Current-Voltage (I-V) Characteristics

Explained

Current voltage graphs for three components

An I to V graph shows how the current through a component changes as the potential difference across it changes. Three shapes are examinable, and each shape is telling you something about resistance rather than just about current.

Getting the graph

Connect the component with an ammeter in series and a voltmeter across it, and include a variable resistor in the circuit. The ammeter must be in series because it measures the current flowing through; the voltmeter must be in parallel because it measures the difference across.

Change the variable resistor, record the current and the potential difference at each setting, and plot current against potential difference. Reverse the connections to the supply to get negative values, which matters for the diode.

The fixed resistor

A straight line through the origin. Current is directly proportional to potential difference, which means the resistance stays constant. A component that behaves this way is called ohmic.

This only holds while the temperature is constant, which is why the definition includes that condition.

The filament lamp

A curve that starts steep and flattens as potential difference increases. Flattening means the current is rising more slowly than the voltage, so the resistance is increasing.

The reason is temperature. As current flows the filament gets hot, the metal ions vibrate more, and the electrons collide with them more often, so the resistance rises. A filament lamp is therefore not ohmic.

The diode

Almost no current until a small forward potential difference is reached, then a sharp rise. In the reverse direction, effectively no current at all.

A diode allows current in one direction only, which is what makes it useful for converting alternating current into direct current. Its resistance in the reverse direction is very high.

Reading resistance off the graph

Resistance is potential difference divided by current, so at any point on the graph you can read off both values and divide.

Be careful with the gradient. For a straight line through the origin, the gradient is current divided by voltage, which is one over the resistance rather than the resistance itself. A steeper line means a lower resistance. Questions rely on that being counterintuitive.

Describing the shapes in words

Say what the line does, then say what that means for resistance, then give the reason. "The curve flattens, so the resistance increases, because the filament heats up." Three clauses, and each is usually a mark.