Capstone: The Lemon Battery
Zinc screw + copper coin + citrus = a real galvanic cell. Stack a few and light an actual LED with fruit.
Builds on: 6.2 Batteries: The Galvanic Cell6.1 Redox: Electron Bookkeeping
The shopping list
3–4 lemons (or potatoes — the vegetable is only the electrolyte, not the fuel!), galvanized (zinc-coated) screws, copper coins or stripped copper wire, alligator leads or taped wire, and one red LED — red, because it has the lowest turn-on voltage (~1.9 V; blue needs ~3 V and will humble your fruit).
What you are actually building
Each lemon is a Zn/Cu galvanic cell from lesson 6.2: the zinc screw is the anode (oxidized, dissolving invisibly slowly), the copper coin the cathode, the citric acid the electrolyte-slash-salt-bridge. Theory says ~1.1 V; a real lemon delivers about 0.9 V — and, crucially, only a trickle of current, because ions crossing lemon pulp meet enormous internal resistance (your Spark Academy vocabulary, now with juice).
One lemon (0.9 V) cannot open a 1.9 V LED — the electron push is simply too weak, no matter how long you wait. Two lemons (1.8 V) sit agonizingly at the doorstep. At three lemons (~2.7 V) the LED opens and glows — dimly, honestly, because the stacked internal resistance chokes the current to some tens of microamps. That dim glow is the point: you can reason about its dimness, quantitatively, with two courses’ worth of understanding.
Dark LED? Check polarity first (long leg = +, toward the last copper). Then squeeze and roll each lemon to break cell walls, push the electrodes deeper (but never touching each other inside), sand the zinc shiny, and make sure each link goes Zn → Cu, not Zn → Zn. An LED is a diode — backwards it blocks completely, as a Spark graduate would expect.
You have now travelled the whole arc: atoms → bonds → reactions → moles → acids → energy → electrons doing work in a wire. The lemon on your desk is the course diploma before the certificate: a chemical energy source you understand from the electron shell up. Check every box below — the certificate unlocks when both capstones are done.
⚗️ Lab — Digital Twin: The Lemon Stack
Simulate the build before you juice up the real one.
- One lemon: 0.9 V — dark. Two: 1.8 V — still dark. Find the magic number.
- Watch the current meter: even lit, it’s microamps. Blame the internal resistance.
- Would six lemons make it dazzling? Check how much the brightness actually gains.