The Three Key I-V Characteristic Shapes
Part of I-V Characteristics — GCSE Physics
This deep dive covers The Three Key I-V Characteristic Shapes within I-V Characteristics for GCSE Physics. Revise I-V Characteristics in Extra Topics for GCSE Physics with 13 exam-style questions and 11 flashcards. This is a high-frequency topic, so it is worth revising until the explanation feels precise and repeatable. It is section 3 of 12 in this topic. Use this deep dive to connect the idea to the wider topic before moving on to questions and flashcards.
Topic position
Section 3 of 12
Practice
13 questions
Recall
11 flashcards
⚡ The Three Key I-V Characteristic Shapes
Figure 2: I-V characteristic graphs for an ohmic resistor, filament lamp, and diode
1. Ohmic Resistor (at Constant Temperature)
Shape: Straight line through the origin
What this means: Current is directly proportional to voltage. When voltage doubles, current doubles. Resistance is constant regardless of voltage or current. This component obeys Ohm's Law (V = IR where R is constant).
The gradient of the line equals 1/R. A steeper line means lower resistance (more current per volt).
2. Filament Lamp
Shape: Curve through the origin that gets less steep at higher voltages
What this means: As voltage and current increase, the filament gets hotter. Hotter metal means more vibration of ions in the lattice, which causes more collisions with conduction electrons, increasing resistance. The curve gets flatter because resistance is increasing, so each extra volt of voltage produces less extra current.
The filament lamp is not ohmic — it does not obey Ohm's Law because its resistance changes with temperature.
3. Diode
Shape: Almost flat (negligible current) for negative voltage, then sharp upward curve from about +0.6 V
What this means: The diode only conducts in one direction (forward bias). Below about 0.6 V in the forward direction, it barely conducts. Above the threshold voltage (~0.6 V), resistance drops dramatically and current flows freely. In the reverse direction (negative voltage), the resistance is extremely high and virtually no current flows.
Quick Check: A student plots an I-V graph for a component. The graph is a straight line through the origin. What does this tell them about the component?
The component is an ohmic conductor. Current is directly proportional to voltage, meaning resistance is constant (does not change with temperature or current). It obeys Ohm's Law (V = IR where R is constant).