Introduction: Electricity & Light From a Lemon
Just over 200 years ago the Italian physicist Alessandro Volta invented the first true battery. In this classroom science experiment we can re-create a very similar battery that Volta invented using nothing more than a lemon and two pieces of metal. Its powerful enough to light up an LED, we are really creating light from a lemon !
By the way... Volta's battery used copper, zinc and a cloth soaked in salt water. In our experiment we will use copper, magnesium and a lemon but the theory is the same, we are using a chemical reaction to make electricity.
The project is intended for students between the ages of 10-15 (US grade 5-9). Older students should be able to complete the project unaided and work out why the circuit does not work (for example connection is not good between the lemons etc).
The project is perfect for Physics or General Science classes but could also be extended to an IT class. It will get your students thinking about where do their mobile phones get their power from. The class shows that a battery uses a chemical reaction to create electrical current.
- Half a lemon cut into 3 segments (i.e 3 x 1/6's of a lemon)
- Some copper wire (about 12" (20cm) in total) - this is the wire used in your home electricity sockets. If you know an electrician they are sure to have many offcuts which you can use. Otherwise its available in every hardware store.
- Some magnesium ribbon (about 3" (10cm) in total) - this is available online for around $3 for a yard (1m). If you cant get it then "galvanized" nails will also work (but not as good), these are nails covered in zinc, hardware stores will have them. They look grey and dull to look at (i.e not shiny).
- An LED (normal 3v LED), avoid Blue as they sometimes needed more power to light them.
Step 1: Prepare the Materials and Make the Cells
Take the 1/2 lemon and cut into 3 segments as shown in the picture
Next cut 2 pieces of copper wire about 1" long. Make sure there is no rubber shielding around the cable, it should be "copper" coloured :-)
Finally 3 pieces of magnesium ribbon about 1" long (its simple to cut with scissors)
We are going to create 3 small batteries (or "cells"). Each battery consists of a lemon segment, a copper terminal and a magnesium terminal.
Why do we need 3 batteries you ask ? Well each battery will produce about 1 volt of electricity, but an LED needs around 3 volts of electricity to work. So if we wire up 3 batteries in a row we'll have 3 volts, it should be perfect to light the LED.
Step 2: Wire Up the 3 Batteries in a Row
So we have the 3 batteries, now we need to connect them in a row.
Whats important at this stage is that the copper terminal from one battery connects to the magnesium terminal of the next battery. The easiest way to do this is to bend the copper wire so that it squeezes onto the magnesium to make a tight connection.
If you accidentally connect the copper to copper or magnesium to magnesium of each battery the batteries will basically cancel each other out, its like putting one of the batteries in your TV remote control in the wrong way round, the remote wont work.
So now we have the 3 batteries in a row.
Step 3: Connect the LED and Let There Be Light !
Finally we can connect the LED to the very left terminal of the left battery and the very right terminal of the right battery so that the electrical circuit is made.
But hold on - the LED is very particular on how its connected. You will see that one of the legs on the LED is longer than the other, this is called the "Anode", this needs to connect to the Positive (+) side of the battery. The shorter leg is called the "Cathode", this needs to connect to the Negative (-) side of the battery.
But which is the positive and which is the negative terminal on the lemon battery ?
.....the copper is positive (+), so connect the long leg of the LED to the copper wire and connect the short leg of the LED to the magnesium terminal.
And hey presto the LED should light. If you give the lemon segments a squeeze you might see the LED glow brighter as more juice will be released making a better connection to the terminals.
So whats the science behind this magic ?
Well a chemical reaction is taking place between the two different metal terminals (called the "electrodes"), the lemon juice helps in the reaction (its called the "electrolyte"). When the chemical reaction takes place some extra "electrons" are created which flow along the circuit into the LED. The LED then converts these electrons into Light.
See what happens to the terminals if you leave the LED connected for a few hours - Im afraid you have not invented a battery that will last forever !
You can also try with just 2 cells, the LED should light but will be dimmer. With just one cell for sure the voltage will be too low to light the LED but go ahead and try.
Batteries are becoming more and more critical to power our power mobile devices & electric cars, this class shows that battery technology has come a long way in the last 200 years but still there is plenty of room for improvement...maybe soon your mobile phone will only need charging once per year !
If you cant find magnesium ribbon:
Finally, you if you dont have any magnesium you can also try the experiment using Zinc just like Alessandro Volta did instead of magnesium (some zinc plated (called "galvanized") nails can be used) but you may need to use more than 3 cells since the Zinc will only produce about 0.9 volts per cell compared to over 1 volt with the magnesium.
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