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Illustration 26.3: Capacitor with a Dielectric
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This animation shows a parallel-plate capacitor connected to a battery that is not shown. The battery maintains a constant electric potential difference between the plates even when you move the dielectric. The red and blue circles on the plates and on the dielectric represent the charge on the plates and the dielectric (position is given in meters, electric field strength is given in newtons/coulomb, and electric potential is given in volts). A dragable dielectric is outside the plates (drag at the center of the dielectric) and the dielectric has a thickness that is smaller than the separation between the capacitor plates. Restart.
What effect does the dielectric have on the electric field between the plates of the capacitor and the charge accumulated on the plates? Make a prediction and test it by dragging the dielectric into the region between the plates.
What did you find? As you drag the dielectric into the capacitor, the electric field inside the dielectric in between the plates decreases. Why does it decrease? Make sure that the dielectric is in the capacitor. How does the electric field between the plates and the charge accumulated change when the dielectric constant of the material is increased or decreased?Make a prediction and test it by changing the dielectric constant.
What did you find? You should have found that, as you increase the dielectric constant of the dielectric, the amount of charge induced on the plates and the dielectric is increased. In response to the initial electric field between the two plates, bound charges are created inside the dielectric. While charge is not free to move inside a dielectric as charge is in a conductor, the charges polarize. This means that neutral atoms become little dipoles in response to the electric field: The electrons are one pole and the nucleus becomes the other pole. This polarization is due to the initial electric field between the plates, which points upward.As a consequence, the positive charges in the dielectric experience an upward force, and the electrons in the dielectric experience a downward force. Once the charges in the dielectric polarize, the net effect is for bound charge to accumulate on the top and bottom of the dielectric. There is no net effect from the dipoles in the middle of the dielectric since the effect of neighboring dipoles cancel. This bound charge creates its own electric field that reduces the initial electric field between the plates, since it points in the opposite direction from the original electric field. The larger the dielectric constant, the larger the bound charge and the more the electric field between the plates gets reduced.
What would happen if the dielectric constant could get really, really big? The bound charge would get bigger and bigger until there was no electric field in between the plates. Such a material is called a conductor.
Note: if the dielectric had a thickness equal to that of the separation between the capacitor plates, the electric field would not have changed. This is due to the fact that while the charge on the plates has changed, the separation between the plates and the electric potential difference between the plates is the same whether there is a dielectric there or not.
Illustration authored by Melissa Dancy and Mario Belloni.