ELECTRICITY

• live wire – brown

  neutral wire – blue

  earth wire – green and yellow stripes.
• 2 factors that current depends on in a circuit

  potential difference

  resistance
• ohmic conductor

  potential difference and current are directly proportional

  resistance remains constant as current changes

  temperature remains constant
• 4 components for which resistance is not constant as current changes

  lamps

  diodes

  thermistors

  light dependent resistors
• what happens to the resistance of a fiament lamp as temperature increases

  resistance increases

  ions in metal have more energy, so vibrate more, causing more collisions with electrons as they flow through metal creating greater resistance to current flow
• what is different about current flow through diode

  current only flows in one direction

  resistance is very high in the other direction, preventing current flow
• what happens to resistance of a thermistor as temp increases

  thermistors resistance decreases
• 2 examples of when a thermistor may be used

  in a thermostat to turn a heater on below a certain temperature

  in a freezer to turn on a cooler when the temp becomes too high
• resistance of an ldr when light intensity decreases

  resistance increases
• application for a ldr

  street lights

  when light levels get too low light gains sufficient current to turn on
• series: total p.d is shared between each component

  parallel: p.d across each component is the same
• why is advantageous to connect lamps in parallel

  if one lamp blows the rest will be unaffected and can still receive current
• alternating current

  current that continously changes direction at at a specific frequency
• direct current

  one directional current flow
• colour for earth wire

  green and yellow
• colour for live wire

  brown
• colour for neutral wire

  blue
• when the earth wire does and doesnt carry a current

  under normal circumstances no current flows through the earth wire

  if a fault occurs in the appliance current will flow to the ground
• purpose of the neutral wire

  to complete by the circuit by connecting the appliance back to the mains supply
• for metal appliances where is the earth wire connected to

  each wire is connected to the metal casing of the appliance

  if live wire begins to come loose and touches the casing, the current will flow through the earth wire, preventing electrocution
• P = IV

  P = IxI R
• energy transfers in a battery powered torch

  batteries convert chemical energy to electrical energy

  bulb converts electrical energy to light energy as well as waste energy in the form of heating
• energy transfers in a battery powered motor

  battery converts chemical energy into electrical energy

  motor converts electrical energy to kinetic energy as well as waste energy in the form of heating due to friction
• things that determine the power of a circuit device

  potential difference across the circuit

  current through the circuit

  amount of energy transferred in a given time
• purpose of the national grid

  to link power stations to consumers so that they have access to a source of electricity
• two types of transformers in NG

  stepup transformers

  stepdown transformers
• step up transformers

  used when connecting power stations to transmission cables

  they increase potential difference
• step down transformers

  connect transmission cables to domestic buildings

  decrease potential difference
• why do transmission lines transfer electricity at high potentials

  a high potential results in a low current

  the lower the current the less energy that is wasted as heat

  therefore it is more efficient
• why does potential need to be decreased between transmission lines and houses

  lower potentials are safer for domestic use and reduces the likelihood of severe electrocution

  designed for 230V
• why can insulators become electrically charged when rubbed together

  electrons are rubbed from one material onto the other

  the material gaining electrons becomes negatively charged

  the material losing electrons becomes equally positively charged