Electricity

• Current


  The flow of electrical charge
• Current


  • Electrical charge will only flow around a complete (closed) circuit if there is a potential difference
  • Current can only flow if there's a source of potential difference
• Unit of current


  Ampere (A)
• Current in a single, closed loop circuit

  • The current has the same value everywhere in the circuit
• Potential difference (voltage)


  The driving force that pushes the charge round
• Unit of potential difference

  Volt (V)
• Resistance


  Anything that slows the flow down
• Unit of resistance


  Ohm (Ω)
• Current flowing through a component

  Depends on the potential difference across it and the resistance of the component
• Greater the resistance across a component

  The smaller the current that flows (for a given potential difference across the component)
• Total charge through a circuit


  Depends on current and time
• Circuit diagram symbols


  • Wire
  • Switch
  • Cell/battery
  • Resistor
  • Lamp
  • LED
  • Voltmeter
  • Ammeter
  • Diode
  • LDR
  • Thermistor
• Wires in a circuit should be straight lines and the circuit should be closed
• Resistance is directly proportional to wire length
• Ohmic conductor


  Has a constant resistance, current is directly proportional to potential difference
• Non-ohmic conductor


  Resistance changes with potential difference, current is not directly proportional
• 1. I-V characteristic

  Graph showing how current changes as potential difference is
• 3 important I-V characteristics

  • Ohmic conductor (straight line)
  • Filament lamp
  • Diode
• LDR (Light Dependent Resistor)


  Resistor whose resistance depends on light intensity
• LDR


  • Resistance is high in the dark, resistance is low in bright light
• Thermistor


  Temperature dependent resistor
• Thermistor


  • Resistance drops in hot conditions, resistance increases in cold conditions
• LDRs and thermistors can be used in sensing circuits to control components based on light or temperature
• Light-dependent resistor (LDR)


  Component whose resistance decreases when exposed to light
• LDR


  • Used in night lights, outdoor lighting, burglar detectors
• Thermistor


  Temperature-dependent resistor
• Thermistor


  • Resistance drops in hot conditions
  • Resistance increases in cold conditions
  • Used as temperature detectors e.g. car engine temperature sensors, electronic thermostats
• Sensing circuit to control a fan

  1. Fixed resistor and fan connected in parallel

  2. Thermistor also connected in parallel

  3. As room gets hotter, thermistor resistance decreases

  4. More current flows through fixed resistor and fan, making fan go faster
• Using an LDR to control a bulb

  1. LDR and bulb connected in parallel

  2. When dark, LDR resistance is high

  3. More voltage across bulb, making it brighter

  4. When light, LDR resistance is low

  5. Less voltage across bulb, making it dimmer
• Series circuits are "all or nothing" - if one component is removed the whole circuit stops working
• Series circuits


  • Potential difference shared between components
  • Same current flows through all components
  • Resistances add up
• Parallel circuits


  • Each component has full source potential difference
  • Current is shared between branches
  • Adding resistors in parallel reduces total resistance
• Most everyday circuits use a mixture of series and parallel connections
• Mains electricity supply is alternating current (AC), battery supply is direct current (DC)
• The more resistors you add


  The less the overall resistance becomes
• These results are consistent with what you learned about resistance in series and parallel circuits on pages 28 and 29
• There are two types of electricity supply - alternating current (AC) and direct current (DC)
• Mains supply is AC, battery supply is DC
• Alternating current


  The current is constantly changing direction, produced by alternating voltages where the positive and negative ends keep alternating
• Direct current


  A current that is always flowing in the same direction, created by a direct voltage