i-v curves of non-ohmic devices — VCE physics year 11, live

I-V curves of non-ohmic devices

An I-V graph plots the current through a component against the voltage across it. An ohmic conductor, such as a fixed resistor or a potentiometer left at one setting, gives a straight line through the origin because its resistance stays constant. Non-ohmic devices bend that line: a filament bulb flattens as it heats, a diode or LED passes almost no current in reverse or below a switch-on voltage, and a thermistor or LDR changes its slope with temperature or light. At every point the resistance is still R = V/I and the power is P = VI. This is the real knowscape from knowhere, not a picture of one. Drag it. Watch what actually changes.

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why this one carries the topic.

Charge is already in every wire and is never used up: it all moves together, round the loop. The supply gives each coulomb energy, measured as voltage, and the loads take it off. Current is coulombs per second; power is voltage times current. Series, parallel and safety switches are that one trade.

Pick a component and change the voltage: its I-V curve traces out from the origin, an amber chord from the origin shows R = V/I at that point, and a small test circuit shows charge flowing at a rate set by the current. Changing the temperature, light level or knob setting tilts the thermistor, LDR and potentiometer lines.

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An ohmic device, such as a fixed resistor or a potentiometer at one setting, has a straight-line I-V graph through the origin, so R = V/I stays constant.

A filament bulb's I-V curve flattens at higher voltage because the hot filament has more resistance, so equal steps in V give smaller steps in I.

A diode or LED conducts one way only: almost no current flows until a switch-on voltage (about 0.7 V for silicon, about 1.8 V for a red LED), then current rises steeply.

An NTC thermistor's resistance falls as temperature rises and an LDR's falls as light increases, so their I-V lines get steeper when hotter or brighter.

At any point on any curve, find resistance with R = V/I from that point's coordinates (not the slope of a tangent) and power with P = VI.

In the lab, use an ammeter in series, a voltmeter in parallel, a variable supply or potentiometer to step V, and a protective resistor with diodes and LEDs.

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knowhere maps every concept to what it rests on and what rests on it — 4 underneath this one, 0 built on top. Each one says why, in a sentence, not as an arrow on a diagram.

this concepti-v curves of non-ohmic devicesPick a component and change the voltage: its I-V curve traces out from the origin, an amber chord from the origin shows R = V/I at that point, and a small test circuit shows charge flowing at a rate set by the current.

sits under itohm's law and resistancebecause An I-V curve is read against Ohm's law: a straight line means constant R, a bend means R is changing.

sits under itcurrent as flowing chargebecause Current is the rate charge flows, which the moving charges in the test circuit show.

sits under itelectrical power and energy in circuitsbecause Power P = VI at each point explains why a bulb heats up and its resistance rises.

sits under itseries and parallel circuitsbecause Tracing a curve needs the ammeter in series and the voltmeter in parallel with the component.

5 more, same treatment.

Each one is its own knowscape in the app — built for how a particular student takes things in, not one explanation handed to everybody.

household circuit with fuses and earthin the appcurrent as flowing chargein the appohm's law and resistancein the appelectrical power and energy in circuitsin the appseries and parallel circuitsin the app

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