The science idea
Every battery you have ever used, from the cell in a clock to the battery in a phone, is a way of turning chemical energy into electrical energy. The first cells, made by Alessandro Volta around 1800, were just two different metals separated by a damp pad. A lemon cell is the same idea in a fruit: two different metals dipped into an acid.
Zinc is more reactive than copper, which means that zinc atoms give up their electrons more easily. At the zinc, each atom lets go of two electrons and goes into the juice as a zinc ion: Zn → Zn²⁺ + 2e⁻. The electrons are left on the zinc, which makes it the negative terminal. If a wire joins the zinc to the copper, the electrons travel through the wire towards the copper, and that flow of electrons is the electric current. At the copper, the hydrogen ions (H⁺) from the acid collect the electrons and form hydrogen gas: 2H⁺ + 2e⁻ → H₂. The copper is the positive terminal. The copper itself does not take part; it only provides a surface for the reaction.
The juice has two jobs. It contains ions (citric acid in water), which move through the fruit to carry charge between the electrodes. If the ions could not move, charge would pile up and the reaction would stop. So the fruit is not the source of the energy; it is the electrolyte, the conducting liquid. The energy comes from the zinc, which is slowly used up. A galvanised nail is steel coated with zinc, which is why it works as the zinc electrode.
One lemon gives only about 0.8 to 1 volt, and it can supply only a very small current, about a milliampere at most, because the juice and the thin electrodes have a high resistance (the internal resistance of the cell). That is why a single lemon cannot light an LED, which needs about 1.8 to 2 V. Cells joined in series add their voltages: connect the copper of one lemon to the zinc of the next. Three or four lemons in series give 2.5 to 3.5 V and make an LED glow faintly in a dim room, and two lemons run a small digital clock that is made for a 1.5 V cell.
The reading changes with the fruit, the metals, the depth of the electrodes and the distance between them, which makes this an excellent investigatory project: you can change one variable at a time, measure the voltage with a multimeter, and draw graphs. The voltage also falls as the cell works, because hydrogen bubbles collect on the copper and the zinc surface changes.
Materials and tools
Cost: Under Rs 100
Lemons, a galvanised nail and a piece of bare copper wire are household or hardware-shop items. A red LED costs a few rupees. A multimeter and a small digital clock can be borrowed from school or home.
| What you need | How many | Free substitute or where to find it |
|---|---|---|
| Lemons (or any other juicy sour fruit) | 4 (plus other fruits to compare) | From the kitchen. Soft, juicy and not dried-out ones are best. |
| Galvanised (zinc-coated) iron nails, about 5 cm long | 4 | From the toolbox or a hardware shop. They should look silvery-grey. Plain shiny steel nails are not zinc-coated. |
| Bare copper wire or strips (about 1.5 mm thick, 8 cm long) | 4 pieces | Strip the plastic from a piece of old thick electrical wire or take a strip of old copper sheet. Ask an adult to strip it with a cutter. |
| Connecting wires with crocodile (alligator) clips, or thin insulated wire | 6 to 8 pieces | Thin insulated wire with 1 cm bare at both ends, held with tape or clothes-pegs. |
| Red LED (5 mm) | 1 | Salvage from an old toy, or buy one (a few rupees). A high-brightness red LED glows best. |
| Small LCD digital clock or calculator that runs on one 1.5 V cell (optional) | 1 | An old wall clock or a cheap digital clock. Take out its cell and note the + and − contacts. |
| Digital multimeter (optional but very useful) | 1 | Borrow from school or a relative. Set it to the DC volts range, 2 V or 20 V. |
| Sandpaper or a rough stone | 1 small piece | To brighten the copper wire. |
Tools
- Knife for any fruit cutting (adult only)
- Ruler to mark spacing
- Notebook and pen for readings
- Dish soap and cloth to clean the nails
- Sticky tape or clothes-pegs to hold the wires
Safety first
Read this before you start
- This is a very low-voltage project: the voltage is under 4 V and the current is tiny, so there is no risk of electric shock. Never connect it to any mains supply.
- Do not eat or drink anything that has had a nail or copper in it. The metals (and the zinc coating) get into the juice. Throw the used fruit away after the experiment and wash your hands.
- Lemon juice can sting your eyes and cuts on your hands. Wash it off with water if it goes into your eyes, and ask an adult to use any knife.
- Do not put a clock or a toy that was made for a single 1.5 V cell on more than two lemon cells. A higher voltage can damage it.
- Keep copper wire ends and nails away from small children. Sharp points of nails can hurt.
Parts and layout
These drawings show the parts and how they fit together. Keep them beside you while you build.
Scroll sideways to see the whole diagram.
Scroll sideways to see the whole diagram.
Step-by-step build
12 steps. Do them in order, and test each part before you go on to the next.
Soften the lemons
Roll each lemon firmly on the table, pressing with your palm, for about 30 seconds. This breaks the juice sacs inside so that the juice flows and carries ions between the electrodes.
Tip: Do not cut the lemon open. Only the metals need to go inside, and an uncut lemon holds the electrodes firmly.
Clean the electrodes
Wash the galvanised nails with soap and water and dry them. Rub the copper wire with sandpaper until it is bright. Do not sand the zinc coating off the nails.
Common mistake: Using nails that are not zinc-coated. A bright steel nail is not galvanised, and the cell will give a much lower voltage.
Insert the electrodes
Push a nail into one end of the lemon about 3 cm deep, and a copper strip about 3 cm away on the same side, also about 3 cm deep. Do not let them touch each other inside the lemon.
Common mistake: Pushing the nail and copper too close together or through the other side. If they touch, the cell is short-circuited and gives no useful voltage.
Measure one cell
Set the multimeter to the DC volts range. Touch the red probe to the copper and the black probe to the zinc nail. Write the reading in your table. If the reading is negative, the probes are the wrong way round.
Tip: A good single cell is about 0.8 to 1 V. Do not push the probe deep into the fruit: touch the metal above the fruit.
Try one lemon on the LED
Connect the LED across the copper and zinc of one lemon with the long leg to the copper. Watch it in a dim room. It will not light, because one cell is below the LED’s start voltage.
Tip: This is a result, not a failure. Record it: one lemon cannot light an LED.
Make three cells
Prepare three lemons the same way (steps 1 to 4) and check the voltage of each. Lemons that differ a lot probably have electrodes inserted differently.
Common mistake: Reusing the same hole in a lemon that has been used before. The metals need fresh juice.
Join the cells in series
With a wire, connect the copper of lemon 1 to the zinc of lemon 2, and the copper of lemon 2 to the zinc of lemon 3. Leave the zinc of lemon 1 and the copper of lemon 3 free. Measure the voltage between these two free ends.
Tip: Copper joined to zinc, again and again. If you join copper to copper or zinc to zinc the voltages cancel.
Light the LED
Connect the LED between the two free ends, with the long leg to the copper end (the + end). In a dim room, look closely. It will glow faintly. If it does not, add a fourth lemon.
Common mistake: Connecting the LED the wrong way round. An LED lights only one way, so reverse it if nothing happens.
Run a clock (optional)
Connect two lemon cells in series to the contacts of a 1.5 V LCD clock, + to + and − to −. If it works, the display will appear and keep time.
Tip: Use only two cells for a clock made for one 1.5 V cell. Press the wire ends firmly onto the contacts.
Investigate one variable
Choose one thing to test: the type of fruit, the depth of the electrodes, or the distance between them. Keep everything else the same. For each setting, measure the voltage three times in the table, using fresh holes.
Common mistake: Changing the fruit and the electrode depth together. Change one variable at a time, or you cannot tell what caused the difference.
Plot your results
Draw a bar chart (for different fruits) or a line graph (for depth or distance) with the average voltage on the vertical axis.
Tip: Label both axes with units, and write a title above the graph.
Clean up
Remove the electrodes, wash and dry them, and throw the lemons into the compost or the dustbin. Wash your hands.
Common mistake: Eating the fruit. It has been in contact with zinc and copper.
How it should look when it is finished
Three or four lemons in a row on a tray, each with a shiny grey nail on one side and a bright copper strip on the other, linked by short wires. A red LED at the end glows faintly in a dim room, and the multimeter shows about 2.7 V across the chain, while a single lemon shows about 0.9 V.
This is a description, not a photograph. Compare your own model with it and with the figures above.
How it works
Zinc is more reactive than copper, so zinc atoms in the nail’s coating tend to lose electrons and dissolve as Zn²⁺ ions into the juice. This leaves the zinc with extra electrons, so it becomes the negative terminal. The copper has no such tendency, so it is the positive terminal. The difference between the two metals is what makes a voltage.
When a wire connects the two metals through a meter or an LED, electrons flow from the zinc through the wire to the copper. At the copper they combine with hydrogen ions from the acid to make hydrogen gas. Inside the fruit, ions move through the juice to complete the circuit, so charge keeps flowing round a closed loop for as long as the reactions continue.
In series, the zinc end of one cell is wired to the copper end of the next, so the voltage of each cell adds to the last. A small, high-brightness LED needs about 1.8 to 2 V and a very small current, so three or four cells in series can light it faintly.
Scroll sideways to see the whole diagram.
Troubleshooting
| What you see | Likely cause | What to do |
|---|---|---|
| The meter reads 0 V or close to it | The electrodes are touching, a probe is not on the metal, or the nail is not zinc-coated. | Move the electrodes apart, press the probes on the clean metal and check that the nail is galvanised. |
| The meter shows a negative voltage | The probes are reversed. | Swap the probes: red on copper, black on zinc. |
| The LED does not light | Too few cells, the LED is reversed, or the connections are loose. | Use four cells, turn the LED round, and press the wire ends firmly. Use a dim room and a high-brightness red LED. |
| The voltage falls quickly | Hydrogen bubbles collect on the copper and the electrodes are used. | Take the electrodes out, clean them and re-insert them in fresh juice. |
| The clock does not work | Wrong polarity, too little voltage, or poor contact. | Check + and − against the clock’s cell contacts, use two cells, and press the wires firmly. |
| Readings for the same fruit differ a lot | The depth and distance of the electrodes were not the same. | Mark the nail and copper at 3 cm with tape and use the same distance every time. |
Test and record
Use the same nail and copper, the same depth (3 cm) and the same distance (3 cm). Measure three times with fresh holes. Use the DC volts range.
| Fruit or setting | Voltage 1 (V) | Voltage 2 (V) | Voltage 3 (V) | Average (V) |
|---|---|---|---|---|
| Lemon | ||||
| Orange | ||||
| Potato | ||||
| Tomato | ||||
| 2 lemons in series | ||||
| 3 lemons in series |
Copy the table into your notebook and write each reading as you take it, in pencil, with the date. Repeat every test at least three times and use the average. Then draw a graph or chart of your results.
Viva questions and answers
A teacher or a judge will ask you how your project works. Practise these eight questions and answer them in your own words.
- Which metal is the positive terminal and which is the negative terminal in the lemon cell?
Copper is positive (+) and zinc is negative (−).
- What does the lemon do?
It is the electrolyte: its acid juice has ions that carry charge between the two metals. The energy comes from the chemical reaction of the zinc, not from the lemon.
- Why do we use two different metals?
Two different metals have different tendencies to lose electrons, and the difference creates a voltage. Two identical metals would give no voltage.
- Write the reaction at the zinc electrode.
Zn → Zn²⁺ + 2e⁻ (oxidation).
- What happens at the copper electrode?
Hydrogen ions from the acid pick up electrons and form hydrogen gas: 2H⁺ + 2e⁻ → H₂ (reduction).
- Why can one lemon not light an LED but several in series can?
One lemon gives about 0.9 V, less than the 1.8 to 2 V an LED needs to glow. In series the voltages add, so three or four cells reach that value.
- Why is the current from a lemon cell so small?
The cell has a high internal resistance because the electrodes are small and the juice is a poor conductor compared with a metal wire.
- How does the voltage change when you use a different fruit, and why?
Give your own readings. Fruits with more acid and more ions in the juice usually give a larger voltage and current; a potato (less acidic) usually gives less.
Written report outline
Use these headings in your project file, and write each part in your own words. Add a title page, your diagrams, the precautions you took and the books or sites you used.
- Aim
- To make a lemon cell and to investigate how the voltage depends on the fruit (or the number of cells in series).
- Apparatus
- Lemons and other fruits, galvanised nails, copper wire or strips, connecting wires, red LED, multimeter, sandpaper.
- Procedure
- Describe how you inserted the electrodes, measured the voltage of one cell, joined cells in series, tried the LED and then changed one variable and measured three times. Add the diagrams.
- Observation
- Your table of voltages, the number of lemons the LED needed, and any bubbles seen at the copper.
- Result
- One lemon gave about ... V. Three lemons in series gave about ... V and made the LED glow faintly. State which fruit or setting gave the highest average voltage.
- Conclusion
- A chemical cell turns chemical energy into electrical energy. The voltage depends on the metals and the electrolyte, and cells in series add their voltages. Explain your best result using what you know about the reactions.
Take it further
- Measure the internal resistance: measure the open-circuit voltage E, then the voltage V across a known resistor R (such as 1 kΩ). Then r = R × (E ÷ V − 1).
- Compare other metal pairs: zinc and iron, or copper and aluminium foil. Which pair gives the highest voltage?
- Join three lemons in parallel (copper to copper, zinc to zinc). What happens to the voltage?
- Add salt or lemon juice to a glass of water and test the cell in the liquid. How does the concentration change the voltage?
- Time how long the voltage of one cell takes to fall by 10 percent, and plot the fall as a graph.
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