IV Graphs (Edexcel IGCSE Physics)

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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 that is not a straight line
    • the resistance is not constant

Linear and non-linear IV graphs

Linear & Non-linear elements, downloadable IGCSE & GCSE Physics revision notes

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

Filament lamp IV graph, downloadable AS & A Level Physics revision notes

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

Investigation of Current and Voltage, downloadable IGCSE & GCSE Physics revision notes

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

  • 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 & Variable Resistor, downloadable IGCSE & GCSE Physics revision notes

Fixed and variable resistor circuit symbols

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Ashika

Author: Ashika

Expertise: Physics Project Lead

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.