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The amount of stored charge is determined
by the amount of electrode material. The more electrode material contained in
the cell the greater its stored charge. Galvanic cells are usually operated
until the amperage is half of its initial value, by that time the internal resistance
has doubled and the power of the galvanic cell is not high enough anymore to
operate an appliance. When the amperage is half of its initial value approximately
50% of a galvanic cell’s stored charge was used during discharging.

The calculation of stored charge on the
basis of the CASSY lab curves are just approximations for the total amount of
charge available for discharging. By changing the amperage to the average
amount of charge (Coulomb) discharged period and assumption based on the definite
integral of the curve can be made.

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This can be compared to the theoretical
amount of available charge in the galvanic cell. By the means of the amount of
electrode material in the cell and the critical point of 50% of the stored
charge, a conclusion on the amount of available charge can be made.

Theoretically the total amount of stored charge
can be calculated as follows:

: number of electrons;

: volume;

concentration of reducing agent;

: electrons transferred during

: amount of charge;

: Faraday-constant (96485.33 C/mol)

3.2.5 Charge density

The charge density is the amount of stored
charge divided by its volume. It is measured in coulombs per cubic meter (C/m?3)

: charge

amount of charge;

 = volume

3.2.6 Electric power

Electric power is the rate, per unit time,
at which electrical energy is transferred by an electric circuit. The SI unit
of power is the watt, one joule per second.

The electric power in watts, produced by an
electric current I consisting of a charge of Q coulombs every t seconds passing
through an electric potential (voltage) difference of V, is:

: electric power;

: voltage;

: amount of charge;

: time;

: amperage

3.2.7 Daniell cell

The Daniell Cell is a type of galvanic cell
invented in 1836 by John Frederic Daniell. It consisted of a copper pot filled
with copper (II) sulfate solution in which was immersed a container filled with
zinc sulfate and a zinc electrode. The zinc electrode functions as an anode and
the copper as a cathode. The Daniell cell represented a great improvement over
the existing technology used in the early days of battery development, for
example the Volta pile.


            Zn(s) ?
Zn2+(aq) + 2e?  (Standard electrode
potential à -0.7618 V)


            Cu2+(aq) +
2e? ? Cu(s)  (Standard electrode potential à +0.340 V)

Overall reaction:

(s) + Cu2+ (aq) ? Zn2+ (aq)
+ Cu (s) (open-circuit voltage à 1.1018

Figure 1: Schematic sketch of the Daniell cell with separated two

During this Matura project the system of
the Daniell cell was preferably used. The electrodes and electrolytes were most
of the time zinc and copper respectively copper (II) sulfate and zinc sulfate.

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