4.2ElectricityFoundation

Resistance and I-V Characteristics

Plot current against voltage for different components and you get very different shapes of graph — a straight line for a resistor, a curve for a filament lamp, and something closer to a one-way valve for a diode. Each shape tells a physical story about how that component’s resistance behaves.

40 min Video by Physics Online Module 4.2 Energy, power and resistance

IV Characteristics of Resistors, Filament Lamps, Diodes and LEDs — A Level PhysicsWatch the full walkthrough before the notes below.

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What you'll be able to do

Define resistance and use $R = \frac{V}{I}$
State Ohm’s law
Interpret I–V characteristic graphs for a resistor, filament lamp and diode
Explain the shape of each I–V graph in terms of the component’s physical behaviour

Resistance

$Resistance$ measures how strongly a component opposes the flow of current for a given potential difference, measured in ohms (Ω).

R = \frac{V}{I}

Resistance = potential difference ÷ current.

1Use

R = \frac{V}{I}

R = \frac{9.0}{0.30}

Answer

R = 30 Ω

Ohm’s law

$Ohm’s law$ states that, for a conductor at constant temperature, current is directly proportional to potential difference. A component obeying this — an $ohmic$ conductor — gives a straight-line I–V graph through the origin, with constant resistance (equal to $1/ gradient$ ) at every point.

Tip — A fixed resistor at constant temperature is the standard example of an ohmic component — but the moment temperature changes significantly (as in a filament lamp), Ohm’s law stops applying.

The filament lamp

A filament lamp’s I–V graph curves and flattens as current increases: rising current heats the filament, its metal ions vibrate more vigorously, and electrons collide with them more often — increasing resistance. This means current grows more slowly than a straight-line relationship would predict, so the lamp is non-ohmic.

The diode

A diode conducts almost exclusively in one direction: in forward bias, negligible current flows until a threshold voltage is reached (about 0.6 V for a typical silicon diode), after which current rises very steeply; in reverse bias, the diode’s resistance is enormously high and essentially no current flows at all, regardless of the applied p.d.

Tip — A diode’s huge resistance difference between forward and reverse bias is exactly why it is used to allow current to flow one way only — for example, converting AC to DC in a rectifier circuit.

Equation recap

R = \frac{V}{I}

Resistance.

Common mistakes to avoid

Assuming a filament lamp obeys Ohm’s law because it still has an I–V graph.

Ohm’s law requires a straight-line, proportional I–V graph — the filament lamp’s curved graph (due to its changing temperature) means it is non-ohmic.

Reading resistance directly off the gradient of a curved I–V graph.

Resistance at any point is always V/I at that specific point — the gradient of a curve only tells you how resistance is CHANGING, not its actual value.

Believing a diode conducts equally well in both directions.

A diode’s resistance is vastly different in forward bias (low, above the threshold voltage) compared with reverse bias (extremely high) — it is a strongly directional component.

Key takeaways

Resistance R = V/I; Ohm’s law: I ∝ V for a conductor at constant temperature (straight-line I-V graph through the origin).
A filament lamp’s resistance rises with temperature, giving a curved, flattening I-V graph.
A diode conducts (with low resistance) only above a threshold voltage in forward bias, and has very high resistance in reverse bias.

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Resistance and I-V Characteristics

40 min Video by Physics Online Module 4.2 Energy, power and resistance