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A diode allows current to flow in only one direction, like a one-way valve. Free GCSE AQA Physics notes with key definitions and 15 questions answered.
Non-Ohmic Components: Lamps and Diodes is part of Current, potential difference and resistance in GCSE AQA Higher Physics.
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A diode allows current to flow in only one direction, like a one-way valve. Free GCSE AQA Physics notes with key definitions and 15 questions answered.
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Use this lesson to stay inside Current, potential difference and resistance while connecting the detail back to the wider Electricity topic.
Read the core explanations from Non-Ohmic Components: Lamps and Diodes before testing yourself from memory.
You've learned that for resistors, doubling the voltage doubles the current. The resistance stays constant - that's Ohm's law in action.
But here's where it gets interesting. Some components don't play by these rules. Their resistance changes as you change the current flowing through them.
These rule-breakers are called non-ohmic components. The two most important ones you need to know are filament lamps and diodes.
Think about an old-fashioned light bulb. Inside is a thin tungsten wire called a filament. When you switch it on, current flows through this wire.
Here's what happens: the current heats up the filament until it glows white-hot. But as the temperature rises, something crucial changes - the resistance increases.
This happens because hot metal atoms vibrate more violently. These vibrations get in the way of the flowing electrons, making it harder for current to flow.
The I-V graph for a filament lamp shows this beautifully. At low voltages, the curve is nearly straight - the filament is cool and behaves almost like a normal resistor.
But as voltage increases, the curve starts to bend. The current doesn't increase as much as you'd expect because the resistance is climbing with the temperature.
A diode is like a one-way valve for electricity. It allows current to flow in one direction but blocks it in the other direction.
When connected in the forward direction, a diode has low resistance and current flows easily. But flip it around - connect it in reverse - and it has extremely high resistance.
This creates a very distinctive I-V characteristic. In the forward direction, once you reach about 0.7V, current starts flowing and increases rapidly.
In the reverse direction, virtually no current flows - the diode acts like an open switch. This is why diodes are used in circuits to prevent current flowing the wrong way.
I-V characteristics are graphs that show how current changes with voltage for different components. They're like fingerprints - each type of component has its own distinctive curve.
For a resistor, you get a straight line through the origin. The gradient equals 1/R, so steeper lines mean lower resistance.
For a filament lamp, you get a curve that starts steep but gradually flattens out. This shows the resistance increasing as the current (and temperature) rises.
For a diode, you get a dramatic step. Almost no current flows until you reach the threshold voltage (about 0.7V), then current increases rapidly.
Worked example: Calculating Resistance from an I-V Graph
A filament lamp has 2.0V across it and 0.4A flowing through it. Calculate its resistance at this operating point.
Answer: 5.0 Ω
Understanding non-ohmic behaviour isn't just academic - it explains everyday phenomena you've probably noticed.
Ever wondered why old light bulbs often blow when you first switch them on? When cold, the filament has low resistance, so there's a surge of current. This sudden heating can cause the thin wire to snap.
Diodes are everywhere in modern electronics. Your phone charger uses them to convert AC mains electricity to DC. LED lights are actually diodes that emit light when current flows through them.
Solar panels also rely on diodes. They prevent current flowing backwards into the panel at night, which would drain your battery.
In car alternators, diodes convert the AC generated by the spinning coils into DC to charge the battery. Without them, your car's electrical system wouldn't work.
Examiners love testing your understanding of I-V characteristics. They'll show you a graph and ask you to identify the component or explain the shape.
The key is recognising the patterns: straight line = resistor, curved line that flattens = filament lamp, step function = diode.
When calculating resistance from graphs, always use R = V/I for the specific point they're asking about. Don't assume the resistance is constant.
For 'explain' questions about filament lamps, you must mention that heating increases atomic vibrations, which impede electron flow. Just saying 'it gets hot' won't get full marks.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
A diode allows current to flow in only one direction, like a one-way valve. In the forward direction it has low resistance once the voltage reaches about 0.7V, and in the reverse direction it has very high resistance, so it is used to prevent current flowing the wrong way.
The I-V graph of a filament lamp is a curve that starts steep and gradually flattens out. At low voltages it is nearly straight because the filament is cool, but as voltage increases the resistance climbs with temperature, so the current does not increase as much as expected.
The I-V graph for a diode shows a dramatic step: almost no current flows until the threshold voltage of about 0.7V, then current increases rapidly. In the reverse direction virtually no current flows, so the diode acts like an open switch.
A filament lamp is a light bulb containing a thin tungsten wire filament that heats up and glows when current flows through it. Its resistance increases as the filament gets hotter.
The threshold voltage of a diode is about 0.7 V. Almost no current flows until this voltage is reached in the forward direction, and then the current increases rapidly, giving a step-shaped I-V graph.
Diodes are used to prevent current flowing the wrong way. Phone chargers use them to convert AC mains electricity to DC, solar panels use them to stop current flowing backwards at night, and car alternators use them to convert AC to DC to charge the battery.
An LED is a diode that emits light when current flows through it. Like other diodes, it allows current to flow in only one direction.
The resistance of a filament lamp increases because the current heats the filament, and hot metal atoms vibrate more violently. These vibrations get in the way of the flowing electrons, making it harder for current to flow, so resistance rises with temperature.
Old light bulbs often blow when first switched on because the cold filament has low resistance, so there is a surge of current. This sudden heating can cause the thin wire to snap.
A non-ohmic component is a component whose resistance changes with the current flowing through it, so it does not obey Ohm's law. The two most important examples are filament lamps and diodes.
A diode connected in the forward direction has low resistance, so current flows easily once about 0.7 V is reached. Connected in reverse, it has extremely high resistance and virtually no current flows, so it acts like an open switch.
An I-V characteristic is a graph showing how current (I) varies with voltage (V) for a particular component. Its shape reveals whether the component obeys Ohm's law, like a fingerprint for each type of component.
Components can be recognised from I-V graphs by their shape: a straight line through the origin is a resistor, a curve that flattens out is a filament lamp, and a step with almost no current until about 0.7 V is a diode.
Resistance is calculated from an I-V graph using R = V/I at the specific point asked about, not by assuming it is constant. A filament lamp with 2.0 V across it and 0.4 A through it has R = 2.0 ÷ 0.4 = 5.0 Ω.
A resistor has a straight-line I-V graph through the origin because its resistance stays constant, so doubling the voltage doubles the current. The gradient equals 1/R, so steeper lines mean lower resistance.