Since these two cases cause the same outcome, there is no need to be concerned as to which is
responsible for the current. In electrical circuits, current is carried by electrons in metallic
inductors.
Consider the waveform in figure 18, with the current entering into terminal1 in the circuit on
the right, the current in the time interval 0 to 1.5 second, is positive and enters terminal1. The
current in the time interval 1.5 to 3 second, is negative and enters terminal2 with positive value.
If there are no current changes in the time interval 0 to 3s, the curve of current will be a line.
Then the electrical circuit in figure 18 is constant which is called direct current (DC) indicating
that it does not change with time. We denote a time-varying current with lowercase letter, such
as
i
or just
i(
t)
.
Kirchhoff’s Current Law
Current can only flow in a closed circuit. Kirchhoff’s current law is used to ensure the rela‐
tionship among every branch of circuit at same point. For current is continuous, any a point
in circuit can not accumulate charge. Hence, at any time and any node, the sum of the currents
which flow same node is equal to the sum of the currents which outflow from same node. This
principle is known as Kirchhoff’s current law (KCL).
In circuit as illustrated in figure 19, the current at the node,
a
, can be written as:
I
1
+
I
2
=
I
3
or, above formula is adjusted into the following equation:
I
1
+
I
2
−
I
3
=0
Namely,
∑
I =0
At any time, the algebraic sum of the currents at a node is equal to zero. It should be clear for
all currents whether they are all leaving or entering the node.
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