Edexcel A Level (IAL) Physics-2.39 Current–Potential Difference Graphs- Study Notes- New Syllabus
Edexcel A Level (IAL) Physics -2.39 Current–Potential Difference Graphs- Study Notes- New syllabus
Edexcel A Level (IAL) Physics -2.39 Current–Potential Difference Graphs- Study Notes -Edexcel A level Physics – per latest Syllabus.
Key Concepts:
- understand how to sketch, recognise and interpret current-potential difference graphs for components, including ohmic conductors, filament bulbs, thermistors and diodes
I–V Characteristics of Ohmic Conductors, Filament Bulbs, Thermistors and Diodes
The current–potential difference (I–V) graph shows how current through a component changes with applied voltage. Different components have characteristic shapes depending on how their resistance behaves.
Ohmic Conductor (e.g., metal wire at constant temperature)
- Obeys Ohm’s law.
- Current is directly proportional to voltage.
- Resistance remains constant.
Graph Shape: Straight line through the origin.
\( I \propto V \quad \Rightarrow \quad R = \dfrac{V}{I} = \text{constant} \)
Filament Bulb
- As current increases, the filament gets hot.
- Resistance increases with temperature.
- The graph curves — becomes less steep at higher voltages.
Graph Shape: A curve that gets shallower at high voltages (non-linear).
More current → higher temperature → higher resistance → smaller increase in current.
Thermistor (NTC — negative temperature coefficient)
- Resistance decreases as temperature increases.
- When voltage increases, current increases rapidly at first.
- Graph becomes steeper — opposite of filament bulb.
Graph Shape: A curve that gets steeper as voltage increases.
Higher current → higher temperature → lower resistance → even higher current.
Diode
- Conducts only when forward biased.
- Has a “threshold” or “switch-on” voltage (typically about 0.6–0.7 V for silicon).
- In reverse bias, current is almost zero.
Graph Shape:
- Forward bias: almost no current until threshold, then rapid increase.
- Reverse bias: near-zero current (almost flat line).
Shows highly non-ohmic behaviour.
Summary Table
| Component | I–V Graph Shape | Behaviour |
|---|---|---|
| Ohmic Conductor | Straight line through origin | Constant resistance, obeys Ohm’s law |
| Filament Bulb | Curves, slope decreases | Resistance increases with temperature |
| Thermistor (NTC) | Curves, slope increases | Resistance decreases with temperature |
| Diode | Flat until threshold, then steep rise | Conducts only when forward biased |
Example (Easy)
A component has an I–V graph that is a straight line through the origin. What type of component is it?
▶️ Answer / Explanation
An ohmic conductor — because \( I \propto V \).
Example (Medium)
The I–V graph of a component curves and becomes less steep as V increases. What does this tell you about the component?
▶️ Answer / Explanation
- It is a filament bulb.
- Increasing V → more current → higher temperature → higher resistance → slower current increase.
Example (Hard)
A component’s I–V graph shows almost no current until 0.65 V, after which current rises sharply. In reverse bias the current is nearly zero. Identify the component and explain the behaviour.
▶️ Answer / Explanation
It is a diode.
- Forward direction requires a threshold voltage (~0.6–0.7 V) to allow conduction.
- Once this is reached, the current increases rapidly → low resistance.
- In reverse bias the diode blocks current → very high resistance.