Determining Field from Potential

LEARNING OBJECTIVES

By the end of this section, you will be able to:

• Explain how to calculate the electric field in a system from the given potential
• Calculate the electric field in a given direction from a given potential
• Calculate the electric field throughout space from a given potential

Recall that we were able, in certain systems, to calculate the potential by integrating over the electric field. As you may already suspect, this means that we may calculate the electric field by taking derivatives of the potential, although going from a scalar to a vector quantity introduces some interesting wrinkles. We frequently need

to calculate the force in a system; since it is often simpler to calculate the potential directly, there are systems in which it is useful to calculate

and then derive

from it.

In general, regardless of whether the electric field is uniform, it points in the direction of decreasing potential, because the force on a positive charge is in the direction of

and also in the direction of lower potential

. Furthermore, the magnitude of

equals the rate of decrease of

with distance. The faster

decreases over distance, the greater the electric field. This gives us the following result.

RELATIONSHIP BETWEEN VOLTAGE AND UNIFORM ELECTRIC FIELD

In equation form, the relationship between voltage and uniform electric field is

where

is the distance over which the change in potential

takes place. The minus sign tells us that

points in the direction of decreasing potential. The electric field is said to be the gradient (as in grade or slope) of the electric potential.

For continually changing potentials,

and

become infinitesimals, and we need differential calculus to determine the electric field. As shown in Figure 3.4.1, if we treat the distance

as very small so that the electric field is essentially constant over it, we find that

(Figure 3.4.1)

Therefore, the electric field components in the Cartesian directions are given by

   (3.4.1)

This allows us to define the “grad” or “del” vector operator, which allows us to compute the gradient in one step. In Cartesian coordinates, it takes the form

   (3.4.2)

With this notation, we can calculate the electric field from the potential with

   (3.4.3)

a process we call calculating the gradient of the potential.

If we have a system with either cylindrical or spherical symmetry, we only need to use the del operator in the appropriate coordinates:

   (3.4.4)

   (3.4.5)

EXAMPLE 3.4.1

Electric Field of a Point Charge

Calculate the electric field of a point charge from the potential.

Strategy

The potential is known to be

, which has a spherical symmetry. Therefore, we use the spherical del operator in the formula

.

Solution

Performing this calculation gives us

This equation simplifies to

as expected.

Significance

We not only obtained the equation for the electric field of a point particle that we’ve seen before, we also have a demonstration that

points in the direction of decreasing potential, as shown in Figure 3.4.2.

EXAMPLE 3.4.2

Electric Field of a Ring of Charge

Use the potential found in Example 3.2.5 to calculate the electric field along the axis of a ring of charge (Figure 3.4.3).

Strategy

In this case, we are only interested in one dimension, the

-axis. Therefore, we use

with the potential

found previously.

Solution

Taking the derivative of the potential yields

Significance

Again, this matches the equation for the electric field found previously. It also demonstrates a system in which using the full del operator is not necessary.

CHECK YOUR UNDERSTANDING 3.11

Which coordinate system would you use to calculate the electric field of a dipole?

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