Leyden Jar, Electric Charge, Galvanic Cell, Voltaic Pile, Anions, Cations, Daniell Cell, Zinc-Carbon Battery and Alkaline Battery

Posted by PITHOCRATES - October 2nd, 2013

Technology 101

(Originally published December 19th, 2012)

Luigi Galvani made a Dead Frog’s Leg Twitch when he hit it with the Electric Discharge Shock from a Leyden Jar

The field of electricity took off with friction generators.  Dragging something across another substance to produce an electrical charge.  Like sliding out of your car on a dry winter day.  Producing an electric discharge shock just before your hand touches the metal door to close it.  Atoms in materials are electrically neutral.  There are an equal number of positive particles (protons) and negative particles (electrons).  Friction can transfer some of those electrons from one surface to another.  Leaving one surface with a net positive charge.  And the other with a net negative charge.  These charges equalize after that electric discharge shock.  Returning the atoms in these materials to an electrically neutral state.

Further exploration of static electric charge led to the development of the Leyden jar.  A precursor to the modern capacitor.  A glass jar with metal foil on the inside and outside of a glass bottle.  The foil sheets act as plates.  The glass as a dielectric.  An electrode attached to one plate received an electric charge from a friction generator.  The other plate was grounded.  The dielectric helped the plates hold an electric charge.  Benjamin Franklin did a lot of experiments with the Leyden jar.  He noted how multiple Leyden jars could hold a greater charge.  Commenting that it was like a battery of cannons.  Giving us the word battery for an electrical storage device.

Luigi Galvani made a dead frog’s leg twitch when he zapped it with the electric discharge shock from a Leyden jar.  Furthering his experiments Galvani found that he could reproduce the twitching by placing the frog’s leg between two different types of metals.  Creating a galvanic cell.  Which created an electric current.  Alessandro Volta recreated this experiment while substituting the frog tissue with cardboard soaked in salt water (an electrolyte).  Creating the voltaic cell.  Piling one voltaic cell onto another created a Voltaic Pile.  Or as we call it today, a battery.

A Daniell Cell created a Current by Stripping away Electrons from one Electrode and Recombining them on Another

What Galvani and Volta discovered was a chemical reaction that caused an electric current.  The Voltaic Pile, though, had a limited life.  To improve on it John F. Daniell added a second electrolyte.  Creating the Daniell Cell.  Which extended the life of a battery charge.  Allowing it to do useful work.  Becoming the first commercially successful battery.  Powering our first telegraphs and telephones.  Even finding their way into our homes operating our doorbells for a century or so before Nikola Tesla brought alternating current electric power to our homes.

The chemical reaction in a Daniell Cell created an electric current by stripping away electrons from one metal electrode in a solution (anode oxidation).  And recombining electrons onto another electrode of a different metal in a different solution (cathode reduction).  Each electrode is in an electrolyte solution.  In a copper-zinc Daniell Cell the anode is typically in a solution of zinc sulfate.  And the cathode is in a copper sulfate solution.  A salt bridge or porous membrane connects the different electrolytes.  When an electric load is connected across the ‘battery’ electrodes it completes the electrochemical system.

Each electrolyte contains ions.  Atoms with a net positive or negative charge.  Positive ions are cations.  Negative ions are anions.  The cathode attracts cations.  Where they combine with free electrons to return to a neutral state.  The anode attracts anions.   Where they give up their extra electrons to return to a neutral state.  This chemical activity dissolves the zinc electrode.  And deposits copper on the copper electrode.  (This electrolysis is the basis for the metal plating industry.)  It is the dissolving of the anode that gives up electrons that travel from one electrode through the electric load to the other electrode.  Doing work for us.  By lighting our flashlights.  Or powering our portable radio.  When the anode dissolves to the point that it cannot give up anymore electrons the chemical reaction stops.  And we have to replace our batteries.

An Alkaline Battery will produce more Useable Power and have a longer Shelf Life than a Zinc-Carbon Battery

Of course, the zinc-carbon batteries we use for our flashlights and radios are not wet cells.  They’re dry cells.  Instead of an electrolyte solution the common battery is made up of dry components.  The zinc anode is the battery casing.  Just inside the battery zinc casing is a paper layer impregnated with a moist paste of acidic ammonium chloride.  This separates the zinc can from a mixture of graphite powder and manganese (IV) oxide (pyrolusite).  In the center of the battery is a carbon rod.  The zinc casing is the negative electrode (anode) and the carbon rod is the positive electrode (the cathode).  The chemical reactions are the
same as they are with the wet cell.  The zinc casing (the anode) becomes thinner over time.  When holes begin to appear the battery will leak creating a sticky mess.  As you no doubt experienced when taking an old set of batteries out of a flashlight that hasn’t been used in years.

An alkaline battery looks similar to a zinc-carbon battery.  But there are many differences.  Instead of an acidic ammonium chloride electrolyte an alkaline battery uses an alkaline potassium hydroxide electrolyte.  The little nub (positive terminal) on top of the battery does not connect to a carbon rod in the center of the battery.  It connects to the outer casing.  Inside this casing is a mixture of graphite powder and manganese (IV) oxide (pyrolusite).  Then a barrier to keep the anode and cathode materials from coming into contact with each other.  But lets ions pass through.  On the other side of the barrier is the anode.  A gel of the alkaline potassium hydroxide electrolyte containing a dispersion of zinc powder.  In the middle of the battery is a metal rod that acts as a current pickup that connects to the bottom of the battery (the negative terminal).

Alkaline batteries are the most popular batteries today.  Because they have a higher energy density than a zinc-carbon battery.  Meaning that an alkaline battery will produce more useable power than a comparable sized zinc-carbon battery.  And they have a longer shelf life.  But with these benefits comes costs.  They can leak a caustic potassium hydroxide.  An irritant to your eyes and skin.  As well as your respiratory system.  As they age they can produce hydrogen gas.  Which can rupture the casing.  If a battery leaks potassium carbonate (a crystalline structure) can grow.  If this crystalline structure reaches the copper tracks of a circuit board it will oxidize the copper and metallic components.  Damaging electronic devices.  But the benefits clearly outweigh the risks.  As about 80% of all batteries sold in the U.S. are alkaline batteries.



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