The defibrillator capacitor
discharges in 11 ms. How much
current does it send through the
heart?
C = q/ǻV
q = CǻV = (68 × 10í^6 F)(2.5 × 10^3 V)
q = 0.17 C
I = ǻq/ǻt
I = (0.17 C) / (0.011 s)
I = 15 A
26.7 - An insulator (dielectric) in an electric field
The simplest capacitor is made up of two plates separated by a vacuum. In this section
we will discuss what happens when an insulating material, a dielectric, is placed
between the plates.
Like a vacuum, this insulating material is intended to prevent the flow of charge
between the plates, but it has another effect, as well: It reduces the overall strength of
the field between the plates, and this proves to be a desirable effect.
To understand how a dielectric reduces the net field, let’s first consider what happens
when an atom is placed in the electric field generated by two charged plates. It might
seem that nothing would happen because the atom is electrically neutral.
However, an atom is made up of a positive nucleus surrounded by negative electrons. If
the field is very strong, it can ionize atoms, separating electrons from the atoms. At this
point, the dielectric material becomes a conductor: Current can flow between the plates,
causing the capacitor to break down. Although an interesting phenomenon to witness,
this outcome is not relevant to the rest of this section.
Less extreme fields can polarize an atom, in essence stretching it so that its electrons
tend to be on one side and its positive nucleus is on the other. This turns the atom into
a dipole, a body with positively and negatively charged regions.
Some kinds of molecules, such as water molecules, are always dipoles. This type of
molecule (called a polar molecule) has regions of positive and negative charge based
on its structure. When no external electric field is present, the dipoles of substances
such as water are randomly aligned, and they create no net electric field.
When a substance containing dipoles is placed in the electric field between a charged
capacitor’s plates, the field will cause some of the dipoles to align so that their positive
poles point toward the negative plate and their negative poles point toward the positive
plate.
Many, but by no means all, of the dipoles that compose the substance will align in this
manner with the external field. Their random thermal motion provides a constant
counterbalance to the organizing tendency of the field. The effect of the alignment that
does occur is to weaken the overall field between the plates. There are two ways to
understand the weakening of the capacitor field in the presence of a dielectric.
First, consider the net effect of the individual fields of all the aligned dipoles. Each
aligned dipole has its own electric field, oriented in the direction opposite to the field
created by the plates. This is shown in Concept 1. The aligned dipoles’ fields reduce the overall field between the plates because they point
oppositely to the capacitor’s field.
You can also look at the diagram in Concept 2 to understand why the overall field is diminished. In the bulk of the dielectric the net charge is
zero. This is because every aligned positive pole there is next to an aligned negative pole on a neighboring dipole, which it balances out.
However, on the far left there is an unbalanced layer of negative charge, and on the far right there is an unbalanced layer of positive charge.
Dielectric
When plates are charged:
Dipole alignment creates field in
dielectric
·Dielectric field opposes field of platesDielectric effect
Dielectric weakens overall capacitor
field