P2 - Electricity

• Ohmic conductor


  A resistor in which the current is directly proportional to the potential difference at a constant temperature, so the resistance remains constant as the current changes
• Ohmic conductor


  • Indicated by a linear (straight line) graph
• Filament lamp


  As the current through it increases, its temperature increases, so the resistance increases as the current increases
• Filament lamp


  • Indicated by a curved graph
• Diode


  The current through it will only flow in one direction, and it has a very high resistance in the reverse direction
• Diode


  • Indicated by a horizontal line along the x-axis, which shows that no current flows
• Thermistor

  The resistance decreases as the temperature increases
• Thermistor


  • Useful in circuits where temperature control or response is required, e.g. in a thermostat or an overheating indicator
• Light Dependent Resistor (LDR)


  The resistance decreases as light intensity increases
• Light Dependent Resistor (LDR)


  • Useful where automatic light control or detection is needed, e.g. in dusk till dawn garden lights/street lights and in cameras/phones to determine if a flash is needed
• Electric current


  The flow of electrical charge - the greater the rate of flow, the higher the current
• Resistance


  The measure of how a component resists the flow of charge - the higher the resistance, the more difficult it is for charge to flow and the lower the current
• Potential difference (voltage)


  The difference in electrical potential from one point in a circuit to another - the bigger the potential difference across a component, the greater the flow of charge through the component and the greater the current
• Current does not flow through a voltmeter
• Series circuit


  The current is the same through each component, and the total potential difference of the power supply is shared between the components
• Total resistance of components in series

  The sum of the resistance of each component
• Adding resistors in series


  Increases the total resistance
• Parallel circuit

  The potential difference is the same across each component

  The total current drawn from the power supply is the sum of the currents through the separate components
• Total resistance of components in parallel


  Less than the resistance of the smallest individual resistor
• Adding resistors in parallel


  Reduces the total resistance
• Mains electricity in the UK

  230V and 50Hz
• Mains supply cable wires

  •Live wire: 230V potential

  •Neutral wire: at or close to 0V potential

  •Earth wire: 0V potential
• Mains electricity operation

  The potential difference causes current to flow through the live and the neutral wires, the live wire carries the alternating potential from the supply, and the neutral wire completes the circuit
• Earth wire


  For safety, it stops the exterior of an appliance becoming live
• National grid

  A system of cables and transformers linking power stations to homes and businesses
• Step-up transformers


  • Increase the potential difference from the power station to the transmission cables, reducing the current and the heating effect, making the transmission more efficient
• Transmission cables


  • Transfer the electricity
• Step-down transformers


  • Reduce the potential difference from the transmission cables to a much lower value for domestic use
• Standard circuit symbols

  • Switch (open)
  • Switch (closed)
  • Cell
  • Battery
  • Diode
  • Resistor
  • Variable resistor
  • LED (light emitting diode)
  • Bulb/lamp
  • Fuse
  • Voltmeter
  • Ammeter
  • Thermistor
  • LDR (light dependent resistor)
• • Live wire - brown

  • Neutral wire - blue

  • Earth wire - green and yellow
• Charge flow


  Current x time
• Charge flow (Q)


  Measured in coulombs (C)
• Current (I)


  Measured in amps (A)
• Time (t)


  Measured in seconds (s)
• Potential difference, current and resistance

  Linked by the equation: Potential difference = current x resistance
• Potential difference (V)


  Measured in volts (V)
• Resistance (R)


  Measured in ohms (Ω)
• Current


  The flow of electrical charge, measured in amperes (A)
• Charge

  A property of matter that causes it to experience a force when placed in an electric field; electric current is the flow of charge; charge can be positive or negative and is measured in coulombs (C)
• Resistance


  A measure of how a component resists (opposes) the flow of electrical charge, measured in ohms (Ω)