I-V Characteristics (DP IB Physics)

Ohm's Law

• Ohm's law states that:

  For a component at a constant temperature, the current through it is proportional to the potential difference across it

• It is defined by the equation:

• Where:

  • V = potential difference (V)

  • I = current (A)

  • R = resistance (Ω)

• An electrical component obeys Ohm’s law if its graph of current against potential difference is a straight line through the origin

  • A fixed resistor obeys Ohm’s law i.e. it is an ohmic component

  • A filament lamp does not obey Ohm’s law i.e. it is a non-ohmic component

The current-voltage graph for a fixed resistor is a straight line through the origin. The fixed resistor is an ohmic component

• The resistance of an ohmic component can be calculated from the gradient of a current-voltage graph, since resistance is equal to

• If current  is on the y-axis and potential difference  is on the x-axis, then

• If potential difference  is on the y-axis and current  is on the x-axis, then 

• Any metal conductor at a constant temperature can be considered an ohmic device

  • This is likely to be a fixed resistance

• Non-ohmic devices include:

  • Lamps

  • LEDs

  • Thermistors

I-V Characteristics

• The Ohmic and non-Ohmic behaviour of an electrical conductor can be deduced by looking at its I-V (current-voltage) characteristics

  • This is usually plotted as a graph showing the variation of current against voltage

• The relation between potential difference across an electrical component (e.g. a fixed resistor) and current can be investigated through a circuit such as the one below

Circuit for plotting graphs of current against voltage. The component being investigated here is a fixed resistor

• By adjusting the resistance on the variable resistor:

  • The current in the circuit will change

  • For each value of the current , the potential difference can be recorded

• A graph of current against potential difference can then be plotted

I-V characteristics of common conductors

• Common ohmic conductors include

  • Wires (at constant temperature)

  • Resistors

• Common non-ohmic conductors include

  • Semiconductor diodes e.g. LEDs

  • Filament lamp

  • Thermistors & light-dependent resistors (LDRs)

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

Resistor

• A resistor is an example of an Ohmic resistor

  • This means the current is directly proportional to the potential difference

  • Its I-V graph is a straight line through the origin

Semiconductor Diode

• When the current is in the direction of the arrowhead symbol, the diode is said to be forward-biased

  • There is a sharp increase in current

  • This is shown on the right side of the graph

• When the diode is switched around, it does not conduct and it is said to be reverse biased

  • The current through the diode is zero

  • This is shown on the left side of the graph

• The diode is a non-ohmic component

  • Its I-V graph is not a straight line through the origin

• A specific type of diode is an LED

  • The I-V characteristic graphs looks the exact same for this

I–V characteristics for a filament lamp

Filament Lamp

• For very small voltages, the filament lamp behaves as an ohmic component

  • The middle section of the graph (around zero voltage) is straight and passes through the origin

• As voltage increases:

  • More current flows through the filament lamp and the temperature of the filament in the lamp increases

  • The higher the temperature of the filament, the higher its resistance

  • Since resistance opposes current, the current flows through the filament at a slower rate 

  • This is shown by the curved section of the graph

• For slightly higher voltages, the filament lamp is non-ohmic 

  • The I-V graph is a curve with decreasing gradient