In this science fair project, construct batteries from various fruits and test them to see which one will produce the most electric current. Then, determine if it would be practical to use fruit as a natural source for generating electricity.
An electric current is a flow of electrons and is measured in units called amperes or "amps." Voltage is the force that pushes the electrons through a circuit (like the pressure on water in a pipe) and is measured in volts.
When two dissimilar metals are placed in a common conducting solution, electricity will be produced. This is the basis of the electro-chemical cell, or wet cell. In the early nineteenth-century, Alessandro Volta used this fact of physics to invent the voltaic pile and discovered the first practical method of generating electricity. Constructed of alternating discs of zinc and copper metals with pieces of cardboard soaked in a salt solution between the metals, his voltaic pile produced an electrical current. Alessandro Volta's voltaic pile was the first "wet cell battery" that produced electricity.
A wet cell consists of a negative electrode, a positive electrode and an electrolyte, which conducts ions (atoms with an electric charge). In this science fair project, copper and zinc metals will be used as the electrodes and the citric acid found in fresh fruit is the electrolyte. The chemistry behind the fruit cell is that zinc is more reactive than copper which means zinc loses electrons more easily than copper. As a result, oxidation occurs in the zinc metal strip and zinc metal loses electrons which then become zinc ions. The electrons then flow from the zinc strip to the copper strip through an external circuit. In the copper strip, reduction occurs and the hydrogen ions in the fruit's critic acid juice accept these electrons to form hydrogen gas; this explains why the investigator may observe bubbling of gas produced at the copper strip when the two metals are connected by a wire.
In this project an LED is used to indicate if the fruit-cell is generating an electric current. A Light Emitting Diode (LED) is a semiconductor device which converts electricity into light. An electric current can flow only in one direction through LEDs, which means that they have a positive and negative terminal (also referred to as the anode and cathode). The cathode should be connected to the negative zinc metal strip, and the anode to the positive copper strip.
Safety: The fruits used in this project should not be eaten. Care should be taken when handling the metal electrodes, LED and alligator clip leads.
- Various fruits (such as a lemon, grapefruit, orange, tomato, and kiwi)
- Alligator clip leads
- Tri-fold cardboard display board
- Copper and zinc electrodes
- (optional) Small 1.5 volt electric hobby motor.
- What is a wet cell battery?
- Why do placing two dissimilar metals into a fruit produce an electric current?
- Which fruit-cell produced the most electricity? Which fruit-cell produced the least?
- Did changing how far in the electrodes were make the current increase or decrease?
- Did putting the electrodes closer together make the current increase or decrease?
- Did putting the electrodes farther apart make the current increase or decrease, or stay the same?
- Did the size of fruit make a difference? If so, did the size make the current increase or decrease?
- How long did the fruit-cell provide electricity to light the LED?
- Citrus fruits are acidic, which helps their juice to conduct electricity. What other fruits and vegetables might work as batteries?
- Would fruit juice minus the fruit work as an electrolyte?