Electricity

• Electric current


  The flow of electrical charge
• Units for charge, current and time1. Q = I t

  2. Charge (Coulombs)

  3. Current (Amperes)

  4. Time (Seconds)
• Current is the same at all points in a closed loop
• Factors current in a circuit depends on

  • 1. Potential Difference (V)
  • 2. Resistance (R)
• Units for Equation to calculate potential difference if current and resistance are known.

  2. Potential Difference (V)

  3. Current (A)

  4. Resistance (Ω)
• Ohmic Conductor


  • A conductor for which current and potential difference are directly proportional
  • Resistance remains constant as current changes
  • Temperature must be constant
• Components for which resistance is not constant as current changes

  • Lamps
  • Diodes
  • Thermistors
  • Light Dependant Resistors (LDRs)
• Resistance of a filament lamp as temperature increases

  • Resistance increases
  • Ions in metal have more energy, so vibrate more, causing more collisions with electrons as they flow through the metal, creating greater resistance to current flow
• Current flow through a diode

  • The current only flows in one direction
  • Resistance is very high in the other direction, preventing current flow
• Resistance of a thermistor as temperature increases

  The thermistor's resistance decreases
• 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 temperature becomes too high
• Resistance of a LDR as light intensity decreases


  The LDR's resistance increases
• Application for a LDR


  • Street lights often use LDRs
  • When light levels become too low, the light gains sufficient current to turn on
• Series


  Same loop
• Parallel


  Adjacent loop
• Series connection


  • Total potential difference is shared between each component
• Parallel connection


  • Potential difference across each component is the same
• Two resistors connected in parallel

  Their total resistance is less than the smallest of the two individual resistances
• Two resistors connected in series

  Their total combined resistance is equal to the sum of the two individual resistances
• Current in a series circuit

  • The current is the same at all positions since the charge only has one path to flow through
• Current in a parallel circuit

  • The current is shared between the different branches. When the charge reaches a junction it splits
• Connecting an ammeter in a circuit to measure current


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

  Voltmeters should be connected in parallel to the component that they are measuring the potential difference of
• Connecting lamps in parallel

  If one lamp blows, the rest will be unaffected and can still receive current (i.e. the circuit is still complete)
• Mains electricity


  a.c supply
• a.c.


  Alternating Current
• d.c.


  Direct Current
• Alternating current


  Current that continuously changes direction at a specific frequency
• Direct current


  One directional current flow
• Mains electricity is an a.c supply
• The frequency of the UK mains electricity supply is 50 Hz
• The voltage of the UK mains electricity supply is 230V
• Wires in cables connecting electrical appliances to the mains

  • Live wire
  • Neutral wire
  • Earth wire
• Earth wire insulation colour

  Green and Yellow Stripes
• Live wire insulation colour

  Brown
• Neutral wire insulation colour

  Blue
• When the Earth wire carries a current


  1. Under normal circumstances, no current flows through the Earth wire

  2. If a fault occurs in the appliance (such as a surge or the casing becoming live), current will flow to the ground
• The neutral wire is at 0 Volts
• The potential difference between the live and earth wires is 230 Volts
• Purpose of the neutral wire

  To complete the circuit by connecting the appliance back to the mains supply