is an electric field?

  • a field created around any electrically charged object - the closer you are to the object, the stronger the field is
• How do you draw an electric field?

  • field lines go from positive to negative
  • always at a right angle to surface