P2

• Equation linking charge, current and time

  1. 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)
• Equation to calculate potential difference if current and resistance are known

  1. V = I R

  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
• Components connected in series

  Total potential difference is shared between each component
• Components connected in parallel

  Potential difference across each component is the same
• Parallel resistors

  Total resistance is less than the smallest of the two individual resistances
• Series resistors

  Total 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


  Ammeters should be connected in series with the component that they are measuring current through
• Connecting a voltmeter


  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 in the UK 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
• 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
• Where the Earth wire is connected for metal appliances

  1. Earth wire is connected to the metal casing of the appliance

  2. If live wire becomes loose and touches the casing, the current will flow through the Earth wire, preventing electrocution
• State two equations for the power of a circuit

  1. P = I V

  2. P = I² R
• State an equation linking energy transferred, power and time

  E = P t
• State an equation linking energy transferred, charge flow and potential difference

  E = Q V
• Amount of energy transferred by an appliance depends on

  • How long the appliance is being used for
  • The power of the appliance