AQA GCSE Physics

Revision Notes

2.1.6 I–V Graphs

Ohm's Law

  • Ohm’s Law states that:

The current through a conductor is directly proportional to the potential difference across it

  • Electrical conductors that obey Ohm’s Law are referred to as ohmic conductors
  • Examples of ohmic conductors are:
    • Fixed resistors
    • Wires
    • Heating elements
  • Ohm’s Law is represented by the equation V = IR
    • If V and I are directly proportional, this means that the resistance R remains constant
  • Ohm’s Law is relevant only at constant temperatures
  • An ohmic conductor will have a current-voltage (I–V) graph that is a straight line through the origin

An I-V graph for an ohmic conductor is a straight line graph through the origin

  • Reversing the potential difference (and direction of the current) makes no difference to the shape of the line, the potential difference and current values will just be negative
  • If the axis labels are swapped around (the current on the x-axis and p.d on the y-axis) the graph will still be a straight line through the origin

Filament Lamps

  • A filament lamp is an example of a non-ohmic conductor
  • This means that the current and potential difference are not directly proportional
    • This is because the resistance of the filament lamp increases as the temperature of the filament increases
  • The I–V graph for a filament lamp shows the current increasing at a proportionally slower rate than the potential difference

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

I-V 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

Resistance & Temperature

  • All solids are made up of vibrating atoms
    • The higher the temperature, the faster these atoms vibrate
  • Electric current is the flow of free electrons in a material
    • The electrons collide with the vibrating atoms which impedes their flow, hence the current decreases
  • So, if the current decreases, then the resistance will increase (from V = IR)
  • Therefore, an increase in temperature causes an increase in resistance

Diodes

  • A diode is a non-ohmic conductor that 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 I–V 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

I-V graph for a semiconductor diode

  • An LED is a specific type of diode that emits light and works the same way as a normal diode

Linear & Non-Linear Graphs

  • Circuit elements can be linear or non-linear
    • In maths, linear means the graph is a straight line
  • To know whether a circuit element is linear or non-linear, check whether its I-V graphs is a straight line or not
    • Linear components have an I-V graph that is a straight line through the origin
    • Non-linear components have an I-V graph that is not a straight line
  • Linear components are said to obey Ohm’s Law and have a constant resistance, whilst non-linear do not
  • Some components may be linear at low currents, then become non-linear as the current increases (and therefore a change in temperature)
    • For example, a fixed resistor at room temperature is linear, but when it becomes very hot it will become non-linear

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

Linear and non-linear I-V graphs

  • Linear elements include:
    • Fixed resistors
    • Wires
    • Heating elements
  • Non-linear elements include:
    • Filament lamps
    • Diodes & LEDs
    • LDRs
    • Thermistors

Author: Ashika

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.
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