The voltage-ampere characteristics of an electronic component are its behavior for various values of applied voltage. Put simply; it is the graph between Voltage and Current obtained when current is measured through an electronic component as a voltage is applied across it.
While V-I characteristics are studied it is common to have the Voltage on the x-axis and the current, I on the y-axis because it is easier to control the applied voltage rather than current. This makes the voltage the independent variable and is hence traditionally placed on the x-axis. Thus, in this case the resistance R at any point is the inverse of the slope of the V-I curve because
A linear VI curve has a constant slope and hence a constant resistance. Carbon resistor and metals obey the Ohm’s law and have a constant resistance. This means that the V-I curve is a straight line passing through the origin.
An electronic component may exhibit linear characteristic only in a particular region. For example, a diode has a mostly linear behavior in its operating region.
A circuit component has non-linear characteristic if the resistance is not constant throughout and is some function of voltage or current. The diode, for example, has varying resistance for different values of voltage.
However, it has a linear characteristic for a narrow operating region. Note that in the graph below we can also see the maximum forward and reverse voltage in which the diode can be operated without causing breakdown and burning up of the diode.
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