The amperage describes the electric current
which is passed through a circuit or appliance. It is measured in A ampere,
or coulombs per second. The electric current is the flow of electric charge in
form of ions or electrons. Voltage and resistance of the circuit alter the
amount of electric current which can flow through the circuit; their correlation
is described in Ohm’s law:
Ampere is measured by connecting an ampere
meter in series with a resistor. They are connected in series, so that all of
the current flows through the ampere meter.
If the circuit is closed without a resistor
connected in series to the ampere meter the short-circuit current (I0)
is measured. The short-circuit current is the maximum amount of electric
current which can be produced by the galvanic cell, since no resistor is
connected in series, there is no resistance, except the internal resistance of
the galvanic cell itself and the negligible resistance of the ampere meter
(approximately 15 mV). Therefore, all of the voltage drops at the internal
resistance (except for a negligible amount at the resistance of the ampere
meter) and thus the limiting factor for the amperage is only the reaction rate
and the movement of the ions towards the electrodes.
Resistance is like a counter force to
voltage, voltage is the potential difference and resistance is a measure of the
difficulty to pass an electric current through a resistor. If a resistor has a
low resistance, a voltage is able to move a certain amount x/t of electric
charge through the resistor, but if the resistance is increased the same
voltage can only move a lower current through the resistor. The resistance is
determined by two properties, the material and the shape of the resistor.
The internal resistance of a galvanic cell
is defined by two properties, by the surface of the electrodes, the area where
reactions can take place, and the difficulty to move electrons, respectively
ions, from one electrode through the electrolyte to the other. In a closed
circuit there is also voltage drop at the internal resistance, for this reason
galvanic cells are supposed to have a low internal resistance, so that all of
the voltage can be utilized in the circuit.
The internal resistance can be measured
with an ohm meter or calculated by subtracting the measured voltage from the
open-circuit voltage and then multiplying by the measured amperage.
: internal resistance;
: open-circuit voltage;