ie-Physics

Experiment IV-8

During the seventeenth century there was much confusion about electricity.  What was thought to be electric fluid was created by rubbing insulators and transported using metals.  Pieter van Musschenbroek (1692-1761) in Leyden, Holland, wondered if it might be possible to store electric fluid in water.  In 1745 as he cranked a sulfur ball, electric fluid flowed down a metal gun barrel, then down a metal wire into a flask of water held by his assistant.  When the assistant touched the gun barrel with his free hand, a large spark shocked the assistant.  Soon such devices known as Leyden jars soon became widely used o store electrical fluid.  Electric sparks form Leyden jars killed several people.

Today we understand that the key components of every Leyden jar is an insulator (the glass jar) sandwiched between two conductors ( inside the original was impure water with outside the salty water with the assistant's hand).  It is more common to new make Leyden jars with thin layers of metal inside and outside glass jars.  Because the ability to store charge depends on the conductor surface area, it is common to roll sandwiches of metal sheets and thin insulators into compact devices called capacitors.  Because of the energy such devices can contain, treat capacitors with much caution.

Benjamin Franklin tried to cook a turkey with electricity stored in Leyden jars but accidentally shocked himself.&  He survived but noted the chest pain lasted for days.  Franklin thought that lightning might resemble the sparks he made.  He also survived his famous kite experiment while several others died trying the obtain evidence.

In Bologna, Italy, Luigi Galvani (1737-1799 at left) had been investigating since 1780 the connection between electricity and muscle contraction using frog legs.  To further investigate electricity in the atmosphere, Galvani used frog legs suspended by brass hooks from the iron lattice supporting hanging gardens around his house.  He found occasional muscle contractions both in thunderstorms and during serene weather.  Brought inside the muscle contractions also occurred when the brass hooks touched an iron plate.  When other metals surprisingly cause more violent contractions, he suspect the electricity was inherent in the animal itself.

Alesandro Volta (1745-1827 at right) read Galvani's report and thought the twitching muscles were not due to animal electricity, but rather electricity caused by a reaction of the metal hooks with a second metal of the cupboard something similar to the Leyden jar.  In 1800 when a sandwich of two different metal disks such as copper and zinc separated by a wet pasteboard spacer provided inconclusive results, he pilled up a battery of sandwiches.  (See below.)  Simultaneously touching both the top and bottom metal disks a distinct shock was felt.  Volta's battery revolutionized the study of electricity, provided a powerful new tool for chemistry, and start the radically changes society that continues today.  For the first time it was possible to make a steady current flow of electricity rather than rely on erratic, short lived, occasional sparks.

Experiment

• various kinds of metals
• electrical multimeter such as RadioShack 15-Range Digital Multimeter Model: 22-810 or Analog Display Compact 8-Range Multimeter Model: 22-218 both under $20
• paper towels or cloth soaped in salt water

There are a number of features of electricity worth understanding.  The following procedures are designed to help that understanding.

1. During the time period when much was learned about electricity such things as light, heat, and electricity were believe to be fluids.  This was a good model for electricity because it shares many properties in common with other fluids.  For example, fluids are composed materials that are conserved.  Electric charge is also conserved.  For example the amount of electric current that flows in a second into one end of a metal wire is identical to the flow at every other point in that wire unless there are branches to allow flow in or out.  The flow of electric current in a conductor is measured in units called Amperes.  One Ampere is roughly the amount of current that flows through a 100 Watt light bulb each second.  Use the multimeter set to measure a few milli-Amperes to measure the current from several drycells, and several sandwiches of various metals as described above.  Do NOT try to measure electricity supplied by power companies such as house hold or industrial electrical power.  Electric current could flow steady in one direction through a wire.  This is called direct current and abrievated as D.C.  Batteries produce D.C. current flow.  Power companies usually push electric current one way through a wire for 1/120 second, the reverse the direction of flow.  This is called alternating current and abrievated as A.C.  Most commercially available power alternates at a frequence of 60 Hertz (i.e., cycles per second).
2. Create a sandwich of two different metals separated by wet paper or cloth.  Set the multimeter to measure at most a Volt or two.  The flowing electric current could carry small or large amounts of energy just like water under high pressure carries more energy than water flowing under low pressure.  The energy per unit charge is measured in units of Volts.  Assemble several electrical sandwiches from different combinations of metals and measure their voltages.
3. If several sandwiches are combined in a battery, do the voltages combine also?  Do the currents combine?

Finally, record your procedures, measurements, and findings in your journal.  If you need course credit, use your observations recorded in your journal to construct a formal report