An ohmic conductor has a constant resistance — doubling the voltage exactly doubles the current, giving a straight-line IV graph through the origin. Non-ohmic components do not obey this simple relationship: their resistance changes with voltage, current, light level, or temperature, giving characteristic curved or stepped IV graphs that are distinct for each type.
What is an ohmic conductor and how does it differ from a non-ohmic component?
Ohm's law states that, for a conductor at constant temperature, current is directly proportional to voltage: I = V/R, where R is constant.
An ohmic conductor — such as a fixed resistor or a metal wire at constant temperature — produces a perfectly straight IV graph passing through the origin. The gradient (I/V) is constant, so resistance R = V/I is the same at every point.
A non-ohmic component has a resistance that changes, so the IV graph is curved or asymmetric. The four non-ohmic components you need for GCSE are:
- Filament lamp — resistance increases with temperature.
- Diode — conducts in one direction only.
- Thermistor (NTC) — resistance decreases as temperature increases.
- Light-dependent resistor (LDR) — resistance decreases as light intensity increases.
What does the IV graph for a filament lamp look like?
A filament lamp has a tungsten wire that gets extremely hot when carrying current (around 2500 °C at full brightness). As temperature rises, the atoms in the metal vibrate more and impede electron flow — resistance increases.
IV graph shape: The graph curves — starting steeply (low resistance when cool), then flattening as current and temperature increase (higher resistance). The graph is symmetrical about the origin (current flows in either direction).
At low voltages the wire is cool and resistance is low. At high voltages the wire is white-hot and resistance is high. This is why the IV graph is S-shaped rather than straight: the gradient (which represents 1/R) decreases as voltage increases.
What is a diode and how does it behave?
A diode is a semiconductor component that allows current to flow in one direction only — it acts as a one-way valve for current.
Forward bias: When connected with its positive terminal to the positive supply (forward bias), a diode begins to conduct significantly only above a threshold voltage of approximately 0.6–0.7 V. Above this threshold, the resistance drops dramatically and current flows freely.
Reverse bias: When connected the other way (reverse bias), the diode has an extremely high resistance — effectively a break in the circuit — and current is negligible.
IV graph shape: The graph has two very different regions:
- In the positive direction (forward bias): nearly zero current up to ~0.7 V, then a steep rise in current.
- In the negative direction (reverse bias): a flat line at (or very close to) zero current.
Uses of diodes:
- Rectification — converting alternating current (AC) to direct current (DC) by allowing only one direction of current through.
- LED (light-emitting diode) — a diode that emits light when forward-biased.
What is a thermistor (NTC) and how is it used?
An NTC thermistor (Negative Temperature Coefficient) has a resistance that decreases significantly as temperature increases. At low temperatures, resistance can be hundreds of kilohms; at high temperatures, resistance drops to hundreds of ohms or less.
| Temperature (°C) | Approximate resistance of typical NTC thermistor |
|---|---|
| 0 | ~30 kΩ |
| 25 | ~10 kΩ |
| 50 | ~4 kΩ |
| 100 | ~0.7 kΩ |
How thermistors are used in circuits:
In a voltage divider (potential divider) circuit, a thermistor is paired with a fixed resistor. As temperature rises, the thermistor's resistance falls, altering the voltage share between the two components. This voltage signal can trigger a transistor switch or be read by a microprocessor:
- Thermostat — switches heating off when temperature rises above a set point.
- Temperature sensor in phones, ovens, car engines, and medical devices.
- Fire alarm — high temperature triggers the alarm circuit.
What is an LDR and how does it work?
A light-dependent resistor (LDR) has a resistance that decreases as light intensity increases. In darkness the resistance can be megaohms; in bright sunlight it falls to hundreds of ohms.
| Light condition | Approximate LDR resistance |
|---|---|
| Darkness | 1–10 MΩ |
| Dim room | 10–100 kΩ |
| Bright daylight | 100–500 Ω |
Uses of LDRs:
- Automatic street lights — when light level falls (dusk), LDR resistance rises, triggering the light to switch on.
- Camera exposure sensors — measuring ambient light to set aperture and shutter speed.
- Burglar alarms — a beam of light keeps the LDR resistance low; breaking the beam raises resistance and triggers the alarm.
- Brightness-adaptive screens — adjusting screen brightness based on ambient light.
Like the thermistor, an LDR is most useful in a potential divider circuit where its changing resistance produces a changing voltage signal.
Frequently asked questions
What is the difference between an ohmic and a non-ohmic component?
An ohmic component obeys Ohm's law: its resistance remains constant regardless of voltage or current, and its IV graph is a straight line through the origin. A non-ohmic component has a resistance that changes with conditions — temperature, light level, or direction of current — so its IV graph is curved or asymmetric. Common examples of non-ohmic components at GCSE include filament lamps, diodes, thermistors, and LDRs.
Why does the resistance of a filament lamp increase with voltage?
As current flows through the tungsten filament, it heats up. Higher temperature causes the metal ions in the lattice to vibrate more vigorously, increasing the frequency of collisions with charge-carrying electrons and so increasing resistance. This explains the curve in the IV graph: at low voltages the filament is cool and resistance is low (steep gradient), but at high voltages it is very hot and resistance is much higher (shallow gradient), so the graph curves rather than remaining straight.
What is the threshold voltage of a diode?
A diode begins to conduct significantly only when the forward voltage across it exceeds approximately 0.6–0.7 V (for a silicon diode). Below this threshold, the resistance is so high that current is negligible. Above this voltage, the resistance drops dramatically and current rises steeply. In reverse bias the diode maintains an extremely high resistance, preventing current from flowing in the opposite direction. This directional behaviour makes diodes essential for rectification and in LEDs.
How do thermistors and LDRs differ in what they sense?
A thermistor responds to temperature — its resistance decreases as temperature increases (for an NTC type). An LDR responds to light intensity — its resistance decreases as light level increases. Both are used in potential divider circuits to convert a physical stimulus (heat or light) into a changing voltage signal that can trigger or control other parts of a circuit. Thermistors are found in thermostats and temperature alarms; LDRs are found in automatic lights and camera exposure sensors.
For Socratic GCSE physics with Professor Newton — predicting IV curve shapes from first principles before sketching a single graph — visit aitutors.me.