2.39 Current-Potential Difference Graphs

• As the potential difference (voltage) across a component is increased, the current also increases (by Ohm’s law)
• The precise relationship between voltage and current is different for different components and can be shown on a current-potential difference or I-V graph
  • For an ohmic conductor, the I–V graph is a straight line through the origin
  • For a semiconductor diode, the I–V graph is a horizontal line that goes sharply upwards
  • For a filament lamp, the I–V graph has an 'S' shaped curve

I–V characteristics for an ohmic conductor (e.g. resistor), semiconductor diode and filament lamp

Ohmic Conductor

• The I–V graph for an ohmic 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

Diode

• The I–V graph for a 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
  • The threshold voltage at which a diode starts to conduct is typically around 0.6V

Filament Lamp

• The I–V graph for a filament lamp shows the current increasing at a proportionally slower rate than the potential difference
• This is because:
  • As the current increases, the temperature of the filament in the lamp increases
  • Since the filament is a metal, the higher temperature causes an increase in resistance
  • 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
• The filament lamp obeys Ohm's Law for small voltages

Thermistor

• The I–V graph for a thermistor is a shallow curve upwards

• The increase in the potential difference results in an increase in current which causes the temperature of the thermistor to rise
• As its temperature rises, its resistance decreases
  • This means even more current is able to flow through

• Since the current is not directly proportional to the potential difference (the graph is still curved), the thermistor does not obey Ohm's Law

• The I–V graph for a thermistor shows the current increasing at a proportionally slower rate than the potential difference
• This is because:
  • As the current increases, the temperature of the thermistor increases
  • Which causes an increase in resistance
  • Resistance opposes the current, causing the current to increase at a slower rate

      ANSWER: C

Step 1: Consider the characteristics of graph X

• • 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

Step 2: Consider the characteristics of graph Y

• • 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 characteristic of a device that only has current in one direction e.g a semiconductor diode

Step 3: Compare this information with the statements A-D

• • A. Resistance is constant
    • therefore this statement is incorrect
  • B. At 2V the current of X is 0.5 A and the current of Y is 0 A.
    • therefore this statement is incorrect
  • D. X is an ohmic component such as a resistor, however Y is the graph for a diode not a filament bulb.
    • therefore this statement is incorrect

Step 4: State the correct answer

• • The correct answer is C