IV Graphs (Edexcel IGCSE Physics)

• When the voltage V across a component is varied, the current I flowing through it may vary linearly or non-linearly
• The relationship between current and voltage of a component can be shown on an IV graph

• When the relationship between current and voltage is linear:
  • the IV graph is a straight line which passes through the origin
  • the resistance is constant

• When the relationship between current and voltage is non-linear:
  • the IV graph that is not a straight line
  • the resistance is not constant

Linear and non-linear IV graphs

Linear IV graphs are straight lines through the origin, indicating a constant resistance. Non-linear IV graphs are curved, indicating a variable resistance

• Components with linear IV graphs include:
  • fixed resistors (at constant temperature)
  • wires (at constant temperature)

• Components with non-linear IV graphs include:
  
  • filament lamps
  • diodes
  • LDRs
  • thermistors

IV graph for a wire or a resistor

• The relationship between current and voltage for a wire or fixed resistor is linear, or directly proportional, which means
  • the IV graph is a straight line, so voltage and current increase (or decrease) by the same amount
  • the slope of the graph is constant, so resistance is constant

The current is directly proportional to the potential difference (voltage) as the graph is a straight line through the origin

IV graph for a filament bulb

• The relationship between current and voltage for a filament lamp is non-linear, or not directly proportional, which means
  • the IV graph is not a straight line, so voltage and current do not increase (or decrease) by the same amount
  • the slope of the graph is not constant, so resistance changes

• The IV graph for a filament lamp shows as voltage increases
  • the current increases at a proportionally slower rate
  • the resistance increases; the flatter the slope, the higher the resistance

IV graph for a filament lamp

• As current through a filament lamp increases, the resistance increases because:
  • the higher current causes the temperature of the filament to increase
  • 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
  • since resistance opposes the current, this causes it to increase at a slower rate

IV graph for a diode

• 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

• When the current is in the direction of the arrowhead symbol, this is forward bias
  • On the IV graph, this is shown by a sharp increase in voltage and current on the right side of the graph
  • This shows the resistance is very low

• When the diode is switched around, this is reverse bias
  • On the IV graph, this is shown by a zero reading of current or voltage on the left side of the graph
  • This shows the resistance is very high

IV graph for a semiconductor diode

Investigating the relationship between current and voltage

• 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
  • a 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