Biomedical Engineering Reference
In-Depth Information
Closed Surface
i
7
Circuit
Element
Circuit
Element
E
Circuit
Element
i
6
i
2
i
3
i
5
B
Circuit
Element
Circuit
Element
D
A
i
1
i
4
i
8
Circuit
Element
Circuit
Element
Circuit
Element
C
FIGURE 9.6
A circuit with a closed surface.
Kirchhoff's current law is also applicable to any closed surface surrounding a part of the
circuit. It is understood that the closed surface does not intersect any of the circuit elements.
Consider the closed surface drawn with dashed lines in Figure 9.6. Kirchhoff's current law
applied to the closed surface gives
i
þ
i
þ
i
þ
i
¼
0
1
4
5
7
9.3.3 Voltage
Voltage represents the work per unit charge associated with moving a charge between
two points (A and B in Figure 9.7) and is given as
v
¼
dw
dq
ð
9
:
4
Þ
The unit of measurement for voltage is the volt (V). A constant (DC) voltage source is
denoted by
V
, while a time-varying voltage is denoted by
v(t),
or just
v
.InFigure 9.7, the
voltage,
, between two points (A and B) is the amount of energy required to move a charge
from point A to point B.
v
i
1
A
+
v
−
Circuit
Element
2
B
FIGURE 9.7
Voltage and current convention.
