2. Electricity

• Current

  The rate of flow of charge at a point in the circuit, measured in amperes (A)
• Conventional current

  The rate of flow of positive charge, in the opposite direction to the flow of electrons
• Current is conserved at a junction in a circuit because charge is always conserved
• Potential difference

  The work done per unit charge in moving between two points in a circuit, measured in volts (V)
• The higher the potential difference

  The greater the current (V = IR)
• Resistance

  The measure of how hard it is for current to flow through a component, measured in ohms (Ω)
• Ohmic conductor

  • Current is directly proportional to voltage (constant resistance)
• Non-ohmic conductor

  • Resistance changes as voltage and current change
• Thermistor

  A resistor whose resistance decreases as temperature increases
• Light dependent resistor

  A resistor whose resistance decreases as light intensity increases
• Series circuit

  • Components are connected end to end in one loop
  • The same current flows through every component
  • The potential difference is shared across each component
• Parallel circuit


  • Components are connected to the power supply in separate branches
  • The current is shared between each branch
  • The potential difference is the same across every branch
• Connecting lamps in parallel is advantageous because if one breaks, current can still pass through the rest
• Fuse


  A thin piece of wire which overheats and melts if the current is too high, protecting the circuit
• Circuit breaker


  An automatic electromagnet switch which breaks the circuit if the current rises over a certain value
• Earth wire


  Creates a safe route for current to flow through in the case of a short circuit, preventing electric shocks
• Double insulation


  Appliances have either plastic casings completely covering their electrical components, or have been designed so that the earth wire cannot touch the metal casing, preventing electric shocks
• Power

  The rate of energy transfer, measured in watts (W)
• Direct current

  Current only flows in one direction
• Alternating current

  Current continuously changes direction
• Charge

  Measured in coulombs (C), there are positive and negative charges which attract and repel
• Atoms

  • Composed of protons (+1), electrons (-1), and neutrons (0)
• Conductor

  Allows electrons to flow through, such as metals
• Insulator


  Impedes the flow of electrons, such as plastics
• Charging a body


  Involves the addition or removal of electrons, creating ions
• Consequences of static electricity

  • Lightning
  • Charged balloon on a wall
  • Comb picking up bits of paper
• Dangers of electrostatic charges include electric shock and risk of fire/explosion when fuelling vehicles
• Earthing


  Offering electrons an alternative pathway to the earth, preventing too much electrostatic charge from building up
• Uses of electrostatic charges

  • Inkjet printer
  • Photocopier