A-Level Physics (IGCSE Edexcel) – I-V characteristics

4.4 - I-V Characteristics

1How to experimentally obtain an current-voltage (I-V) characteristic
2I-V characteristics in various components such as resistors, filament lamps, diodes
3The impact of temperature on the resistivity in metals and filament lamps

4 min

Understanding I-V graphs

The term "I-V characteristic" represents a graphical relationship between the electric current (I) flowing through a component and the voltage (V) applied across it.

Key points:

A straight graph line signals a consistent resistance.
A curved graph line indicates a varying resistance.

Steps to create an I-V graph:

Diagram of a circuit setup for creating an I-V graph, including a component, ammeter, voltmeter, and variable resistor.

Set up a circuit with a variable resistor in series with the component and power supply.
Record the voltage and current values displayed by the voltmeter and ammeter respectively.
vary the resistance of the variable resistor.
Record the new current and voltage values.
Repeat steps 3 and 4 until a full set of readings is obtained.
Plot these values to show current (I) against voltage (V).
Draw a line of best fit.

Resistor I-V characteristic

Ohmic conductors, like metals and resistors, abide by Ohm's law, particularly at a steady temperature, resulting in a linear I-V graph which implies a uniform resistance.

Graph showing the I-V characteristic of an ohmic conductor with current on the y-axis and potential difference on the x-axis.

Characteristics of ohmic materials:

Direct proportionality between current and potential difference.
Constant resistance when the temperature remains stable.
The steeper the gradient, the lower the resistance.

I-V Graphs in filament lamps

The graph below shows the I-V graph obtained for a filament lamp.

I-V graph showing the relationship between current and potential difference for a filament lamp.

Filaments are thin coils of wire. When a current flows a filament the temperature increases.

As the filament temperature increases:

Positive ions in the metal vibrate more vigorously.
This results in more collisions for electrons.
Electron movement becomes more challenging.
Consequently, the electrical resistivity of the filament increases (shown by a decreasing gradient on the I-V graph).

Diode I-V characteristics

Diodes, including LEDs, are unique in that they conduct current primarily in one direction, known as the forward bias.

Graph showing the current versus potential difference for a diode, illustrating the forward and reverse bias characteristics.

When the voltage across the diode is negative:

The resistance is extremely high.
The current is almost negligible.

When the potential difference across the diode is positive:

Resistance drops significantly above a threshold of approximately 0.6V.
Beyond this threshold potential difference, the current flow increases rapidly.