I-V Characteristics (Cambridge (CIE) A Level Physics)

I–V characteristics

• As the potential difference (voltage) across a component is increased, the current also increases (in accordance with Ohm’s law)

• The precise relationship between potential difference and current is different for different components and can be shown on an I-V graph:

I-V Characteristics of different components

I-V characteristics for metallic conductor (e.g. resistor) and semiconductor diode 

• The I-V graph for a metallic conductor at constant temperature e.g. a resistor, is very simple:

  • The current is directly proportional to the potential difference

  • This is demonstrated by the straight line graph through the origin

• The I-V graph for a semiconductor diode is slightly different. A diode is used in a circuit to allow current to flow only in a specific direction:

  • 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, it does not conduct and is called reverse bias. This is shown by a zero reading of current or potential difference on the left side of the graph

Resistance in a filament lamp

• The I-V graph for a filament lamp has a distinctive shape

I-V characteristics for a filament lamp

A graph showing the I-V characteristics for a filament lamp.

• This is because:

  • As the current increases, the temperature of the filament in the lamp increases

  • The higher temperature causes an increase in resistance

  • Resistance opposes current, causing the current to increase at an increasingly slower rate

• Where the graph is a straight line, the resistance is constant

• The resistance increases as the graph curves

Resistance and 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 through a conductor

• The electrons collide with the vibrating metal ions that make up the wire

• The vibrating ions impedes the flow of electrons (charge)

• Therefore, the current decreases

  • As the temperature increases, the resistance increases

  • And as the resistance increases, the current decreases