MadSci Network: Physics

Re: Variables in a Voltaic Pile?

Date: Tue Dec 9 04:18:16 2003
Posted By: Ewen McLaughlin, Lecturer, Chemistry, Swansea College
Area of science: Physics
ID: 1069441472.Ph


I've answered your questions in order:

Q1 The highest voltage will come from a combination of a very reactive 
metal with a very unreactive metal but why?

A1 Metals react by losing electrons: the more reactive a metal, the more 
easily it will lose electrons. This is usually measured by the reduction 
potential – you can think of this as the voltage required to force 
electrons to react with the positive metal ion. Here are the reduction 
ppotentials for the metals you have. I took the values from Huheey’s 1983 
Mg+2 --> Mg	-2.363 V
Al+3 --> Al	-1.662 V
Zn+2 --> Zn	-0.7628 V
Pb+2 --> Pb	-0.126 V
Cu+2 --> Cu	+0.337 V
You can see that Mg is the most reactive metal at your disposal. A cell 
made with Mg and Cu should give 2.7 V (subtract the reduction ppotential 
of the more reactive elements from that of the less reactive element). 
This is under standard conditions i.e. 25 C, 1 mol dm-3 solutions, etc.

Q2 Also, if voltage = current*resistance, then shouldn't it be that the 
more layers in the pile, the higher the voltage?

A2The voltage increases with each layer you add, but this is not because 
voltage = current * resistance. Each layer is a cell, producing a small 
potential difference. Stacking the layers connects the cells in series to 
make a battery. The total potential difference will be the potential of 
each cell times the number of cells in the battery.

Q3 Lastly, do the concentration of the saltwater and the thickness of the 
filter-paper between the metals affect the voltage? If so, how?

A3 The saltwater-soaked filter paper/cardboard/felt allows ions to pass 
from one plate of the cell to the other – the ‘salt bridge’ of classic 
electrochemical cells. Without it, or without the salt – no voltage. This 
is a vital part of the circuit – not only does charge flow, as electrons, 
in the external circuit (lighting a bulb, powering a calculator, whatever) 
but charge also must flow within the cell. This charge flows as ions, 
positive ions such as sodium will flow in the opposite direction to the 
negatively-charged electrons in the outer circuit; negatively-charged 
ions, such as chloride, will flow in the same direction as electrons. 
Hydrogen ions are highly conductive, so an acid should be better than 
sodium chloride.
The thickness of the filter paper should be minimised, so that its 
conductivity is increased. However, direct contact between the metal 
plates will allow electrons to pass from one to the other, short-
circuiting the cells. The filter paper must be robust enough to stop this 

Ewen McLaughlin

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