Question

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A proton is released from the positively charged plate and an electron is released from the negatively charged plate. When they reach the opposite plate, the work done by the electric field on the electron is

The magnitude of the force experienced by the proton and the electron due to the electric field is the same. The distance travel by the proton and electron is also the same. Therefore, the work done on the electron and proton will be the same

Answer 1

The charge , Q, causes every other charge, q, to have electric potential energy. Q is called the source charge as it is considered to be the cause of the electric potential energy, while q is called a test charge.

A common definition for electric potential is the work done per unit charge moving a test charge from infinity to a point in an electric field. The units are J/C, or volts (V).

V = U/q

Therefore, U = Vq

That is, the work done on every coulomb of charge of a test charge to bring it from infinity to at a distance r away from the source of the field is

V = kQ/r

The work done moving a charge, q, from one point with potential V₁ to another point with potential V₂ in the field is

U = q(V₂ -V₁)
The electric field orientation of a ( +q ) and a ( -q ) charge separated by a distance L is shown below. Such two equal and opposite charges form the so-called an "electric dipole."

Note: The meaning of each field line is as follows: if a test charge ( +1 unit of charge ), is placed on a field line, it will move on that curved line being constantly repelled from the (+q) and attracted toward the (-q) until it is absorbed by (-q ).

The space around these two charges contains infinite number of points. Each point experiences the effects of two fields, one from the ( +q ) charge and one from the ( -q ) charge.    Since each field acts along the line that connects the charge to a given point, vector addition must be employed in order to find the resultant field. The following example clarifies the need for vector addition:

Homework: In the figure shown, find the resultant field at each point where there is a charge. For example, when finding the field at where q1 is, suppose q1 is nonexistent and find the resultant field by q2 and q3 at that point.