Open circuits and
short circuits can often cause more confusion and difficulty
in the analysis of a system than standard
series or parallel configurations.
What is short circuit?
A short circuit is a very low resistance, direct connection between
two terminals of a network, as shown in
Fig. 1. The current through
the short circuit can be any value, as determined by the system it is
connected to, but the voltage across the short circuit is always zero
volts because the resistance of the short circuit is assumed to be essentially
zero ohms and $V = IR = I(0 Ω) = 0 V$.
Fig. 1: Defining a short circuit.
In summary, therefore,
a short circuit can carry a current of a level determined by the
external circuit, but the potential difference (voltage) across its
terminals is always zero volts.
In
Fig. 1(a), the current through the 2 Ω resistor is 5 A. If a short
circuit should develop across the 2 Ω resistor, the total resistance of the
parallel combination of the 2 Ω resistor and the short (of essentially zero
ohms) will be
$$2 Ω || 0 Ω = {(2 Ω)(0 Ω) \over
2 Ω + 0 Ω} = 0 Ω$$
as indicated in
[Fig .3(b)], and the current will rise to very high levels,
as determined by Ohm's law:
$$I ={E \over R}= {10V \over 0Ω} = \infty A$$
The effect of the 2 Ω resistor has effectively been "shorted out" by
the low-resistance connection. The maximum current is now limited
only by the circuit breaker or fuse in series with the source.
Fig. 2: Demonstrating the effect of a short circuit on current levels.
For the layperson, the terminology short circuit or open circuit is
usually associated with dire situations such as power loss, smoke, or
fire. However, in network analysis, both can play an integral role in
determining specific parameters of a system. Most often, however, if a
short-circuit condition is to be established, it is accomplished with a
jumper-a lead of negligible resistance to be connected between the
points of interest. Establishing an open circuit just requires making sure
that the terminals of interest are isolated from each other.
Do you want to say or ask something?