Electricity

• Current


  Flow of electrical charge
• Potential difference (voltage)


  The driving force that makes the charge flow around a circuit
• Resistance


  Anything that slows the flow of current down
• The current flowing through a component


  Depends on the potential difference across it and the resistance of the component
• The greater the resistance across a component, the smaller the current that flows (for a given potential difference across the component)
• Circuit diagram symbols


  • Battery
  • Switch open
  • Switch closed
  • Filament lamp
  • Fuse
  • LED
  • Resistor
  • Variable resistor
  • Ammeter
  • Voltmeter
  • Diode
  • LDR
  • Thermistor
• A laptop charger passes a current of 4 A through a battery
• The resistance of a circuit can depend on factors like whether components are in series or parallel, or the length of wire used
• Ammeter


  • Measures the current flowing through the test wire
  • Must always be placed in series with whatever is being investigated
• Voltmeter


  • Measures the potential difference across the test wire
  • Must always be placed in parallel around whatever is being investigated
• Investigating resistance and wire length


  1. Attach crocodile clips to wire at different lengths

  2. Close switch and record current and potential difference

  3. Repeat for different wire lengths

  4. Plot graph of resistance against wire length
• Ohmic conductor


  Component where the resistance does not change as the current flowing through it is changed
• Non-ohmic conductor


  Component where the resistance does change as the current flowing through it is changed
• For non-ohmic conductors like filament lamps, as the current increases the resistance increases
• Diodes only allow current to flow in one direction
• 1. V characteristic


  • Graph showing how the current flowing through a component changes as the potential difference across it is increased
• 1. V characteristics


  • Ohmic conductor
  • Filament lamp
  • Diode
• The resistance of a component can be calculated from its I-V characteristic using R=V/I
• LDR (Light Dependent Resistor)


  Resistor whose resistance depends on the intensity of light
• Thermistor


  Resistor whose resistance depends on temperature
• Using LDRs and thermistors in sensing circuits


  1. LDR/thermistor connected in parallel with fixed resistor and load

  2. As resistance of LDR/thermistor changes, potential difference across load changes, affecting its behaviour
• Series circuits


  • All components connected one after the other
  • Potential difference shared between components
  • Current is the same everywhere
  • Resistances add up
• Parallel circuits


  • Components connected separately to power supply
  • Potential difference is the same across all components
  • Current is shared between branches
  • Adding a resistor in parallel reduces the total resistance
• The current will flow through each component
• Adding a Resistor in Parallel Reduces the Total Resistance
• If you have two resistors in parallel, their total resistance is less than the resistance of the smallest of the two resistors
• In parallel, both resistors have the same potential difference across them as the source
• Adding another loop increases the total current that can flow around the circuit
• An increase in current means a decrease in the total resistance of the circuit
• A current shared between identical components is a current halved
• Parallel circuits are trickier, but they're much more useful than series circuits
• Adding resistors in series increases the total resistance of the circuit
• When you add resistors in parallel, the total current through the circuit increases so the resistance of the circuit has decreased
• The more resistors you add in parallel, the smaller the overall resistance becomes
• The UK mains supply (the electricity in your home) is an AC supply at around 230 V
• The frequency of the AC mains supply is 50 cycles per second or 50 Hz (hertz)
• Cells and batteries supply direct current (DC)
• Wires in a 3-core cable


  • Live wire (brown)
  • Neutral wire (blue)
  • Earth wire (green and yellow)
• Live wire


  Provides the alternating current from the mains supply
• Neutral wire


  Completes the circuit, current flows through the live and neutral wires