# Video: Understanding the Electric Field between Two Charged Plates

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?

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### 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.