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Ohm's law lab: slide the voltage, read the ammeter and voltmeter, plot the V-I graph

A virtual circuit for Ohm's law. Slide the battery voltage, read the ammeter and voltmeter, record readings and plot V against I. The slope is the resistance. Compare a resistor with a filament bulb, and work out a wire's resistance from its length, thickness and metal.

Keep it one tap away for everyday use.

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  1. Set volts
  2. Read meters
  3. Plot V-I
  4. Check
Nothing is uploadedWorks offline once open

Set up the circuit

V-I graph

your readingsbest straight linering: the reading now

Try this

  • Take 6 readings with the resistor. The points sit on a straight line through zero, and its slope is the resistance.
  • Switch to the bulb and take 6 readings. The line bends: V/I grows as the filament heats up.
  • With the wire, double the length. The current halves at the same voltage, because R doubles.
  • Change the wire from 0.3 mm to 0.6 mm thick in your head: the area is four times bigger, so R is a quarter. Try 0.2 mm and 0.4 mm here.
  • Try a copper wire: its resistance is so small that the current is huge. That is why copper is used for connecting wires.

Check yourself

What it does

This is a practice bench for Ohm’s law. A battery, an ammeter, a part and a voltmeter are wired up for you. Slide the voltage and watch the ammeter needle move, the electrons drift faster round the circuit and the reading change. Record a few readings and the page plots the V-I graph and draws the best straight line through them. For a resistor the points lie on a straight line through zero, and the slope of that line is the resistance. For a filament bulb they do not: the line bends, because the hot filament has more resistance than the cold one.

It follows the electricity topics of Class 10 Science (Ohm’s law, the factors a wire’s resistance depends on, resistivity) and the current electricity topics of Class 12 Physics (current density, conductivity, drift speed of electrons, materials that do not obey Ohm’s law). The Level switch adds the Class 12 quantities. When you have the idea, connect resistors together in the circuit simulator, or read real resistor values with the resistor colour code.

How to use it

  1. Pick a level: Class 10 shows the readings and V = IR; Class 12 also shows power, and for a wire the current density, conductivity, electric field and (for copper) the drift speed of the electrons.
  2. Pick what is in the circuit: a resistor (choose 1 to 50 Ω), a bulb, or a wire (choose the metal, its thickness and its length).
  3. Slide the battery voltage from 0 to 12 V. The ammeter, voltmeter and the formula line update straight away.
  4. Tap Record this reading at several voltages, or Take 6 readings to record 2, 4, 6, 8, 10 and 12 V in one go. The graph shows your points and the best straight line; the text under it gives the slope (the resistance) or tells you the points bend.
  5. Use Check yourself at the bottom for fresh questions with new numbers each time.

What each part means:

  • Ammeter: measures the current, in amperes, and is connected in series. It reads to 0.001 A below 1 A and to 0.01 A above.
  • Voltmeter: measures the potential difference across the part, in volts, and is connected in parallel with it. It reads to 0.1 V.
  • Resistance now: V ÷ I for the reading you are looking at.

Worked example

Choose the wire: nichrome, 0.3 mm thick, 1.00 m long. The area is A = π d² / 4 = π × (0.0003 m)² / 4 = 7.07 × 10⁻⁸ m², so R = ρL/A = 1.1 × 10⁻⁶ × 1.00 ÷ 7.07 × 10⁻⁸ = 15.6 Ω. At 6.0 V the ammeter reads 0.386 A (6 ÷ 15.6). Take 6 readings and the slope comes out at about 15.6 Ω. Now slide the length to 2.00 m: the resistance doubles to 31.1 Ω and the current at 6 V halves to 0.193 A.

With the bulb, 6 readings give V/I from about 6 Ω at 2 V up to about 19 Ω at 12 V, and the page says the points bend.

Tips

  • The V-I graph here has the current across the bottom and the voltage up the side, so the slope is V/I = R. Some books draw it the other way round; then the slope is 1/R.
  • Changing the part, the resistance or the wire clears the readings, so one graph is always one part.
  • A copper wire has so little resistance that the current gets huge. A real cell could not supply it, and the page warns you.

FAQ

Why does the bulb not obey Ohm’s law? Ohm’s law needs the temperature to stay the same. A filament gets white hot, and the resistance of a metal rises with temperature, so V/I is bigger at higher voltages.

Is the bulb a real one? No. It is a simple teaching model (2 Ω cold, 12 Ω at 6 V) chosen to show the shape of the curve.

Is anything saved? No. Your readings and answers stay on this page and are gone when you close it.

Where the numbers come from

  • Ohm's law V = IR, R = ρL/A, current density J = I/A = σE and drift speed v = I/(neA): standard school physics results.
  • Resistivities at about 20 °C rounded to two figures (copper 1.7 × 10⁻⁸ Ω m, nichrome 1.1 × 10⁻⁶ Ω m and so on) and one free electron per copper atom (8.5 × 10²⁸ per m³): general textbook values, rounded.
  • The filament bulb is a made-up teaching model (2 Ω cold, 12 Ω at 6 V), not data for a real bulb.
  • Checked against an independent NumPy script (np.polyfit for the best line, the wire and drift-speed formulas) and the textbook drift speed of 1.1 × 10⁻⁴ m/s for 1.5 A in 1 mm² of copper.

Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.

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Last updated 2026-11-09.

Change log for Ohm's law lab

  1. Add #1041 Ohm's law lab Details on GitHub

Every change to fibub

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