I-V Characteristics (WJEC GCSE Physics: Combined Science)

• I-V characteristics are just the relationship between current, , and voltage, , for different components
• These relationships are often shown by I-V graphs

• Circuit components can be linear or non-linear
  • In maths, linear means the graph is a straight line
• 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 and Non-linear I-V Graphs

Linear I-V graphs have a straight line going through the origin, this shows a constant resistance. Non-linear I-V graphs have curved lines showing a changing resistance

• If current and voltage are directly proportional, then as one increases, the other increases by the same amount
• When I-V  graphs are directly proportional, this means that the resistance R  remains constant

• Linear components include:
  • Fixed resistors (at constant temperature)
  • Wires (at constant temperature)
• Non-linear components include:
  • Filament lamps
  • Diodes & LEDs
  • LDRs
  • Thermistors

Resistance in Filament Lamps & Diodes

• In order to investigate the variation of resistance in a filament lamp or diode, the following circuits should be set up:

Circuits Used to Investigate Resistance of a Filament Lamp and a Diode

These circuits enable the variation of resistance in a filament lamp or a 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

I-V  Characteristics of a Filament Lamp

• For a filament lamp the current and voltage are not directly proportional
• The I–V graph shows the current increasing at a proportionally slower rate than the voltage

I-V Graph of a Filament Lamp

The resistance changes with temperature 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 voltage reverses the current and makes no difference to the shape of the curve

I-V Characteristics of a Diode

• A diode is a non-linear conductor that allows current to flow in one direction only
  • The direction is shown by the triangular arrow of the diode symbol 
  • 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 has a unique shape
  • When the diode is in forward bias, the graph shows a sharp increase in voltage and current (on the right side of the graph)
  • When the diode is switched around, in reverse bias, the graph shows a flat line where current is zero at all voltages (on the left side of the graph)

I-V Graph of a Diode

A diode only allows current to flow in one direction which gives the I-V graph a distinct shape

Resistance in LDRs & Thermistors

• In order to investigate the variation of resistance in an LDR or a thermistor, the following circuits should be set up:

Circuits Used to Investigate Resistance in an LDR and a Thermistor

These circuits enable the variation of resistance in a LDR or a thermistor to be investigated

• For the LDR circuit:
  • Begin with the lamp turned off in a dark room
  • Record the reading on the voltmeter and ammeter
  • Slowly increase the light intensity of the lamp using the dimmer switch
  • Record the reading on the voltmeter and ammeter for each increase in light intensity

• For the thermistor circuit:
  • Begin with the heater turned off
  • Record the reading on the voltmeter and ammeter
  • Slowly increase the temperature of the heater using the dimmer switch
  • Record the reading on the voltmeter and ammeter for each increase in temperature of the heater

• In both situations, make sure the lamp and heater are close to, but not touching, the LDR and thermistor respectively
• Wait a few seconds before taking the voltmeter and ammeter readings to allow the LDR and thermistor to react to the change in the environment
• Plot an I-V graph to show the resistance of each component