I-V Characteristics

Overview

This note supports Current Electricity Fundamentals by focusing on the graph shapes that tell you whether a component is ohmic or non-ohmic.

Use it as the graph-reading layer on top of the hub, not as a replacement for the main definitions there.

Related topics:

• Resistivity and Materials
• Electrical Power and Ratings
• Current Electricity Common Exam Traps

Definition

An I-V characteristic is a graph showing how current $I$ through a component varies with potential difference $V$ across it.

Unless a question states otherwise:

• horizontal axis: $V$
• vertical axis: $I$

Always check the axes before you interpret gradient or curvature.

Why It Matters

These graphs help you see:

• whether a component obeys Ohm’s law
• whether resistance is constant
• how the component behaves as temperature, bias, or light level changes
• the current for a given voltage

Key Representations

Ohmic Behaviour

A component is ohmic if, at constant temperature,

$$V\propto I$$

So the graph is a straight line through the origin and the resistance is constant.

For an ohmic conductor,

$$V=IR$$

Non-Ohmic Behaviour

A component is non-ohmic if the graph is curved or asymmetric, or if the gradient changes with operating point.

Common examples:

• filament lamp
• diode
• thermistor
• LDR

Reading Resistance from a Graph

Resistance at a chosen point is still found from

$$R=\frac{V}{I}$$

Large $V/I$ means large resistance. Small $V/I$ means small resistance.

Gradient Meaning

If the graph is $I$ against $V$,

$$\text{gradient}=\frac{\Delta I}{\Delta V}=\frac{1}{R}$$

So:

• steeper line means lower resistance
• shallower line means higher resistance

If the graph is $V$ against $I$,

$$\text{gradient}=R$$

Component Patterns

Metallic Conductor

A metallic resistor at constant temperature obeys Ohm’s law and gives a straight line through the origin.

Filament Lamp

As current increases, the filament heats up. The temperature rise increases lattice vibrations, so collisions with electrons become more frequent and resistance rises.

The curve therefore becomes less steep at higher voltages.

Diode

A diode conducts much more easily in forward bias than in reverse bias.

Forward bias:

• almost no current until the turn-on region
• current rises rapidly after the threshold

Reverse bias:

• current is very small for normal reverse voltages
• resistance is very large

Thermistor

For an NTC thermistor,

$$T\uparrow \Rightarrow R\downarrow$$

The dominant effect is that more charge carriers become available as temperature rises.

LDR

For an LDR,

$$\text{light intensity}\uparrow \Rightarrow R\downarrow$$

More light creates more mobile charge carriers, so the resistance falls.

Quick Checks

• Check the axis labels before using a gradient.
• Use $R=V/I$ for the resistance at a point.
• Do not assume every curve is a failure of Ohm’s law in the same way; the physical cause matters.
• Keep the graph reading tied back to the main hub page and the common-exam-traps note.

Links

• Current Electricity Fundamentals
• Resistivity and Materials
• Electrical Power and Ratings
• Current Electricity Common Exam Traps
• DC Circuits