Lemon Battery:


Does anyone have any tips on how to make the lemon battery work?
We are using Jacaranda Science Book 2, page 231.


For best results with a lemon battery use BROAD pieces of zinc and copper (1-2cm wide, roughly polished with sandpaper). Use a high impedance meter (any digital multimeter) to monitor the voltage produced (it will be around 0.9 volts per battery). [Do not use a school quality analogue meter (those with a needle) because they draw too much current, which causes the voltage produced across the zinc and copper electrodes to drop right off].
YOU CANNOT MAKE A GLOBE GLOW, because of this same problem. You can however use a Light Emitting Diode (LED) instead; because they draw much less current (these are readily available at Dick Smith). But it must be connected the right way around or it will not work (just reverse the connection).
AT LEAST THREE, THOUGH PREFERABLY 4 OR 5, LEMON BATTERIES IN SERIES** WITH EACH OTHER IS REQUIRED TO LIGHT THE LED. Get the students to link them together if not enough sets to go around. Shroud the LED to see the glow more clearly.
** To connect in series, means to connect the zinc electrode of one battery to the copper one of another, and so forth. Connecting like electrode to like electrode (i.e. a parallel connection) will increase the current, BUT NOT the voltage – A THRESHOLD VOLTAGE OF 1.7 VOLTS is required to activate an LED# #.
A lemon battery can produce 0.9volts, but minuscule current, because of the lemon battery’s high internal resistance.
Good old Ohms Law comes into play, ie Volts=Current x Resistance, or more appropriately expressed here, Current = Volts/Resistance.
From this equation you can see that a high resistance will give a very low current.
# # THRESHOLD VOLTAGE OF 1.7 VOLTS: Since 1.7 volts is required, one needs to put at least three, but preferably 4 or 5 Lemon Batteries in series with each other to get the required voltage. [Just two is not sufficient, because, as explained above, whilst each battery produces 0.9 volts, the voltage will drop off significantly when connected to the load (the LED in this case). This effect can be observed by simply connecting a digital multimeter (on DC Volts setting) across the battery’s electrodes, and noting how the voltage goes down when the LED is connected. This is the case with ANY type of battery, though significantly more pronounced with a lemon battery.]

Set up 3 Coca Cola cells in series using copper and zinc electrodes. It will light an LED eg 5 Amp, available from Altronics, Perth. Connect the LED the correct way around.

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