Fruit Battery

Can a lemon light a lamp? Push zinc and copper into some fruit, wire the cells together and try to light a red LED or a torch bulb.

Fruit or vegetable
Try to light
Battery voltage
2.70 V
Inside resistance
5.40 kΩ
Current
175 µA
LED
Very dim (try a dark room)

The maths, live

Series: voltages add E = n × E₁E = 3 × 0.9 = 2.70 V
Parallel rows share the load r = n × r₁ ÷ rowsr = 3 × 1800 ÷ 1 = 5400 Ω
Current I = (E − V(LED)) ÷ rI = (2.70 − 1.76) ÷ 5400 = 175 µA

A red LED needs about 1.8 V before it lights at all. How many lemons in series does it take? Why does the torch bulb never light, however many you add?

How it works

Zinc and copper in the juice make a simple cell. The zinc gives up electrons, which flow through the wire to the copper. One lemon gives about 0.9 V, but its juice has a very high internal resistance (nearly 2000 Ω), so it can only push out about half a milliamp.

Cells in series add their voltages, and a red LED needs about 1.8 V before it lights at all, so you need at least 2 or 3 lemons. Putting rows side by side (in parallel) shares the work and lets more current flow. A torch bulb needs about 0.1 A just to glow, a hundred times more than the fruit can give, so it never lights.

Try this

  1. What is the smallest number of lemons in series that makes the LED glow?
  2. Add a second row of lemons in parallel. What happens to the voltage and the current?
  3. Which is the best battery: lemons, potatoes or apples?