3. Charge, current, voltage, power and energy Many biomedical instruments utilize a sensor to convert a signal created by the body into an
electrical signal. In medicine, the electrical circuits and electrical components are often utilized
to detect the biomedical signal by sensor. After basic electrical components and biomedical
sensors are connected together, a bioinstrumentation is then formed. Hence, describing a
bioinstrumentation could begin with charge, current, voltage, power and energy. In this
section, these basic variables will be introduced and explained.
3.1. Charge and its conversion In our life, there are two kinds of charge, negative and positive, and they are carried by the
protons and electrons, respectively. The negative charge,
q e , carried by the electron is the
smallest amount of charge that exists and is measured in unit called coulombs(C):
q e = −1.6×10
−19
C The symbol,
q(t)
, is used to represent the charge that change with time, and the symbol,
Q , is
used for constant charge. The charge carried by a proton is the opposite of a electron.
3.2. Current and voltage 3.2.1. Current Electrical current,
i(t)
, is defined as the change in the amount of charge that passes through a
given point or area in a given time period. Current is measured in amperes (A). By the
definition, one ampere equals one coulomb/second (C/s):
i(t)=
dq dt Advances in Bioengineering
200
and
q(t)=
∫ t 0
t i(λ)dλ + q (
t 0
)
Figure 17. A simple circuit illustrating current flowing around a closed loop
In addition to the above definition, current also depends on the direction of flow, as illustrated
in figure 17 Current is defined as positive if
