Electric Cells and the Lemon Battery, Grade 9
The lemon battery fails in almost every classroom for one reason: the nail.
Textbooks and internet guides tell you to use a steel or iron nail. Iron with copper gives about 0,5 V per lemon, so three lemons make about 1,5 V, and a red LED needs about 1,8 V before it lights at all. Nothing happens, and the class decides the experiment is a myth.
Use a galvanised nail and do not sand it. Galvanised means zinc-coated, zinc with copper gives about 0,9 V a lemon, and three of them light an LED. This page covers how a cell actually works, how to build one that works, and why voltage is not the amount of energy stored in a battery.
Where this fits in the curriculum
| Subject | Natural Sciences |
|---|---|
| Grade | 9 |
| Term | 3 |
| Strand | Energy and change |
| Topic | Topic 11, Electric cells as energy systems |
| Status | No formally assessed task |
| Marks | None prescribed. Our worksheet is 40 marks |
What a cell is made of
| Part | Its job | In a lemon cell |
|---|---|---|
| Negative electrode | Gives up electrons | The zinc coating on a galvanised nail |
| Positive electrode | Takes up electrons arriving through the circuit | Copper wire or a copper coin |
| Electrolyte | Lets charge move inside the cell | The lemon juice |
Two different metals plus an electrolyte is a cell. The lemon is not the battery, it is only the electrolyte, which is why a potato, an apple or a glass of salt water works just as well.
Building a lemon battery that works
What you need: three or four lemons · galvanised nails, one per lemon · bare copper wire or copper coins · crocodile leads · a voltmeter · a red LED
- Roll each lemon firmly on the bench to break the juice sacs inside
- Push a galvanised nail into one side and a piece of copper into the other, about 3 cm apart and not touching
- Connect the voltmeter: red lead to the copper, black to the nail. Read it
- Connect the copper of one lemon to the nail of the next to put them in series
- Read the voltage again after each lemon you add
- With three lemons, connect the LED in a darkened corner. Long leg to the copper end
| Lemons in series | Voltage | LED |
|---|---|---|
| 1 | about 0,9 V | No |
| 2 | about 1,8 V | Only just, if at all |
| 3 | about 2,7 V | Yes, dimly |
Measure it with a voltmeter before you try the LED. A number on a dial is a better result than a faint glow, and it is the same measurement the next topic asks for.
Why the nail decides everything
| Metals in the lemon | Per lemon | Three in series | Will it light a red LED? |
|---|---|---|---|
| Plain steel or iron with copper | about 0,5 V | about 1,5 V | No |
| Galvanised nail with copper | about 0,9 V | about 2,7 V | Yes |
Zinc gives up electrons far more readily than iron does, so the zinc-copper pair produces a much bigger voltage. That pair is what a real cell uses, and it is why the experiment became famous in the first place.
Do not sand the nail. Several instructions tell you to. Sanding removes the zinc coating and turns a galvanised nail back into a plain steel one.
Voltage is not the energy stored in a battery
Many textbooks define voltage as "the amount of energy stored in a cell". One comparison disproves it.
| AA cell | D cell | |
|---|---|---|
| Voltage | 1,5 V | 1,5 V |
| Energy stored | about 2 500 mAh | about 12 000 mAh |
| Size | Small | Much larger |
Same voltage, roughly five times the energy. If voltage measured stored energy the D cell would read about 7,5 V, and it does not.
Voltage is the energy given to each unit of charge. How much charge the cell can supply before it goes flat is a separate quantity, and that is what makes a D cell last longer.
The lemon makes this concrete: 0,9 V, not far off a real cell, and it cannot run anything for more than a few minutes.
If it does not work
| Problem | Cause | Fix |
|---|---|---|
| The LED does not light | Plain steel nails, or only two lemons | Galvanised nails, three lemons, dark corner |
| Only 0,4 to 0,5 V per lemon | The nail is not galvanised, or it was sanded | Fresh galvanised nail, unsanded |
| The voltmeter reads backwards | Leads swapped | Copper is the positive terminal |
| The reading falls while you watch | Normal. The cell polarises as it works | Record the first steady reading |
| No reading at all | The metals are touching inside the lemon | Keep them about 3 cm apart |
| Every lemon reads low | Dry or old fruit | Roll them hard first. Fresh lemons have more juice |
| The LED lights and then fades | Correct. A lemon supplies almost no current | Nothing to fix. Explain why |
Safety
- Nobody eats the lemons afterwards. They have had metal pushed into them
- Wash hands. The juice is acidic and the nails are zinc-coated
- This is under 3 V and it is electrically harmless
- If you also run the laboratory zinc and copper sulfate cell: copper sulfate is harmful if swallowed and toxic to aquatic life. Goggles on, and it does not go down the drain
Free worksheet and marking memo
Both free, no sign up, straight to the PDF.
- Learner worksheet, 40 marks. Part A is how a cell works, including the voltage question. Part B is the lemon battery investigation with your own readings
- Marking memorandum, with expected voltages and the three answers that look right and score nothing
Related practicals
- Series and parallel circuits, Grade 9. Cells in series and in parallel, with real cells
- Electrochemical cells, Grade 12. Where this goes: the same zinc and copper, with half-reactions and standard potentials
- Internal resistance of a battery, Grade 12. Why the lemon's voltage sags the moment you draw current from it
What you need to run it
About R60, and most of it comes from a supermarket and a hardware store.
| Item | Where | Roughly |
|---|---|---|
| Four lemons | Any shop | R15 |
| Galvanised nails | Hardware store | R20 a bag, and they last for years |
| Bare copper wire | Hardware store | R25 a metre |
The one thing worth buying properly is the voltmeter. A single lemon reads about 0,9 V, so a meter whose lowest range is 0 to 6 V puts that reading in the bottom sixth of the dial where it cannot be read usefully. A dual range meter with a 0 to 1 V scale reads it properly, then switches to 0 to 5 V for three lemons in series. It is R350, and it is used again in Grade 10, Grade 11 and Grade 12.
The rest is in the electricity collection: crocodile leads at R58 for ten, and LEDs at R1 each.
We are out of stock on zinc and copper electrodes, which is what the textbook's laboratory version of this cell needs. Given that the laboratory version produces 1,1 V and cannot light an LED, while three lemons produce 2,7 V and can, we would suggest the lemons even if the electrodes were on the shelf.