What Changes the Resistance of a Wire, Grade 9
The standard Grade 9 resistance experiment cannot produce a result, and it is worth knowing why before you set it up.
It asks a class to compare 5 cm each of copper, steel, aluminium and nichrome in a circuit with a bulb. Three of those four wires are electrically invisible next to the bulb, so three groups get the same reading and the ranking question has no answer.
This page gives the version that works, using nichrome and a lot more of it, plus the four factors that change resistance and why the current against length graph in most textbooks is the wrong shape.
The four factors
| Factor | Increase it and resistance ... | Why |
|---|---|---|
| Length | Increases | The charge travels further and collides with more particles on the way |
| Thickness | Decreases | More room to flow. A wide pipe carries more water than a narrow one |
| Material | Depends | Silver, copper and gold conduct best. Nichrome resists about 65 times more than copper, which is why heaters are made of it |
| Temperature | Increases, for metals | Hotter particles vibrate more and get in the way |
Where this fits in the curriculum
| Subject | Natural Sciences |
|---|---|
| Grade | 9 |
| Term | 3 |
| Strand | Energy and change |
| Topic | Topic 12, Resistance |
| Status | No assessed task. Two Skills focus sections: measuring, and analysing data |
| Marks | None prescribed. Our worksheet is 40 marks |
Why the textbook version fails
Worked for 5 cm of 0,2 mm wire, in a circuit whose bulb is about 8 Ω:
| 5 cm sample | Its resistance | Effect on the ammeter |
|---|---|---|
| Copper | about 0,03 Ω | None you can measure |
| Aluminium | about 0,05 Ω | None you can measure |
| Steel | about 0,2 Ω | Barely |
| Nichrome | about 1,8 Ω | The only one that moves the needle |
Three of the four change the total circuit resistance by under 3 %, which is smaller than the reading error on a school ammeter. The same problem sinks the thickness investigation, which uses four copper wires.
The version that works
Use nichrome, use 50 cm of it, and tape it along a ruler.
| Wire | Resistance per metre | At 50 cm |
|---|---|---|
| 0,15 mm nichrome | about 62 Ω | about 31 Ω |
| 0,25 mm nichrome | about 22 Ω | about 11 Ω |
Length
- Clip one lead at 0 cm on the taped wire
- Slide the second clip to 10 cm and read the ammeter
- Repeat at 20, 30, 40 and 50 cm
- Plot current against length
Thickness
Same length, 50 cm, of each gauge. The thicker wire passes about three times the current. That is a result a class can see without squinting at a needle.
Taping the wire to a ruler is the trick that makes this practical work. It turns "measure the length" into "read the scale", it stops the wire kinking, and a kink is a permanent thin spot that overheats.
Your graph will be a curve, and the textbook's is not
Many Grade 9 textbooks teach data analysis with this example: current against length of copper wire, falling in a straight line from 5 A at 3 cm to 1 A at 15 cm, reaching zero at about 18 cm.
Every part of that is impossible.
| The claim | The problem |
|---|---|
| The line is straight | Current = voltage ÷ total resistance. As the wire gets longer the current falls steeply at first and then flattens. It is a curve |
| It reaches zero at 18 cm | Current never reaches zero while the circuit is complete. Past 18 cm the straight line predicts negative current |
| 5 A at 3 cm | Not from a 4,5 V battery through a bulb |
| 18 cm of copper stops the current | 18 cm of copper is about 0,1 Ω. It stops nothing |
The conclusion the book draws is right: longer wire, more resistance. The data it uses to teach analysis would fail its own analysis, and asking a class to spot that is a better lesson than copying the graph.
Types of resistor worth knowing
| Component | What it does | Where you meet it |
|---|---|---|
| Fixed resistor | One set value | Inside every appliance |
| Variable resistor | Adjustable by a slider or dial | Volume controls, dimmer switches |
| LDR | Resistance falls in bright light, rises in the dark | Streetlights, security lights, phone screen brightness |
| Diode | Conducts in one direction only | Power supplies. An LED is a diode, not a resistor |
Cover an LDR with your hand and the resistance climbs. That is how a streetlight knows it is dark, and it takes five seconds to demonstrate.
Safety
- Nichrome gets hot. At short lengths on 4,5 V it will be warm and can burn. Take the reading, then disconnect
- Do not leave the circuit closed between readings
- Never connect an ammeter directly across a battery. An ammeter is almost zero resistance, so that is a short circuit through the instrument. Most textbooks do not say this, and it is how school ammeters die
- Tape bare wire to a ruler rather than leaving it loose on the bench
If it does not work
| Problem | Cause | Fix |
|---|---|---|
| Every material reads the same | The 5 cm textbook design | Nichrome, 50 cm |
| The current does not change with length | The clip is gripping tape or oxide | Sand the wire lightly and clip firmly |
| The reading drifts down as you watch | The wire is heating up | That is the fourth factor appearing in your data. Read quickly, then disconnect |
| The needle slams across | Too short a length | Start at 50 cm and work down |
| The graph looks straight | Too few points, all bunched together | Five points spread across the full range |
| No current at all | Broken connection | Check each clip is on bare metal |
Free worksheet and marking memo
Both free, no sign up, straight to the PDF.
- Learner worksheet, 40 marks. Part A is the four factors, conductors and insulators, and special resistors. Part B is the length investigation, the graph, and why a straight line cannot be right
- Marking memorandum, with typical nichrome readings and the three answers that look right and score nothing
Related practicals
- Series and parallel circuits, Grade 9. The next topic, with the same battery and meters
- Ohm's law, Grade 11. Where R = V/I properly belongs, with the graph to prove it
- Resistors in series and parallel, Grade 10
- Types of forces, Grade 9. The other Term 3 topic
What you need to run it
A roll of nichrome, an ammeter, a battery and some clips.
Bare nichrome wire is R40 for 10 m of 0,15 mm and R48 for the 0,25 mm. One roll cuts into sixteen 60 cm samples, so the wire costs about R2,50 a group. Buy both gauges: the thickness comparison needs two, and they are the only part of the practical that cannot be improvised.
The ammeter is the real purchase. R295, and it is used again by the Grade 9 circuits practical, Grade 10 electricity, Grade 11 Ohm's law and Grade 12 internal resistance. Everything else is in the electricity collection: battery holders at R6,50 and crocodile leads at R58 for ten.
We do not currently stock LDRs, diodes or small fixed resistors. The chapter teaches all three, and a component pack is on our list.