The Voltaic Cell

Definition of Voltaic or Galvanic Cell:

When two different conducting materials are immersed in an electrolyte, as shown in figure (a), the chemical action of forming a new solution results in the separation of charges.  This method for converting chemical energy into electric energy is a voltaic cell. It is also called galvanic cell, named after Luigi Galvani (1737 – 1798).

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(a) Electrodes or plates in liquid electrolyte solution

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(b) Schematic of a circuit with a voltaic cell as a dc voltage source V to produce current in Load

Referring back to figure (a), the charged conductors in the electrolyte are the electrodes or plates of the cell.  They serve as the terminals for connecting the voltage output to an external circuit, as shown in figure (b).  Then the potentials difference resulting from the separated charges enables the cell to function as a source of applied voltage.  The voltage across the cell’s terminals forces current to flow in the circuit to light the bulb.

Current output of Voltaic Cell:

Electrons from the negative terminal of the cell flow through the external circuit with RL and return to the positive terminal.  The chemical action in the cell separates charges continuously to maintain the terminal voltage that produces current in the circuit.

The current tends to neutralize the charges generated in the cell.  For this reason, the process of producing load current is considered discharging of the cell.  However, the internal chemical reaction continues to maintain the separation of charges that produces the output voltage.

Current inside the Voltaic Cell:

The current through the electrolyte is a motion of ion charges.  Notice in figure (b), that the current inside the voltaic cell flows from the positive terminal to the negative terminal.  This action represents the work being doe by the chemical reaction to generate the voltage across the output terminals.

The negative terminal in figure (a) is considered to be the anode of the voltaic cell because it forms positive ions for the electrolyte.  The opposite terminal of the voltaic / galvanic cell is its cathode.

Internal Resistance of Voltaic Cell:

Any practical voltage source has internal resistance, indicated as ri.  This limits the current it can deliver.  For a chemical cell, as in above figures, the ri is mainly the resistance of the electrolyte.  For a good cell, ri is very low, with typical values less than 1Ω. As the cell deteriorates, though ri increases, preventing the cell from producing its normal terminal voltage when load current is flowing.  The reason is that the internal voltage drop across ri opposes the output terminal voltage.  This factor is why yu can often measure normal voltage on a dry cell with a voltmeter, which drains very little current, but the terminal voltage drops when the load is connected.

The voltage output of a cell depends on the elements used for the electrodes and the electrolyte.  The current rating depends mostly o the physical size.  Larger batteries can supply more current.  Dry cells are generally rated up to 250 mA. While the lead acid wet cell can supply current up to 300 A or more.  Note that a smaller ri allows a higher current rating.

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