IV Graphs (Edexcel IGCSE Physics)

Spec point — 2.9 · describe how current varies with voltage in wires, resistors, metal filament lamps and diodes, and how to investigate this experimentally

IV graphs

• 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 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 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 resistance changes

• The IV graph for a filament lamp shows as voltage increases

  • the current increases at a proportionally slower rate
  • the resistance increases

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

Exam Hint It is a common misconception among students that the gradient of an IV graph is equal to the reciprocal of the resistance. This is only true for linear IV graphs with straight lines through the origin.

To calculate the resistance of a component with a non-linear IV graph at a given point, read the values of current and voltage for that point and use the definition of resistance as the ratio of voltage to current.

Investigating the relationship between current and voltage

• In order to investigate the relationship between current and voltage of 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

Spec point — 2.10 · describe the qualitative effect of changing resistance on the current in a circuit

• 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