It is generally agreed that there are six basic sources of electricity. These are: heat, light, friction, pressure, magnetism and chemical action. Of these, magnetism is the most important, contributing by far the largest portion of electrical production worldwide. Power generators, whether hydroelectric or fueled by coal, oil, gas or nuclear power, all use magnetism as the actual means of electricity generation. Light, acting upon solar panels, is slowly gaining in importance, but has yet to make great inroads in commercial electricity production because of its cost. Electricity generated directly by heat, pressure and friction tend to be either very small--the microvolt output of a thermistor when exposed to heat, or the equally small output of a crystal microphone when subjected to sound pressure--or uncontrollable, as in the case of lightening, caused by friction. Chemical action in the form of batteries is both the oldest means of producing electrical current, and has had a great impact on our modern way of life.
In 1791 Luigi Galvani discovered electrical activity in the nerves of the frogs that he was dissecting. He thought that electricity was of animal origin and could be found only in living tissues. A few years later, in 1800 Alessandro Volta discovered that electricity could be produced through inorganic means. In fact, by using small sheets of copper and zinc and cloth spacers soaked in an acid solution, he built a battery - the first apparatus capable of producing electricity. Many were quick to predict that electricity would never serve a useful purpose. Some still do. However, electricity has a central role in our lives today
While listening to lessons about chemistry, many students may wonder why it was ever invented, if it was really ever necessary to invent it and if the world would be better off without it. The small experiments that follow are intended to interest these students in the study of chemistry and electrical phenomena. These simple and (it is hoped) interesting experiments can teach the fundamental concepts of electricity and chemistry without asking much of the student. Many of these demonstrations are easily adapted to various configurations and each can be done independently or as part of a full curriculum.
POROUS VASE - An actual porous vase made for the purpose may be hard to find. Its purpose is to prevent the quick mixing of various solutions, while permitting the exchange of ions. For these purposes you can adapt a terra-cotta (clay) pot of the type used in gardening simply by plugging the hole in the bottom with molten wax and allowing it to cool. Another even more economical answer lies in constructing a barrier of paper. As shown in figure 4, roll the paper to form a cylinder and glue it in place on the bottom of the main container using a silicone adhesive such that liquids cannot pass between the two areas defined by the paper. A barrier of just one sheet would be too permeable, therefore use at least three layers of paper when building this device.
DISTILLED WATER - Don't use de-ionized water in place of actual distilled water. Many times water sold for household use is de-ionized rather than distilled water. Be sure the label indicates "distilled water" rather than "purified water". What is the difference? Many substances are soluble in water and a few of these substances separate into positive and negative ions in water. Generally these are made up of molecules that have ionic bonds, while non-ionic molecules remain intact, just in solution. For example, sugar dissolves easily in water and the sugar molecules remain intact as sugar. De-ionized water can have any number of dissolved substances that do not result in ions, yet are present nonetheless. Apart from this, a poor quality sample of de-ionized water can contain significant amounts of ions. On the other hand, distilled water is usually very pure - containing only actual water molecules.
SOLUTIONS (IMPORTANT) - The dilution of acids is dangerous. If water is added to a concentrated acid, it can explode violently causing severe injuries. Never pour water into concentrated acid. Always add the acid to the water. If you need to dilute an acid, get help from your instructor. The use of fruit juices and table vinegar will provide all the acid strength really necessary in these experiments. Only in a chemistry class, with proper safety training and equipment and instructor supervision, may other acids be used, if desired. When making a solution of copper sulfate or zinc sulfate, add these chemicals to water rather than adding the water to the chemical.
OTHER PRECAUTIONS (IMPORTANT) - Many chemicals, even household materials, while not extremely dangerous can be irritating to the skin, eyes and respiratory tract. Do not allow any of the chemicals discussed in these experiments to get on your hands or skin. Do not put them in your mouth or eat them. Avoid breathing any vapors from these chemicals. Do not keep them in bottles or containers that could be confused with food or drink containers. Do not leave these chemicals in such a place that they might be confused with food or drink (such as on a kitchen table or counter or in the refrigerator). Never eat or drink in a working chemistry or electronics laboratory. Store chemicals in a separate, controlled place away from foods and out of the reach of children. Label each container clearly with the name of the contents and as a non-food item.
- a lemon
- a strip of copper
- a strip of zinc
- a voltmeter
- two cables with alligator clips
- a thermometer or clock with an LCD display
Roll the lemon firmly with the palm of your hand on a tabletop or other hard surface in order to break up some of the small sacks of juice within the lemon. Insert the two metal strips deeply into the lemon, being careful that the strips not touch each other. Using the voltmeter, measure the voltage produced between the two strips (figure 3). It should show about one volt.
It would be nice to be able to illuminate a light bulb using your new lemon powered battery, but unfortunately it is not strong enough. If you were to try to light a bulb using this setup, the voltage across the strips would fall immediately to zero. Given this, if you want to demonstrate that the current produced by this battery is capable of powering something, try with a small device that uses an LCD display. A clock or a thermometer usually works well. An LCD display consumes an extremely small amount of current and your lemon battery is able to adequately drive this type of device. Remove any conventional battery that is in your clock or thermometer and power it with your lemon battery. You should see the device recommence functioning normally. If not, try swapping the polarity of the electricity from your lemon battery. This system allows you to demonstrate that the battery is producing energy even if you don't have a voltmeter.
How does this battery work? The Copper (Cu) atoms attract electrons more than do the Zinc (Zn) atoms. If you place a piece of copper and a piece of zinc in contact with each other, many electrons will pass from the zinc to the copper. As they concentrate on the copper, the electrons repel each other. When the force of repulsion between electrons and the force of attraction of electrons to the copper become equalized, the flow of electrons stops. Unfortunately there is no way to take advantage of this behavior to produce electricity because the flow of charges stops almost immediately. On the other hand, if you bathe the two strips in a conductive solution, and connect them externally with a wire, the reactions between the electrodes and the solution furnish the circuit with charges continually. In this way, the process that produces the electrical energy continues and becomes useful.
As a conductive solution, you can use any electrolyte, whether it be an acid, base or salt solution. The lemon battery works well because the lemon juice is acidic. Try the same setup with other types of solutions. As you may know, other fruits and vegetables also contain juices rich in ions and are therefore good electrical conductors. You are not then, limited to using lemons in this type of battery, but can make batteries out of every type of fruit or vegetable that you wish.
Like any battery, this type of battery has a limited life. The electrodes undergo chemical reactions that block the flow of electricity. The electromotive force diminishes and the battery stops working. Usually, what happens is the production of hydrogen at the copper electrode and the zinc electrode acquires deposits of oxides that act as a barrier between the metal and the electrolyte. This is referred to as the electrodes being polarized. To achieve a longer life and higher voltages and current flows, it is necessary to use electrolytes better suited for the purpose. Commercial batteries, apart from their normal electrolyte, contain chemicals with an affinity for hydrogen which combine with the hydrogen before it can polarize the electrodes
•Citrus fruits are acidic, which helps their juice to conduct electricity. What other fruits and vegetables might you try that would work as batteries?
•If you have a multimeter, you can measure the current produced by the battery. Compare the effectiveness of different types of fuits. See what happens as you change the distance between the nails.
•Do acidic fruits always work better? Measure the pH (acidity) of the fruit juice and compare that with the current through the wires or brightness of the light bulb.