Electrical and electromagnetic methods - Geophysics for the Mineral Exploration Geoscientist - page 242

Resistor

e)

a) e -

Pump

I

Rubber
barrier

Capacitor

I I

54321

Current flow continues until the
capacitor is fully discharged

Water flows whilst barrier
is deforming

f)

b)

Pump

Pump

Rubber
barrier

Lower water
pressure

Higher water
pressure

Alternating water flow direction
repeatedly deforms the barrier in
opposite directions

Further water flow prevented
because barrier cannot be further deformed

c)

g)

e -

Pump (off)

a.c. generator

I

Rubber
barrier

Capacitor

I I

1 2 3 4 5

Capacitor repeatedly
charged and discharged
as current alternates

Water flows as barrier
returns to its undeformed state

V

d)

Battery (d.c.)

e -

I

Capacitor

I I

Current flow continues until
capacitor is fully charged

V

Figure 5.6 Effects of a capacitor in electrical circuits demonstrated using simple plumbing systems. (a - c) Plumbing system analogous to a d.c.
circuit during charging of a capacitor. Labels 1 to 5 indicate stages of deformation of the rubber barrier with increasing time: (a) charging, (b)
fully charged and (c) discharging. (d) and (e) Charging and discharging of a capacitor in a d.c. circuit. (f) Plumbing analogy for a capacitor in
an a.c. circuit. (g) Alternate charging and discharging of a capacitor in an a.c. circuit. V - voltage, and I - current.

Consider now the situation for alternating current. In
our plumbing analogy the elastic rubber barrier is now in a
system where the direction of water flow is continuously
alternating ( Fig. 5.6f ). Its response to the flow will vary
according to the rate or frequency of the alternation. The
degree of deformation will be greater for lower-frequency
alternations because there is more time for expansion to
occur, and the more the barrier is deformed the greater the
pressure required for the process to continue. Logically, the
amount of deformation will decrease as the frequency of

alteration increases, because then there is less time avail-
able between pressure pulses for expansion to occur. So for
higher frequencies the average resistance, or impedance, to
flow presented by the barrier will be less, owing to the
lesser effort required to partially deform the barrier. The
analogous electrical circuit is shown in Fig. 5.6g .
It is clear then that a capacitor in a d.c. circuit, after a
brief charging period, blocks the flow of current. However,
the back-and-forth flow of a.c. is not prevented, although it
is hindered. This opposition to current flow is known as