IV Graphs (Edexcel IGCSE Physics)

• As the potential difference across a component is increased, the current also increases
  • This is because potential difference and current are proportional

• The precise relationship between voltage and current is different for different components and can be shown on an IV graph, including in:
  • Fixed resistors & wires
  • Filament lamps
  • Diodes

Fixed Resistors & Wires

• The current through a fixed resistor or a wire increases as the potential difference (or voltage) across it increases
• In other words, current is directly proportional to the potential difference for a fixed resistor (or a wire)
  • This relationship is true because the resistance of the fixed resistor (or wire) stays constant

• An IV graph shows that the line is straight and goes through the origin, as shown in the image below:

IV graph for a fixed resistor. The current is directly proportional to the potential difference (voltage) as the graph is a straight line through the origin

Filament Lamps

• For a filament lamp, current and voltage are not directly proportional
  • This is because the resistance of the filament lamp increases as the temperature of the filament increases

• The IV graph for a filament lamp shows the current increasing at a proportionally slower rate than the potential difference

IV graph for a filament lamp

• This is because:
  • As the current increases, the temperature of the filament in the lamp increases
  • The higher temperature causes the atoms in the metal lattice of the filament to vibrate more
  • This causes an increase in resistance as it becomes more difficult for free electrons (the current) to pass through
  • Resistance opposes the current, causing the current to increase at a slower rate

• Where the graph is a straight line, the resistance is constant
  • The resistance increases as the graph curves

• Reversing the potential difference reverses the current and makes no difference to the shape of the curve

Diodes

• A diode allows current to flow in one direction only
  • This is called forward bias

• In the reverse direction, the diode has very high resistance, and therefore no current flows
  • This is called reverse bias

• The IV graph for a diode is slightly different:
  • When the current is in the direction of the arrowhead symbol, this is forward bias
    • This is shown by the sharp increase in potential difference and current on the right side of the graph

  • When the diode is switched around, this is reverse bias
    • This is shown by a zero reading of current or potential difference on the left side of the graph

IV graph for a semiconductor diode

Investigating IV Graphs Experimentally

• In order to investigate the relationship between current and voltage different components, the following equipment is required:
  • An ammeter - to measure the current through the component
  • A voltmeter - to measure the voltage across the component
  • A variable resistor - to vary the current through the circuit
  • Power source - to provide a source of potential difference (voltage)
  • Wires - to connect the components together in a circuit

• The image below shows the circuits set up to obtain IV graphs for a filament lamp and a diode

These circuits enable the investigation of current and voltage for a filament lamp or diode to be investigated

• The current is the independent variable
  • The variable resistor is used to change the current flowing through the filament lamp / diode

• The voltage is the dependent variable
  • The voltmeter is used to measure the voltage across the filament lamp / diode

• Recording measurements of current and voltage as the current increases enables an IV graph to be plotted for each component

• Resistance is the opposition to the flow of current
  • The higher the resistance of a circuit the lower the current

• Resistors come in two types:
  • Fixed resistors
  • Variable resistors

• Fixed resistors have a resistance that remains constant
• Variable resistors can change the resistance by changing the length of wire that makes up the circuit
  • A longer length of wire has more resistance than a shorter length of wire

Fixed and variable resistor circuit symbols