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1.5.3 
Ideal Short-circuit Element and Ideal open-circuit Element
Ideal two-terminal short-circuit element is the element that is used to model a piece of connecting wire 
in Circuit Theory. It is also used to model an ideal switch in closed condition. It has no resistance, no 
inductance and no charges distributed on it. The voltage across its terminals is constrained to remain 
at zero. It can carry an arbitrary current that is decided by rest of the circuit. Thus, definition of ideal 
short-circuit element parallels that of an ideal independent voltage source. Hence an ideal short-
circuit element may be thought of as a special case of an ideal independent voltage source with E(t) 
=
0 for all t. It is described by the following equations.
v(t) 
=
0 V,
 i(t) 
=
Arbitrary, decided by the rest of the circuit in which this source is connected.
Similarly, an ideal open-circuit element is equivalent to an ideal independent current source with 
I
s
(t) 
=
0.
It is described by the following equations.
i(t) 
=
0 A,
v(t) 
=
Arbitrary, decided by the rest of the circuit in which this source is connected.
In practice, a short-circuit element has a little resistance and inductance in series. A practical
open-circuit has a small capacitance shunting its terminals.
1.6 
PowEr and EnErgy rElatIonS For two-tErmInal ElEmEntS
An ideal two-terminal circuit element has a unique voltage variable assigned at its terminals and a 
unique current variable assigned to its terminals. The electrical behaviour of such an element can be 
described in terms of these two variables at all instants. Electromagnetic disturbances are assumed to 
travel instantaneously to all parts of such an element. This results in an electrical description that is 
Fig. 1.5-3 

IdealIndependent
CurrentSource
–
–
+
+
(b)
(a)
i
(
t
)
i
(
t
)
v
(
t
)
I
s
V
I
s
(
t
)
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1.30


CircuitVariablesandCircuitElements
independent of space variables for the element. Such an electrical description for an element is termed 
as lumped parameter description.
Further, ideal two terminal elements have only one kind of electrical phenomena taking place 
inside them. The capacitive and inductive effects in a practical resistance are neglected in order to 
arrive at an ideal two-terminal resistance model. The resistive and inductive effects in a physical 
capacitor are neglected to model it by an ideal two-terminal capacitor. The capacitive and resistive 
effects in a physical inductor are neglected to arrive at the ideal two-terminal inductance model.
Moreover, lumped two-terminal elements confine the 
electromagnetic fields associated with them to the space 
inside them and in the immediate vicinity.
Such a two-terminal element can be represented in 
general by the symbol below in Fig. 1.6-1. The variable 
assignment for the element is also shown in the figure.