P2: Electricity

• Current


  Flow of electrical charge
• Electric current will only flow round a complete (closed) circuit if there is a potential difference
• Potential difference (voltage)


  The driving force that pushes the charge round
• 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
• Investigating the effect of wire length on resistance

  1. Attach crocodile clip to wire at 0 cm

  2. Attach second crocodile clip, e.g. 10 cm away

  3. Close switch, record current and potential difference

  4. Open switch, move second clip, close switch, record new length, current and potential difference

  5. Repeat for different wire lengths

  6. Calculate resistance for each length

  7. Plot graph of resistance against wire length
• Ohmic conductors


  • Resistance doesn't change with current
  • Current is directly proportional to potential difference
• Non-ohmic conductors


  • Resistance changes with current
  • Filament lamp resistance increases with temperature
  • Diode resistance depends on direction of current
• Investigating I-V characteristics


  1. Set up test circuit

  2. Vary variable resistor to change current

  3. Take readings of current and potential difference

  4. Reverse current direction

  5. Plot graph of current against voltage
• 1. V characteristics


  • Ohmic conductor (straight line)
  • Filament lamp (curved)
  • Diode (only conducts in one direction)
• LDR (Light Dependent Resistor)


  Resistor with resistance that depends on light intensity
• Thermistor


  Temperature dependent resistor
• Sensing circuit to control a fan

  1. Fixed resistor and fan in parallel

  2. Thermistor in parallel

  3. As room gets hotter, thermistor resistance decreases, more current flows through fan
• Connecting a component across an LDR

  The greater the pd across the component, the more energy it gets. So a bulb would get brighter as the room gets darker.
• Series circuits


  • Components connected end to end
  • If one component is disconnected, the whole circuit stops working
  • Potential differences add up
  • Current is the same everywhere
  • Resistances add up
• Parallel circuits


  • Components separately connected to power supply
  • Removing one component hardly affects the others
  • Potential difference is the same across all components
  • Current is shared between branches
  • Adding a resistor reduces the total resistance
• Parallel circuit


  Junctions where the current either splits or rejoins. The total current going into a junction has to equal the total current leaving.
• If two identical components are connected in parallel then the same current will flow through each component
• Adding a Resistor in Parallel

  Reduces the Total Resistance
• Resistors in parallel


  • The total resistance is less than the resistance of the smallest of the two resistors
• Current shared (between identical components)

  Is a current halved
• Investigating Resistance


  1. Find at least four identical resistors

  2. Build the circuit using one resistor

  3. Measure the current and calculate the resistance

  4. Add another resistor in series

  5. Measure the current and calculate the resistance

  6. Repeat until all resistors added

  7. Plot a graph of number of resistors vs total resistance
• Investigating Resistance in Parallel

  1. Build the initial circuit

  2. Measure the total current and calculate the resistance

  3. Add another resistor in parallel

  4. Measure the total current and calculate the resistance

  5. Repeat until all resistors added

  6. Plot a graph of number of resistors vs total resistance
• Adding resistors in series increases the total resistance of the circuit
• Adding resistors in parallel increases the total current through the circuit, so the total resistance decreases</b>
• The more resistors added in parallel, the smaller the overall resistance becomes
• Alternating current (AC)


  The positive and negative amounts are produced by alternating voltages in which the positive and negative ends keep alternating
• Direct current (DC)


  A current that is always flowing in the same direction, created by a direct voltage
• The UK mains supply is an AC supply at around 230 V with a frequency of 50 Hz
• Wires in a 3-core cable

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


  Provides the alternating potential difference (around 230 V) from the mains supply
• Neutral wire


  Completes the circuit, current flows through the live and neutral wires, around 0 V
• Earth wire


  For protecting the wiring and for safety, stops the appliance casing from becoming live, usually at 0 V
• Touching the live wire can give you an electric shock as your body provides a link between the supply and the earth
• Power of an electrical appliance

  The energy transferred per second
• The power rating of an appliance tells the maximum safe power it can operate at
• Potential difference


  The energy transferred per unit charge
• Transformer


  Has a primary coil and a secondary coil joined with an iron core. Used to increase or decrease the potential difference.
• Transformers are nearly 100% efficient, so the power in the primary coil = power in the secondary coil
• The national grid is a giant system of cables and transformers that covers the UK and connects power stations to consumers