High School Physics

Charge distribution in two oppositely charged parallel metal plates

Last updated on March 16th, 2022 at 12:22 pm

In this post, we will find how the charges are distributed if two parallel metal plates are oppositely charged. We will also know about the Electric field lines between two oppositely charged and parallel plates.

How charge distribution in two oppositely charged parallel metal plates happens?

Electrostatic forces of repulsion of like charges, within each plate, cause the charges to distribute evenly within each plate, and electrostatic forces of attraction of opposite charges on the two plates cause the charges to accumulate on the inner surfaces (see figure 1). Thus, the charges spread and distribute evenly on the inner surfaces of the charged plates.

figure 1: The distribution of net charge on oppositely charged parallel plates

How two parallel plates can be oppositely charged?

Here, one setup is being discussed that can oppositely charge two parallel plates.

This Set up consists of two vertical parallel metal plates connected to the terminals of a power supply that can deliver about 1 kV to the plates. As shown in the figure, one plate gets positively charged and the other one gets negatively charged.

[To Take care when carrying out this experiment (like using the protective resistor in series with the supply if necessary). Also, taking other necessary & standard precautions is required for this kind of setup.]

To begin with, the plates need to be separated by a distance of about one-third of the length of their smaller side.

Now how to know that the plates are charged and there is an electric field between the 2 plates?

A detector is made from a rod of insulator – a plastic ruler or strip of polythene is ideal. Attached to the rod is a small strip of foil: thin aluminium or gold foil or “Dutch” metal are suitable. The dimensions of the foil need to be about 4 cm × 1 cm and the foil can be attached to the insulator rod using adhesive tape. This small piece of foil when charged can be used to detect the presence of an electric field.

See also  Polarization of a Dielectric between parallel plates of a capacitor

If the foil is briefly touched to one of the plates, it will charge the foil. The foil bends away from the plate it touched.
The angle of bend in the foil indicates the strength of the electric field. As the observation is over, supply is to be Turned off.

Electric field lines between two oppositely charged and parallel plates

The magnitude of the resulting electric field can be shown to be the vector sum of each individual field, so it can be shown that the electric field anywhere between the plates is uniform. (figure 2)

figure 2: Electric field lines between two oppositely charged parallel plates
figure 2: Electric field lines between two oppositely charged parallel plates

Thus, between two oppositely charged and parallel plates, electric field lines exist only between the charged plates. These lines extend perpendicularly from the plates, starting at the positively charged plate and terminating at the negatively charged plate. The electric field lines are uniform in both direction and density between the two oppositely charged plates, except near the edges of the plates.

Such a system is called a parallel-plate capacitor.

Use of parallel-plate capacitor

This type of parallel-plate capacitor is found in many different types of electrical equipment, including printers and televisions (where it is part of the “instant-on” feature). It is also used in particle accelerators, such as cathode-ray tubes and mass spectrometers.

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