Polarity of voltage
drops
We can trace the direction that electrons
will flow in the same circuit by starting at the negative
(-) terminal and following through to the positive (+)
terminal of the battery, the only source of voltage in the
circuit. From this we can see that the electrons are moving
counter-clockwise, from point 6 to 5 to 4 to 3 to 2 to 1 and
back to 6 again.
As the current encounters the 5 Ω
resistance, voltage is dropped across the resistor's ends.
The polarity of this voltage drop is negative (-) at point 4
with respect to positive (+) at point 3. We can mark the
polarity of the resistor's voltage drop with these negative
and positive symbols, in accordance with the direction of
current (whichever end of the resistor the current is
entering is negative with respect to the end of the
resistor it is exiting:
We could make our table of voltages a little
more complete by marking the polarity of the voltage for
each pair of points in this circuit:
Between points 1 (+) and 4 (-) = 10 volts
Between points 2 (+) and 4 (-) = 10 volts
Between points 3 (+) and 4 (-) = 10 volts
Between points 1 (+) and 5 (-) = 10 volts
Between points 2 (+) and 5 (-) = 10 volts
Between points 3 (+) and 5 (-) = 10 volts
Between points 1 (+) and 6 (-) = 10 volts
Between points 2 (+) and 6 (-) = 10 volts
Between points 3 (+) and 6 (-) = 10 volts
While it might seem a little silly to
document polarity of voltage drop in this circuit, it is an
important concept to master. It will be critically important
in the analysis of more complex circuits involving multiple
resistors and/or batteries.
It should be understood that polarity has
nothing to do with Ohm's Law: there will never be negative
voltages, currents, or resistance entered into any Ohm's Law
equations! There are other mathematical principles of
electricity that do take polarity into account through the
use of signs (+ or -), but not Ohm's Law.
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