Resistance

• Electrical current


  Transfers energy around circuits
• Types of current


  • Direct
  • Alternating
• Resistance


  The opposition in an electrical component to the movement of electrical charge through it. Measured in ohms.
• Potential difference


  The potential difference (or voltage) of a supply is a measure of the energy given to the charge carriers in a circuit. Units = volts (V).
• To measure the potential difference across a component, a voltmeter must be placed in parallel with that component
• Voltage


  The potential difference across a cell, electrical supply or electrical component. Measured in volts (V).
• When a charge moves through a potential difference

  Electrical work is done and energy transferred
• One volt is the potential difference when one coulomb of charge transfers one joule of energy
• Conductors


  • Have a low resistance
• Insulators


  • Have a high resistance
• Resistance


  The opposition in an electrical component to the movement of electrical charge through it. Measured in ohms.
• Potential difference


  The potential difference (or voltage) of a supply is a measure of the energy given to the charge carriers in a circuit. Units = volts (V). This is the voltage between two points that makes an electric current flow between them.
• Current


  Moving electric charges, eg electrons moving through a metal wire.
• Ammeter


  A device used to measure electric current.
• Voltmeter


  A device used to measure potential difference or voltage.
• Voltage


  The potential difference across a cell, electrical supply or electrical component. Measured in volts (V).
• Experiment to investigate how changing the length of the wire affects its resistance

  1. Connect the circuit

  2. Connect the crocodile clips to the resistance wire, 100 cm apart

  3. Record the readings on the ammeter and voltmeter

  4. Move one crocodile clip closer, reducing the length by 10 cm

  5. Record the new readings

  6. Repeat reducing the length by 10 cm each time down to 10 cm

  7. Calculate the resistance of each length using R = V/I

  8. Plot a graph of resistance against length
• Resistance is directly proportional to length as the graph gives a straight line through the origin
• Hazards and control measures


  • Heating of the resistance wire - Burns to the skin
  • Do not touch the resistance wire whilst the circuit is connected. Allow the wire time to cool.
• Electrical current


  Transfers energy around circuits
• Types of current


  • Direct
  • Alternating
• Resistor


  An electrical component that restricts the flow of electrical charge. Fixed-value resistors do not change their resistance, but with variable resistors it is possible to vary the resistance.
• Potential difference


  The potential difference (or voltage) of a supply is a measure of the energy given to the charge carriers in a circuit. Units = volts (V). This is the voltage between two points that makes an electric current flow between them.
• Current


  Moving electric charges, eg electrons moving through a metal wire.
• Series circuit


  A circuit where one component follows directly from another, eg three bulbs in a row with no junctions are said to be connected in series.
• Parallel circuit


  A circuit where components are connected to the same set of terminals, allowing current to flow through different branches.
• Investigate resistor networks


  1. Set up circuit as shown in figure 1

  2. Turn power supply on and close switch

  3. Record voltmeter and ammeter readings

  4. Calculate resistance using R = V/I

  5. Change resistor and repeat

  6. Arrange resistors in series as shown in figure 2

  7. Record voltmeter and ammeter readings

  8. Determine total resistance of resistors in series

  9. Arrange resistors in parallel as shown in figure 3

  10. Record voltmeter and ammeter readings

  11. Calculate total resistance of resistors in parallel
• In series, the resistance of the network is equal to the sum of the two individual resistances
• In parallel, the resistance of the network is less than either of the two individual resistances
• Placing resistors in series

  Resistance is double that of a single resistor
• Placing resistors in parallel

  Resistance is half that of a single resistor