Introduction to Health Physics: Fourth Edition - Ruang Baca FMIPA UB

where
Impedance
t = time after closing the switch (in seconds),
R = resistance (in ohms),
C = capacitance (in farads),
VC = voltage across capacitor, and
V = battery voltage.
REVIEW OF PHYSICAL PRINCIPLES 41
The product RC is called the time constant of the circuit and represents the time
(in seconds) after closing the switch, until the current decreases to 1/e, or 37%
of the initial current, and the voltage increases to V0(1 − 1/e), or 67% of its final
voltage. A charged capacitor in a circuit with a series resistor may be considered
as the equivalent of a battery whose voltage, Vi, is the voltage across the capacitor.
When such a circuit is closed, the capacitor will discharge through the resistor and
the capacitor’s voltage, VC , will decrease according to
Vc = Vie −t/RC , (2.67)
and the charge on each plate of the capacitor is given by
Q = CVC. (2.68)
While electric current in a circuit is transmitted as electrons flow through conducting
wires, electromagnetic waves are transmitted as disturbances in the electromagnetic
fields established in various media, including a vacuum (free space). The electric
and magnetic components of the field may be considered the analogs of voltage and
current in an electrical circuit. The impedance Z of an alternating current is given
by Ohm’s law:
Z = V
, (2.69)
i
where i is the current flow due to the voltage V . By analogy, the impedance of a
medium through which an electromagnetic wave is propagated is given by
Z = ε0
, (2.70)
H0
where ε0 and H0 are the electric and magnetic field strengths respectively. Using the
relationship given by Eq. (2.55),
√ √
H0 μ = ε0 ∈,
we can rewrite Eq. (2.70) as
Z =
μ
. (2.71)
∈