I-V Characteristics

An I-V characteristic is a graph of current I against potential difference V for a particular component. It is the electrical "fingerprint" of a device — look at the shape of the curve and you can tell whether you are dealing with a resistor, a lamp, a diode or a thermistor.

This lesson is a major OCR H556 required topic (Module 4.2.2, f–g) and appears on every exam paper. You must be able to:

• Sketch the characteristic of five standard components.
• Explain the physical reason for each shape.
• Interpret real experimental data.

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Experimental Setup

To measure an I-V characteristic you need a variable pd across the component and a way to measure both V and I. A typical circuit uses a potential divider (Lesson 12) to provide smoothly variable voltage from zero up to the supply. An ammeter measures I in series with the component; a voltmeter measures V in parallel with it.

flowchart LR
  B[Battery] --> S[Slider] --> DUT[Component]
  A((Ammeter)) --> DUT
  DUT --> B
  DUT -.- V1((Voltmeter))

The variable pd is swept from a negative value through zero to a positive value to reveal any asymmetry in the device's behaviour. For each setting of the slider, you record a pair (V, I). Then you plot I (vertical) against V (horizontal).

Exam Tip: OCR often asks you to describe an experiment to measure the I-V characteristic of a component. A full answer must mention a potential divider (not a rheostat, which cannot go to zero), an ammeter in series, a voltmeter in parallel, reversing the supply to get negative V, and taking repeat readings.

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The Standard Components

OCR H556 requires you to know the characteristics of:

1. Ohmic (metallic) resistor at constant T
2. Filament lamp
3. Semiconductor diode (including LED)
4. NTC thermistor
5. Light-dependent resistor (LDR)

We will look at each in turn.

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1. Ohmic Resistor

flowchart LR
  O[Origin] -->|straight line| P[Positive region]
  O -->|same slope| N[Negative region]

The I-V graph is a straight line through the origin, with the same slope in both positive and negative V regions. The gradient is 1/R, so R is constant. This is the hallmark of an ohmic conductor.

Feature	Interpretation
Straight line	I ∝ V
Through origin	No offset; I = 0 at V = 0
Same gradient both sides	Symmetric — direction of current doesn't matter
Constant gradient	R independent of V

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2. Filament Lamp

The filament lamp's characteristic is S-shaped (also called a "lazy S" or "flattened S"). At very low voltages the filament is near room temperature, and the wire behaves almost ohmically — the I-V curve starts off as an approximately straight line near the origin. As V increases, the filament heats up (possibly to over 2000 °C in a car headlamp), the ions of the tungsten lattice vibrate more violently, and electron scattering increases. The resistance rises.

Hence for the same ΔV, you get a smaller ΔI at higher voltages. The curve flattens.

The symmetry point is the origin — reversing the current direction does not change the filament's physics, so the curve is rotationally symmetric about the origin (odd symmetry).

V (V)	I (A) (typical 12 V bulb)
0	0
1	0.45
3	0.95
6	1.35
9	1.65
12	1.90

Notice the current roughly doubles between 0 and 1 V, but only increases by ~30% from 6 to 12 V.

Exam Tip: The canonical OCR answer: "As current increases, the filament temperature rises, so the lattice ions vibrate more and scatter electrons more frequently. This reduces the mean drift velocity, so the resistance increases and the I-V graph curves over."

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3. Semiconductor Diode

A diode is a non-linear, asymmetric device. It is designed to allow current to flow easily in one direction (forward bias) and almost not at all in the reverse direction (reverse bias).

Features of the diode characteristic:

• Forward bias (V > 0): almost no current flows until V exceeds a turn-on voltage of about 0.6 V (silicon) or 0.3 V (germanium). LEDs turn on at ~2 V depending on colour. Beyond the turn-on voltage, a tiny further increase in V produces a huge increase in I.
• Reverse bias (V < 0): the current is nearly zero (maybe a few microamperes) up to a breakdown voltage at which the diode fails destructively.

Region	Approximate behaviour
V < 0	I ≈ 0 (tiny leakage)
0 < V < 0.6	I ≈ 0 (exponentially small)
V > 0.6	I rises very rapidly