I-V Characteristics

This lesson covers the current-voltage (I-V) characteristics of different components — as required by the Edexcel GCSE Physics specification (1PH0). You need to understand the I-V graphs for resistors, filament lamps and diodes, and know how the resistance of LDRs and thermistors changes. This includes the core practical on investigating I-V characteristics.

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What Are I-V Characteristics?

An I-V characteristic is a graph that shows how the current (I) through a component varies as the potential difference (V) across it changes. By studying the shape of the graph, you can understand how the component's resistance behaves.

How Resistance Relates to the Graph

From V = IR, we get R = V/I.

• If the graph is a straight line through the origin, the resistance is constant — the component is an ohmic conductor.
• If the graph curves, the resistance is changing.
• A steeper line on an I-V graph means a lower resistance (more current flows for the same voltage).

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Core Practical: Investigating I-V Characteristics

Aim

To investigate the I-V characteristics of a resistor, a filament lamp and a diode.

Equipment

• Power supply (or battery)
• Variable resistor (or variable power supply)
• Ammeter (in series)
• Voltmeter (in parallel across the component)
• Component under test (resistor, filament lamp or diode)
• Switch
• Connecting wires

Method

1. Set up the circuit with the component, ammeter in series, and voltmeter in parallel across the component.
2. Include a variable resistor in series to control the current (or use a variable power supply).
3. Close the switch and adjust the variable resistor to change the potential difference across the component.
4. Record the voltmeter reading (V) and ammeter reading (I) for each setting.
5. Take readings for at least six different voltages, including low, medium and high values.
6. Reverse the battery connections and repeat to obtain readings for negative voltages (reverse direction).
7. Plot a graph of current (y-axis) against potential difference (x-axis).

Circuit Diagram

graph TD
    A["Power Supply"] --> B["Switch"]
    B --> C["Variable Resistor"]
    C --> D["Ammeter (A)"]
    D --> E["Component<br/>under test"]
    E --> F["Back to<br/>Power Supply"]
    E -.- G["Voltmeter (V)<br/>(in parallel)"]

    style A fill:#2c3e50,color:#fff
    style B fill:#7f8c8d,color:#fff
    style C fill:#e67e22,color:#fff
    style D fill:#e74c3c,color:#fff
    style E fill:#2980b9,color:#fff
    style F fill:#2c3e50,color:#fff
    style G fill:#27ae60,color:#fff

Exam Tip: This is a core practical — you may be asked to describe the method, draw the circuit, explain how you would obtain results for both positive and negative voltages, and sketch the expected graph. Make sure you mention reversing the connections to get the full I-V graph.

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I-V Graph: Ohmic Conductor (Fixed Resistor)

Shape

A straight line passing through the origin (0, 0).

What This Tells Us

• The current is directly proportional to the potential difference.
• The resistance is constant — it does not change as the current or voltage changes.
• This component obeys Ohm's law.

Key Features

Feature	Detail
Graph shape	Straight line through origin
Resistance	Constant
Relationship	V is directly proportional to I
Temperature	Stays approximately constant

Exam Tip: An "ohmic conductor" is one that obeys Ohm's law, meaning V is directly proportional to I at constant temperature. The condition "constant temperature" is important — if the temperature changes, even a metal wire may not behave as an ohmic conductor.

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I-V Graph: Filament Lamp

Shape

A curved line that starts steep near the origin and becomes less steep (flatter) at higher voltages. The curve is symmetric — it looks the same for negative and positive voltages.

What This Tells Us

• As the voltage increases, the current increases, but not proportionally.
• The filament lamp does not obey Ohm's law.
• As current increases, the filament gets hotter. The increased temperature causes the metal atoms in the filament to vibrate more, making it harder for electrons to flow through — so resistance increases.

Key Features

Feature	Detail
Graph shape	Curved — steep at low V, flatter at high V
Resistance	Increases as current (and temperature) increases
Relationship	V is NOT proportional to I
Temperature	Increases significantly with current

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I-V Graph: Diode

Shape

• In the forward direction (positive voltage): virtually no current flows until the voltage reaches about 0.6–0.7 V (the threshold voltage), then the current increases rapidly.
• In the reverse direction (negative voltage): essentially no current flows — the diode has very high resistance.

What This Tells Us

• A diode allows current to flow in one direction only (forward bias).
• In reverse bias, the diode acts like an open switch.
• The I-V graph is asymmetric — it only shows significant current on one side.

Key Features

Feature	Detail
Graph shape	Current in forward direction only; effectively zero in reverse
Forward bias	Low resistance — current flows after threshold voltage
Reverse bias	Very high resistance — no current flows
Threshold voltage	Approximately 0.6–0.7 V

Exam Tip: When sketching the diode I-V graph, make sure you show no current in the reverse direction and a sharp increase in current in the forward direction after the threshold voltage. Many students lose marks by drawing a gradual curve instead of a sharp turn.

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Comparison of I-V Graphs

graph TD
    A["I-V Characteristics"] --> B["Ohmic Conductor<br/>(Resistor)"]
    A --> C["Filament Lamp"]
    A --> D["Diode"]
    B --> E["Straight line<br/>through origin<br/>Constant R"]
    C --> F["Curved line<br/>Symmetric<br/>R increases with T"]
    D --> G["Current in one<br/>direction only<br/>Threshold ~0.7 V"]

    style A fill:#2c3e50,color:#fff
    style B fill:#2980b9,color:#fff
    style C fill:#e67e22,color:#fff
    style D fill:#27ae60,color:#fff
    style E fill:#1a1a2e,color:#fff
    style F fill:#1a1a2e,color:#fff
    style G fill:#1a1a2e,color:#fff

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LDR (Light-Dependent Resistor)

An LDR is a resistor whose resistance changes with light intensity.

Condition	Light Intensity	Resistance	Current (for given V)
Bright light	High	Low	High
Darkness	Low	High	Low

How It Works

• In bright light, more photons hit the LDR, freeing more charge carriers (electrons) — so the resistance decreases.
• In darkness, fewer photons hit the LDR, so fewer charge carriers are freed — the resistance increases.

Applications