Lemon battery that lights an LED

What they will learn

• That two different metals in an acidic liquid make a battery: the zinc gives up electrons and the copper collects them.
• That when you connect cells in series (+ to −) their voltages add up: 4 × 0.9 V ≈ 3.6 V.
• That an LED only lights in one direction and needs a minimum voltage (≈ 1.8 V for red).
• That voltage is not enough: a lemon gives very little current, which is why the light is faint.

Prepare beforehand

• Strip 4 pieces of copper wire 6 cm long and bend each one into an L, leaving 1 cm for the clip.
• If you use coins, leave them for 5 minutes in vinegar with salt, rinse and dry them: they should come out shiny copper-coloured.
• Take the lemons out of the fridge 1 hour beforehand: cold lemons give less current.
• Test the LED with a 3 V battery (long leg to +) to make sure it works and to see which is the long leg.
• Print base A and get a room ready that can be made dark.

Materials and tools

• Lemons — 4 (+1 spare)
  juicy, thin-skinned, at room temperature
• Galvanised screws or nails — 4 buy
  zinc-plated (matt silvery grey), from 4 × 40 mm to 5 × 50 mm
• Copper electrical wire — 4 pieces 6 cm long
  solid 1.5–2.5 mm² wire, stripped (leftover from house wiring)
• Crocodile clip leads — 5 buy
  30–50 cm long, metal clips
• 5 mm red LED — 1 (+1 spare) buy
  diffused or high-brightness red, forward voltage ≈ 1.8 V
• Corrugated cardboard — 1 piece 10 × 30 cm
  mounting base so the lemons don't roll
• Modelling clay — 4 hazelnut-sized blobs
  so the lemons lying on their side don't roll on the base
• Vinegar and salt — 1 glass with 100 ml of vinegar and 1 teaspoon of salt
  to clean the coins or tarnished copper
• Multimeter — 1 (optional but highly recommended)
  20 V DC range and 2 mA or 20 mA

Tools

• Pointed knife — make a 1 cm cut if you use coins adult
• Wire strippers or scissors — strip the copper wire adult
• Pliers — bend the copper into an L and cut it to 6 cm
• Felt-tip pen — mark + and − on each lemon

Pieces and templates

• A · Mounting base ×1 — corrugated cardboard 3–4 mm
• B · Copper electrode ×4 — solid copper wire 1.5–2.5 mm², stripped
• C · Zinc electrode ×4 — galvanised screw or nail 4–5 mm

Step by step

1. Soften the lemons

   Put each lemon on the table and roll it 10 times, pressing with your palm, without breaking the skin. This breaks the little cells inside and frees the juice around the electrodes.

   Check: The lemon feels soft when you squeeze it.

2. Place the lemons on the base

   Put each lemon on its circle on base A, lying on its side with a blob of modelling clay underneath so it doesn't roll. Leave about 10 mm between lemons.

3. Push in the copper

   In each lemon, push a copper electrode B 3 to 4 cm deep on the side marked «Cu +». Leave the L-shaped bend outside. If you use a coin, ask an adult to make a 1 cm cut with the knife first; then you push the coin in halfway.

   Tip: Mark a «+» on the skin next to the copper with a felt-tip pen.

4. Push in the zinc

   On the side marked «Zn −», 2–3 cm from the copper, push in a galvanised screw C about 2–3 cm. The two metals must not touch, either inside or outside the lemon.

   Check: There are at least 2 cm of skin between the copper and the zinc.

5. Measure one lemon

   If you have a multimeter, set it to 20 V DC. Red probe on the copper and black on the zinc of one lemon. It should read between 0.85 and 1.0 V. Repeat with all four and write down each value.

   Check: If a lemon gives less than 0.7 V, change that screw or move the electrodes closer together.

6. Link the lemons in series

   With a crocodile clip lead, join the zinc of lemon 1 to the copper of lemon 2. Then the zinc of 2 to the copper of 3, and the zinc of 3 to the copper of 4. Use 3 leads and don't close the loop.

   Check: Only the copper of lemon 1 (+) and the zinc of lemon 4 (−) are left free. With the multimeter between them: 3.4–4.0 V.

7. Get the LED ready

   Clip one crocodile lead to the long leg of the LED and another to the short leg, making sure the two clips don't touch each other. Bend the LED legs apart slightly to separate them.

8. Connect the LED

   Join the lead from the LED's long leg to the free copper of lemon 1 and the one from the short leg to the free zinc of lemon 4. Turn off the room light and wait 30 seconds for your eyes to adjust.

   Check: The LED glows a faint red, visible in the dark.

9. If it doesn't light

   First turn the LED round (swap its two leads). If it still doesn't light, use the multimeter to check that the chain gives more than 3 V and that each clip is biting metal, not skin or dried juice.

10. Clear up safely adult

    Disconnect the leads, pull out the electrodes and wash them with water. Throw the lemons in the food waste bin: some zinc has dissolved in the juice and they must not be eaten.

Does it work? The test

With the chain of 4 lemons connected to the red LED, in the dark. If you have a multimeter: measure the voltage of each lemon, of the chain without the LED and with the LED connected, and the current on the 2 mA range (multimeter probes in series with the LED).

What should happen: Each lemon: 0.85–1.0 V. Chain of 4 without LED: 3.4–4.0 V. With the LED connected the voltage drops to ≈ 1.8–2 V and about 0.1–0.5 mA flows (less than when short-circuited, because the LED takes almost 2 V): the red LED glows faintly, visible in the dark. With 3 lemons it sometimes still glows; with 1 lemon, never.

If something goes wrong

• The LED doesn't light at all — LED the wrong way round, chain wrongly connected or a green/blue/white LED. Solution: Turn the LED round. Check that each lead goes from Zn to the Cu of the next lemon. Use a red LED.
• Each lemon gives 0.3–0.5 V — The screw isn't zinc (stainless steel or iron) or the coin is tarnished. Solution: Use matt grey galvanised screws and shiny stripped copper or coins cleaned with vinegar and salt.
• The chain gives 0 V or less than expected — A lemon connected the wrong way round (Cu to Cu) or a lead not making contact. Solution: Measure lemon by lemon with the multimeter and follow the order Cu+ → Zn− in each one.
• The voltage is fine but the LED is barely visible — The lemons give very little current. Solution: Do it in total darkness, push the electrodes in deeper, move them to 2 cm apart or add a fifth lemon. Putting two identical chains in parallel also helps.
• It worked and is fading — The zinc gets covered in hydrogen bubbles (polarisation) and the juice near the electrodes runs out. Solution: Pull out the electrodes and push them back into a fresh spot on the lemon, or wait 5 minutes.
• The clips touch — Short circuit between the LED legs or between electrodes. Solution: Spread the LED legs apart and attach the clips pointing in opposite directions.

Safety

• Don't eat the lemons or the juice after the experiment: they contain dissolved zinc.
• The screws and the stripped copper are sharp: push them in with your palm protected by a cloth.
• The knife for the coin cuts is used by an adult.
• Lemon juice stings in cuts and eyes: wash your hands when you finish and don't rub your eyes.
• This experiment is safe because the current is tiny; never experiment with the sockets at home.

The science behind it

For children

Something invisible happens inside the lemon: the zinc screw keeps letting go of tiny particles called electrons, which want to get to the copper. They can't go through the lemon, so they take the long way round: along the wire. That race of electrons along the wire is electricity! One lemon only pushes a little, so we put four in a row, one after another, so their pushes add up, like four friends pushing a car. Even so, the light is very small: that's why you have to look at it in the dark.

For the adult

Each lemon is a galvanic cell: the zinc (anode, − terminal) oxidises, Zn → Zn²⁺ + 2 e⁻, and the electrons travel through the external circuit to the copper (cathode, + terminal), where they reduce the H⁺ ions from the citric acid: 2 H⁺ + 2 e⁻ → H₂ (the tiny bubbles in the diagram). The copper isn't consumed: it is just a surface where the reduction takes place. The theoretical electromotive force of the Zn/H⁺ couple under these conditions is ≈ 0.9–1.0 V. But each lemon's internal resistance is high (≈ 1–5 kΩ) because the electrolyte is a poor conductor and the electrode surface is small, so the short-circuit current is only 0.2–1 mA. In series, the voltages add up (4 × 0.95 ≈ 3.8 V) but so do the internal resistances. A red LED needs ≈ 1.7–1.8 V to start conducting and at 0.5 mA already gives a light visible in the dark; a green or blue LED needs 2.2–3.2 V and much more current to be seen. Increasing the electrode surface (pushing them in further) and shortening the distance between them reduces the internal resistance and increases the current. The energy doesn't come from the lemon as food but from the oxidation of the zinc: the acid only acts as the electrolyte.

• electrode: the metal placed in the liquid of a battery (here, copper and zinc).
• electrolyte: a liquid that lets electric charges through: the lemon juice.
• volt (V): the unit that measures how hard a battery «pushes» the electricity.
• in series: connecting cells one after another (+ to −) to add up their volts.
• polarity: which side is + and which is −; an LED only works in one direction.

By age

• 5–7 years: The adult pushes in the electrodes and connects the leads. The child softens the lemons, checks with a finger that zinc and copper aren't touching and watches the LED in the dark. Focus on «four lemons push together».
• 8–10 years: They do the whole assembly following base A and measure lemon by lemon with the multimeter. Let them find out for themselves what happens if they turn a lemon round.
• 11–12 years: Have them measure voltage and current, work out the internal resistance of a lemon (r = open-circuit V / short-circuit I) and draw the circuit diagram with a cell symbol for each lemon.

In a group or in class

Teams of 4, each child prepares a lemon and then the team links them up (≈ €1.50 per team for lemons, screws and LED; 5 crocodile clip leads per team). Before the session: strip and cut the copper, buy galvanised screws and test the LEDs. 50-min session: 10 min of explanation, 20 min of assembly and measuring, 10 min with the lights off to see the LEDs, 10 min to join the whole class's chains into one big battery and see if it lights more LEDs. One multimeter for every 2 teams.

Challenges

• Power a calculator that uses LR44 cells (1.5 V) with 2 lemons in series.
• Get the highest possible voltage from a single fruit cut into separate pieces.
• Design a battery with 4 cups of salted vinegar instead of lemons and compare.
• Make the LED visible with the room light on (hint: more lemons in series and in parallel).

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