Note that for nearly all materials the current - voltage relation is
temperature dependent. Thus at low temperatures the curve in Fig.
rises more steeply for positive V
than at higher
temperatures, and at successively higher temperatures the asymmetry in
the curve becomes less and less pronounced.
The current - voltage
relation for a source may also be represented graphically. For a source
represented by Vab=E−Ir
, that is, V=E−Ir
, The graph appears
in Figure (b)
. The intercept on the V−
axis, corresponding to
the open circuit condition (I=0)
, is at V=E
, and the intercept
on the I−
axis, corresponding to a short-circuit situation (V=0)
is at I=Er
This relation may be used to find the
current in a circuit containing a non linear device, as in fig (c)
Its current voltage relation is shown in fig (d)
is also plotted on this graph, each curve represents a
current-voltage relation that must be satisfied, so the intersection
represents the only possible values of V
. This amounts to a
graphical solution of two simultaneous equation of V
one of which is nonlinear.
When a device has a nonlinear
voltage-current relation, the quantity VI
is not constant.
This ratio may still be called resistance, but now it varies with
current; it is constant only for a device obeying Ohm's law. Often a
more useful quantity is dVdI
, which expresses the relation
between a small change in current and the resulting voltage change.
This is called dynamic or incremental resistance. Which of the following vacuum tube devices shows negative resistance?