MadSci Network: Physics
Query:

Re: How does a D-cell battery work?

Date: Sun May 28 12:04:12 2000
Posted By: Sidney Chivers, , Nuclear Engineering, retired
Area of science: Physics
ID: 958778624.Ph
Message:

There is an answer to a related question in the MadSci archives at
 /posts/archives/jan99/916205789.Eg.r.html

In addition, most college chemistry textbooks include a discussion of dry cell and other type batteries.

Current and electrons flow in opposite directions. Some textbooks describe this as due to a historical precedent rather than a technical necessity. Conventional current direction is defined as the direction that positive charges flow. The discovery that electrical current, in most applications, is due to the movement of electrons with negative charge came later.

In a D-cell, in fact in any electrical circuit, current is said to flow from the positive terminal to the negative terminal. To allow current to flow from the positive terminal of a D-cell to its negative terminal, the circuit must be completed by adding a electrically conductive path between the two terminals. That done, the right-hand-rule applies to the conductive path in that with the fingers of the right hand wrapped around the conductive path (i.e. a copper wire) and the thumb pointed in the direction of the current, the magnetic field lines are circling the conductive path in the direction indicated by the fingers of the right hand. Having said that I also add the caution that in demonstrating the right hand rule you don't really want your fingers wrapped around the conductive path, particularly if the copper wire or other conductor used is not insulated, as you would then become part of the conductive path and may find the experience unpleasant.

I used two references that you may want to consult:

- Gary L. Buckwalter and David M. Riban, College Physics, McGraw-Hill, 1987, isbn 0-07-052142-5, page 452-453 (provides a discussion of conventional current direction which should also be available in any other college physics textbook)

- Steven S. Zumdahl, Chemical Principles, D. C. Heath and Company, 1992, isbn 0-669-27871-8, pages 444 to 451 (a discussion of battery types which should also be available in any other college chemistry textbook)

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Admin note:
Jocelyn Wishart adds the following:

Yes, there is a difference. When the idea of current was established in Faraday’s time they had no knowledge of the role electrons played. Arbitrarily it was decided current would flow from the positive to the negative. This is now known as "conventional current". Now we know that electrons really move in the opposite direction.

conventional current

electron flow

The left and right hand rules refer to the directions of conventional current, magnetic field and motion in an electric motor or dynamo respectively. They are not linked to the direction of current in a battery cell.

The direction of current in a cell is caused by the reaction between the chemicals that make up the cell.

In the most common "batteries" the carbon electrode ( positive terminal) is surrounded by ammonium chloride and water paste (the electrolyte) contained within a zinc casing. The zinc is the negative terminal and when connected to the positive terminal say via a bulb gives up electrons as it reacts to become zinc ions. At the carbon rod electrons are taken up in a reaction changing the ammonium ions to ammonia and hydrogen. So a current of electrons flows from the zinc casing to the carbon electrode via the bulb and lighting it. The current is transmitted through the cell itself by the ions to complete the circuit.

So the direction of current is caused by the types of electrodes involved and not any ‘rules’. Electrons are lost by the more reactive of the pair to the other one and true current flows from the electrode made of the more reactive "metal" to the less reactive one. Conventional current as ever flows in the opposite direction.


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