What a learner can do afterwards
- Builds a test circuit with an ammeter in series and a voltmeter across the component
- Plots current against potential difference for both directions and describes the shape
- Explains the lamp's curve by its filament heating up, and the diode's shape by current in one direction only
1 · Read
To test a component you build a circuit with the ammeter in series and the voltmeter across the component. Change the supply step by step and record each current with its voltage. Those pairs become the dots of your current against voltage graph.
A fixed resistor gives a straight line through zero, because current rises in step with voltage. The steepness of the line is set by the resistance. A steep line means low resistance, since more current flows for each volt.
A filament lamp starts like a resistor, then its graph bends flat at higher voltages. The filament heats up, its resistance climbs, and current rises more slowly. A diode is stricter: it passes current one way and blocks it the other, so its graph stays near zero on the reverse side and climbs fast on the forward side.
Reverse the supply and the resistor and lamp graphs mirror through zero. The diode keeps its flat side on reverse, which is how you spot it. A lopsided graph means a diode, not a resistor or a lamp.
Straight means steady resistance, a bend means heating, and lopsided means a diode.
More to read
20.2 Ohm’s Law: Resistance and Simple Circuits
Ohm's law says the current through a metal wire at steady temperature grows in step with the voltage across it. Double the voltage and you double the current, so plotting current against voltage gives a straight line through zero. The steepness of that line is set by the resistance. A steep line means an easy path with low resistance, while a shallow line means a harder path with high resistance.
Not every component behaves this way. A filament lamp heats up as current flows, its resistance climbs, and the graph bends flat at higher voltages. A diode is even stricter: it lets current through one way and blocks it the other, so its graph stays near zero on the reverse side and shoots up on the forward side.
20.1 Current
Current is the rate at which charge flows past a point, measured in amps. In a metal wire the moving charges are electrons drifting from the negative end toward the positive end. The current depends on how many carriers there are, how fast they drift, and how wide the wire is.
To plot an IV graph you need pairs of readings, so you put the ammeter in series with the component and the voltmeter across it. Then you change the supply step by step and record each current with its voltage. Those pairs become the dots on your graph, and the shape of the dots tells you which component you tested.
Diode as a circuit element
A diode is a one way street for current. Forward, with the positive lead on the anode side, it opens up once the voltage passes a small threshold and current climbs fast. Reverse the leads and it slams shut, letting through almost nothing until extreme voltages.
That is why its IV graph looks so lopsided. One side is flat along the axis and the other side curves sharply upward. If you see that shape on a test plot, you are looking at a diode, not a resistor or a lamp.
Basic electrical quantities: current, voltage, power
Voltage pushes, current flows, and resistance gets in the way. For an ohmic resistor these three lock together in Ohm's law, so any one of them follows from the other two. Power is how fast energy moves, found by multiplying voltage by current.
IV graphs turn these links into pictures. If voltage and current stay proportional the picture is a straight line and the resistance is constant. When heating or a junction breaks that proportion, the line bends, and the bend itself is the clue about what is happening inside.
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2 · Watch
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Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.