The penny battery is a

voltaic pile upright=1.2, Schematic diagram of a copper–zinc voltaic pile. The copper and zinc discs were separated by cardboard or felt spacers soaked in salt water (the electrolyte). Volta's original piles contained an additional zinc disk at the bottom, ...

which uses various

coin A coin is a small, flat (usually depending on the country or value), round piece of metal or plastic used primarily as a medium of exchange or legal tender. They are standardized in weight, and produced in large quantities at a mint in order t ...

age as the metal disks (pennies) of a traditional voltaic pile. The coins are stacked with pieces of

electrolyte An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon dis ...

soaked paper in between (see diagram at right). The penny battery experiment is common during

electrochemistry Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference, as a measurable and quantitative phenomenon, and identifiable chemical change, with the potential difference as an outco ...

units in an educational setting. Each

cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery ...

in a penny battery can produce up to 0.8

volt The volt (symbol: V) is the unit of electric potential, electric potential difference (voltage), and electromotive force in the International System of Units (SI). It is named after the Italian physicist Alessandro Volta (1745–1827). Defi ...

, and many can be stacked together to produce higher voltages. Since the battery is a

wet cell An electric battery is a source of electric power consisting of one or more electrochemical cells with external connections for powering electrical devices. When a battery is supplying power, its positive terminal is the cathode and its negati ...

, the effectiveness will be reduced when the electrolyte evaporates.

Coinage selection

As the name implies, Canadian pennies from 1997 to 1999 may serve the zinc

electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials de ...

and 1942-1996 pennies as the

copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...

. Alternatively, American pennies from 1982–present may be used as the zinc electrodes and 1944-1982 pennies as the copper electrodes. A variety of other coins can also be used, with varying results.

Building a penny battery

A penny battery can be useful in producing a small amount of voltage. To make a penny battery it is crucial that there are two different kinds of metals with a substance in between them. To begin, scratch off the copper coating on one side of a penny exposing the metal zinc (silver color). This process will be difficult and will take some time. It is beneficial to have at least 5 pennies so that enough volts can be created. Then cut 5 circle pieces as big as the penny of matboard or cardboard. Soak the matboard in an acid solution. An acid as simple as vinegar and water, or lemon juice could be used. Stack the pennies on top of one another with a piece of matboard in between them. The zinc side should be facing upward. Use a penny that has not been scratched on either side and place it on top. Finally connect an LED with the longer lead attached to the top and shorter lead touching the bottom. The LED should light up proving that the battery works. It is also possible to use a voltmeter to test the amount of volts being produced by the battery cell. Take a AA battery and attach it to voltmeter to ensure that it is working properly before testing out the penny battery. For an alternate way of making this that is slightly weaker, clic
here
This method uses USA pennies from 1945 to 1980 or 10 cent euro coins, alongside aluminum foil. If the LED is not lighting up or if the voltmeter is not registering any electricity then a few problems could have occurred during set up. First, make sure that the matboard or cardboard pieces are moist. Less electrical energy will be produced if less electrolytes are available. Second, ensure that none of the pennies are touching one another and that each matboard only touches two pennies and does not overlap onto other pennies. This would create a short and little to no electrical energy will be produced. Third, check the acidity of the solution that is being used to soak the matboard. The greater the acidity, the greater number of electrolytes, and the greater amount of electricity that can be conducted. Fourth, it can be beneficial to sand down the coins instead of scratching off the copper to reach the zinc layer underneath.

Energy

Batteries convert the chemical energy of the two metals (electrodes) interacting with the acid on the matboard (electrolyte) into electrical energy. In this situation, the metal surface serves as the electrode and an electric current (movement of electrons from one metal to the other) is created when the wire connects both metal surfaces. In the first hour, a five cell penny battery is able to provide about watts. Each cell is defined as a stack of a zinc penny, matboard, and a copper penny. Each cell can provide about 0.6 volts. Indicating that to power an LED light, needing 1.7 volts, only three cells need to be used. As time goes on the amount of energy that the battery can provide decreases. A five cell penny battery can last up to hours providing minimal voltage. The stack of cells is also known as a voltaic pile.

Chemistry

A penny battery functions as a standard

, and is powered by a

redox Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate (chemistry), substrate change. Oxidation is the loss of Electron, electrons or an increase in the oxidation state, while reduction ...

reaction between zinc and acid. Electrons flow through the electrolyte solution from

zinc Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodi ...

toward

because zinc has a higher activity than copper. The acid releases positively charged

hydrogen ion A hydrogen ion is created when a hydrogen atom loses or gains an electron. A positively charged hydrogen ion (or proton) can readily combine with other particles and therefore is only seen isolated when it is in a gaseous state or a nearly particle ...

s that combine with these electrons to form

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...

gas, which escapes to the atmosphere. The release of gas corresponds with a large increase in

entropy Entropy is a scientific concept, as well as a measurable physical property, that is most commonly associated with a state of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodynam ...

, making the reaction irreversible. The reaction can be written as two separate reactions in different regions of the cell, or as one overall reaction. The reactions shown here use

acetic acid Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main component ...

, but a variety of other acids can also be used. ;Reaction at anode:Zn(s) → (aq) + 2e⁻ ;Reaction in electrolyte solution:(aq) + 2e⁻ → (aq) + (g) ;Overall reaction:Zn(s) + (aq) → (aq) + (aq) + (g)

Common misconceptions

Despite often being made of similar materials, this is not the same mechanism that powers a

galvanic cell A galvanic cell or voltaic cell, named after the scientists Luigi Galvani and Alessandro Volta, respectively, is an electrochemical cell in which an electric current is generated from spontaneous Oxidation-Reduction reactions. A common apparatus ...

. Both types of cell can use acid as an electrolyte, copper as a cathode, and zinc as both an anode and as a substance to be oxidized. However they cause different substances to be reduced: voltaic piles reduce acid, and galvanic cells reduce copper. This is because galvanic cells contain dissolved copper ions, which can be reduced to form the more stable copper metal. Voltaic piles such as the penny battery start with all of their metal in solid form, so they don't contain any dissolved copper ions that can be reduced.

References