Electricity (2)

• What is electric current?

  Flow of electric charge.
• In a series circuit:

  current is the same at all points

  Resistance adds to make the total

  potential difference is shared
• In a parallel circuit:

  current adds to make the total

  adding resistance reduces the total resistance

  potential difference is the same
• what two factors does the current in a circuit depend on?

  1. Potential difference (V)
  2. Resistance (R)
• what is an ohmic conductor? State condition required

  a conductor for which current and potential difference are directly proportional

  >resistance remains constant as current changes

  >temperature must be constant
• List 4 components for which resistance is not constant as current changes

  1. lamps
  2. diodes
  3. thermistor
  4. LDRs
• what happens to the resistance of a filament lamp as temp increases

  resistance increases

  > ions in metal have more energy, so vibrate more, causing more collisions with electrons as they flow through the metal, causing greater resistance
• A diode:

  the current only flows in one direction

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

  the thermistor’s resistance increases
• Thermistor uses:

  in a thermostat

  in a freezer
• what happens to the resistance of an LDR as light intensity decreases

  the LDR‘s resistance increases
• LDR uses:

  street lights

  > when light levels become too low, the light gains sufficient current to turn on
• If 2 resistor s are connected in parallel, what can be said about their combine total resistance

  their total resistance is less than the smallest of the two individual resistances
• If 2 resistors are connected in series, what can be said about their total resistance?

  their total combine resistance is equal to the sum of the two individual resistances
• How should an ammeter be connected to measure current?

  in series with the component they are measuring current through
• How should a voltmeter be connected in a circuit to measure potential difference?

  In parallel to the component that they are measuring the potential difference of
• A.C vs D.C

  a.c: alternating current-> used for mains electricity

  d.c: direct current
• What is the frequency and voltage of the UK mains electricity supply?

  frequency: 50 Hz

  voltage: 230V
• Wires connecting an appliance to the mains:

  1. live wire (brown)
  2. neutral wire (blue)
  3. earth wire (green and yellow)
• The earth wire:

  no curent flows through

  if a fault occurs in the appliance (surge or casing becoming live) current will flow to the ground
• The neutral wire:

  0 volts

  completes the circuit by connecting the appliance back to the mains supply
• The live wire:

  carries current
• What two factors does the amount of energy being transferred by an appliance depend on?

  1. how long the appliance is being used for
  2. the power of the appliance
• describe the energy transfers in a battery powered torch

  • battery converts chemical energy into electrical energy
  • bulb converts electrical energy into light energy and wastes energy in the form of heating
• describe the energy transfers in a batter powered motor

  • battery converts chemical energy into electrical energy
  • motor converts electrical energy into kinetic energy and waste energy in the form of heating due to friction
• The National grid

  to link power stations to consumer

  2 types of transformers used

  -> step up transformers: increase P.D to reduce energy waste (high potential, low current, less energy wasted)

  -> step down transformers: decrease P.D to make it safe for domestic use

  the
• What can happen when rubbing insulting materials together?

  they can become (statically) electrically charged
• 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
• non contact forces

  repulsive or attractive forces acting between two electrically charged objects
• how to prevent unwanted static charge build up:

  any surfaces that are rubbing together should be earthed to allow charge to flow off the objects
• Ohmic conductor graph 


  Graph:

  
• Filament lamp graph: 


  Graph:

  
• Diode graph 


  Graph: