Video Transcript
The diagram shows the electric
field created by a pair of parallel charged plates. The red plate represents the
positively charged plate and the blue plate represents the negatively charged
plate. At which of the points 𝑃, 𝑅, or
𝑆 is the electric field strongest?
Looking at the diagram, we see the
red plate which we’re told has a positive charge and the blue plate which we know
has a negative charge. Thanks to that charge difference,
an electric field is set up between the plates. And we see the field lines
represented by these hashed lines. We also see three points between
the plates: point 𝑃, point 𝑅, and point 𝑆. And we want to know at which of
these three points is the electric field strongest. To figure this out, there’s
something we’ll want to recall about electric field lines and how they relate to
electric field strength.
Electric field line density
indicates relative electric field strength. That means that electric field
lines which are spaced closer together indicate a stronger electric field than field
lines that are spaced farther apart. For example, if we were to look at
the electric field created by a positive point charge, if we put a box far away from
this point charge, not many field lines will pass through it. But as we move the box in closer,
more field lines would pass through it. That is, field line density would
increase. And likewise, electric field
strength would increase. And that brings us back to our
diagram.
When we look at these electric
field lines between the parallel plates, we notice that they run parallel to one
another. That is, the spacing between
electric field lines is always the same. They don’t get closer together or
farther apart. In other words, the electric field
line density is constant all throughout this region between the plates. And that tells us that the relative
electric field strength is also a constant in this region. It’s not stronger in one place and
weaker in another. But it’s the same strength
everywhere. This is true whether we’re
considering points 𝑃, 𝑅, and 𝑆 or any point in between these plates. That then tells us our answer about
where the electric field is strongest. The electric field has the same
strength at each point. And as we saw, that includes all
points between the parallel plates.
