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I-V Characteristics (CIE A Level Physics)

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Katie M

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Katie M

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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 componentsIV conductor and diode graphs, downloadable AS & A Level Physics revision notes

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

Worked example

The I–V characteristic of two electrical component X and Y are shown.WE - IV characteristics question image, downloadable AS & A Level Physics revision notesWhich statement is correct?

A.     The resistance of X increases as the current increases

B.     At 2 V, the resistance of X is half the resistance of Y

C.     Y is a semiconductor diode and X is a resistor

D.     X is a resistor and Y is a filament lamp

ANSWER: C

  • The I-V graph X is linear
    • This means the graph has a constant gradient. I/V and the resistance is therefore also constant (since gradient = 1/R)
    • This is the I-V graph for a conductor at constant temperature e.g. a resistor
  • The I-V graph Y starts with zero gradient and then the gradient increases rapidly
    • This means it has infinite resistance at the start which then decreases rapidly
    • This is characters of a device that only has current in one direction e.g a semiconductor diode
  • Therefore the answer is C

Resistance in a filament lamp

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

I-V characteristics for a filament lampFilament lamp IV graph, downloadable AS & A Level Physics revision notes

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

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Katie M

Author: Katie M

Expertise: Physics

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.