The Ratio That Holds Still

You'll learnwhat V over I defines, when it stays put as the voltage moves, how to read resistance off a graph without inverting it, and why doubling a wire's radius quarters its resistance.

Put a source across any device, measure the volts and the amps, and divide: that ratio is its resistance. For some devices the ratio holds still while the voltage sweeps, and that steadiness is Ohm's law. For a lamp filament it does not hold at all. Twelve short steps; press Next for each one.

12 STEPS · 6 QUICK CHECKS · R = V/I · THE LAW · THE SLOPE · LENGTH AND RADIUS · v2

R = V/I defines · Ohm's law says it holds still · steeper line, smaller R

Before you start

What you're looking at

A device under test: an ammeter in series, a voltmeter across it, and a graph of the current it draws against the volts applied.

The question

What does the ratio V over I tell you, and when does it stay put as the voltage changes?

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