Biomedical Engineering Reference
In-Depth Information
components of the system to be adjusted so that the output and input have a known,
measured relationship. Without this information, it is impossible to convert the output of
an instrument system into a meaningful representation of the measurand.
Another important component, a feedback element, is not a part of all instrumentation
systems. These devices include pacemakers and ventilators that stimulate the heart or the
lungs. Some feedback devices collect physiological data and stimulate a response—a heart-
beat or breath—when needed or are part of biofeedback systems in which the patient is
made aware of a physiological measurement, such as blood pressure, and uses conscious
control to change the physiological response.
9.3 CH
ARGE, CURRENT, VOLTAGE, POWER, AND EN
ERGY
9.3.1 Charge
Two kinds of charge, positive and negative, are carried by protons and electrons, respec-
tively. The negative charge carried by an electron,
q
e
, is the smallest amount of charge that
exists and is measured in units called coulombs (C).
10
19
C
q
e
¼
1
:
602
The symbol
q(t)
is used to represent charge that changes with time, and
Q
is used for
constant charge. The charge carried by a proton is the opposite of the electron.
9.3.2 Current
Electric current,
, is defined as the change in the amount of charge that passes through
a given point or area in a specified time period. Current is measured in amperes (A). By
definition, one ampere equals one coulomb/second (C/s).
i(t)
t
Þ¼
dq
dt
i
(
ð
9
:
1
Þ
and
Z
t
q
(
t
Þ¼
i
(
lÞ
d
l þ
q
(
t
Þ
ð
9
:
2
Þ
0
t
0
Current, defined by Eq. (9.1), also depends on the direction of flow, as illustrated in the
circuit in Figure 9.3. Current is defined as positive if
a.
A positive charge is moving in the direction of the arrow.
b.
A negative charge is moving in the opposite direction of the arrow.
Since these two possibilities produce the same outcome, there is no need to be concerned as
to which is responsible for the current. In electric circuits, current is carried by electrons in
metallic conductors.
