Ohm's Law: Current, Voltage and Resistance
Ohm's Law states that the potential difference across a conductor is directly proportional to the current through it, at constant temperature. Those last four words are the whole point, and most explanations leave them out. This page covers the law, the difference between an ohmic and a non-ohmic conductor, and the Grade 11 experiment that proves both at once: a nichrome wire that obeys the law and a light bulb that does not.
The distinction almost everyone blurs
| Always true? | What it is | |
|---|---|---|
| R = V ÷ I | Yes, always | The definition of resistance. It applies to any component at any moment |
| V proportional to I | No | Ohm's Law. Only true for an ohmic conductor at constant temperature |
You can work out the resistance of a light bulb at any instant with R = V ÷ I. But that resistance changes as you change the voltage, so the bulb does not obey Ohm's Law.
Try it: change any two and the third follows
Drag the sliders. This is R = V ÷ I rearranged, and it holds for any component at any moment. It is not Ohm's Law.
V = IR
I = V ÷ R
4.5 V ÷ 18.8 Ω
0.24 A
Three cells across 30 cm of nichrome wire
Ohm's Law is not a law that everything obeys. It describes how some materials behave, under some conditions. That is exactly what the experiment below demonstrates.
Where this fits in the curriculum
| Subject | Physical Sciences |
|---|---|
| Grade | 11 |
| Term | 3 |
| Topic | Electricity and magnetism, electric circuits |
| Marks | 50 |
Ohmic and non-ohmic conductors
An ohmic conductor has a resistance that stays constant over a wide range of voltages and currents. Plot voltage against current and you get a straight line through the origin. Metals at constant temperature are the usual example, and a nichrome wire is the classic one.
A non-ohmic conductor has a resistance that changes with the voltage across it. The graph is curved. A light bulb is the everyday example, and a diode is the extreme one.
The experiment: two parts, one point
Both parts use the identical circuit. Only the component changes.
Apparatus
| Item | Qty |
|---|---|
| Ammeter, 0 to 1 A | 1 |
| Voltmeter reading at least 4,5 V | 1 |
| Knife switch | 1 |
| Connecting leads | 6 |
| Battery holders | 2 |
| Nichrome wire, 30 cm | 1 |
| Small light bulb and holder | 1 |
You supply three 1,5 V torch cells. We do not ship them, because cells posted today are stale by the time the practical runs and every school already has them.
Part 1: the nichrome wire
- Connect one cell, the switch, the ammeter and a 30 cm length of nichrome wire in series, with the voltmeter in parallel across the wire only.
- Close the switch just long enough to read both meters. Record the voltage and the current, then open it again.
- Add a second cell in series. Read and record.
- Add a third cell. Read and record.
- Plot V on the vertical axis against I on the horizontal.
Roughly what to expect with 0,15 mm wire, which is about 19 ohm over 30 cm:
| Cells | Voltage | Current | V ÷ I |
|---|---|---|---|
| 1 | 1,5 V | 0,08 A | 18,8 |
| 2 | 3,0 V | 0,16 A | 18,8 |
| 3 | 4,5 V | 0,24 A | 18,8 |
A straight line through the origin, and V ÷ I is constant. That constant is the resistance, and it is also the gradient of the line. A class that measures the gradient has found the resistance without ever using the formula.
Part 2: the light bulb
Swap the nichrome wire for the bulb. Change nothing else. Repeat with one, two and three cells.
| Cells | Voltage | Current | V ÷ I |
|---|---|---|---|
| 1 | 1,5 V | 0,17 A | 8,8 |
| 2 | 3,0 V | 0,23 A | 13,0 |
| 3 | 4,5 V | 0,27 A | 16,7 |
A curve that gets steeper, and V ÷ I nearly doubles across the range. The current does not double when the voltage does.
Why? More current heats the filament. A hotter metal has a higher resistance. So each extra volt buys less extra current than the one before, and the temperature condition in Ohm's Law has been broken.
A quick check that your readings are sensible. Extend the trend to the rated 6 V and the resistance reaches about 20 ohm, which is exactly what a 6 V, 0,3 A lamp should be.
Put the two graphs side by side. Straight against curved is the entire distinction between ohmic and non-ohmic, and no amount of explanation lands like seeing it.
Two things that decide whether this works
The voltmeter must read to at least 4,5 V
Three cells in series give 4,5 V. A 0 to 3 V meter pins before the last reading, and that is the meter most schools reach for because it is the one used in the Grade 12 internal resistance practical.
Use a 0 to 5 V, 0 to 6 V, or a dual-range meter. A 0 to 10 V meter technically works but 4,5 V on a 10 V scale is too coarse to read accurately.
The bulb must be rated 6 V, not 2,5 V
A 2,5 V bulb will blow at 4,5 V, which is the third reading. A 6 V bulb survives the full range and still shows the resistance climbing clearly.
It should be dim but visibly lit on one cell. If it is completely dark, the cells are flat.
Safety
- Only close the switch while you are taking a reading. The nichrome wire heats up with current flowing, and a hot wire has a different resistance
- The wire gets genuinely hot on three cells. Do not hold it with the switch closed
- Check the ammeter is in series before closing the switch. An ammeter across a battery is a short circuit
- Low voltage cells cannot give a dangerous shock, but a shorted battery gets hot fast
- Flat cells go to battery recycling, not general waste
If it does not work
| What you see | What caused it |
|---|---|
| Readings drift while you watch | The switch has been left closed and the wire is heating. Close, read, open |
| The straight line bends | Same cause. Heating the nichrome makes it non-ohmic too |
| The voltmeter pins at full scale | Wrong range. Three cells give 4,5 V |
| A needle reads backwards | Meter polarity reversed. Positive terminal towards the positive of the battery |
| The ammeter reads nothing | It is wired in parallel instead of in series, or the switch is open |
| Current too small to measure | The wire is too thick or too short. 30 cm of 0,15 mm gives 80 to 240 mA |
| You never quite get 1,5 / 3,0 / 4,5 V | Normal. Torch cells sag under load. It does not matter, because you plot the voltage you actually measured against the current you actually measured |
| The bulb blows | It was a 2,5 V bulb, not a 6 V one |
The failure worth keeping
If a class leaves the switch closed and their straight line bends, that is not a spoiled result.
They have just demonstrated the temperature clause by accident. A nichrome wire is ohmic only while its temperature is constant. Heat it and it stops behaving. That is a better lesson than a clean graph, and it is worth stopping the lesson to point out.
How the 50 marks are made up
| Section | Marks |
|---|---|
| Circuit diagram and method | 9 |
| Part 1, the nichrome wire | 8 |
| Part 2, the light bulb | 6 |
| Graphs | 12 |
| Interpretation | 11 |
| Conclusion and evaluation | 4 |
The mark most often dropped is leaving at constant temperature out of the statement of Ohm's Law. It is worth a mark on its own, and it is the clause this entire experiment exists to demonstrate.
If you have time
Halve the wire. Cut it to 15 cm and the resistance halves, so the current doubles at the same voltage. Resistance is proportional to length.
Try a diode. It is the most extreme non-ohmic component there is: almost no current one way, plenty the other. It makes the bulb's gentle curve look tame.
Free worksheet and marking memo
Both free, no sign up, straight to the PDF.
- Learner worksheet, 50 marks, covering both parts with the circuit diagram space, results tables, graph questions and interpretation set out
- Marking memorandum, with a worked sample set for both components, the mark allocation and a note on the six places learners most often drop marks
Related practicals
- Internal resistance of a battery, Grade 12. Nearly the same box, one grade later. Note that it uses a 0 to 3 V voltmeter, not this one
- Series and parallel circuits, Grade 10. The practical that comes before this one
- Newton's Second Law, Grade 11. The other Grade 11 practical with a graph at the centre of it
Buy this experiment
We are putting together a complete Ohm's Law Kit for Grade 11 with the meters, switch, leads, battery holders, bulbs and nichrome wire in one box, plus a printed teacher guide and the marking memo. Coming shortly.
In the meantime every item in the apparatus table above is in stock and sold separately. The nichrome wire is the one schools forget: a 10 m roll cuts into more than thirty lengths and costs less than a set of leads.