Conceptual Physics

between two charges is proportional to their magnitudes and inversely proportional to

the square of the distance between them. Opposite charges attract one another, and

like charges repel.

The strength of the field equals the amount of force divided by the magnitude of the test

charge. The equation for calculating the field in this fashion is shown in Equation 1.

Since an electric field is used to describe the nature of the force that a test charge

experiences at a point in space, the field has a specific direction and magnitude at each

point. It is a vector quantity.

The strength of the field is independent of the test charge. Dividing out the charge

cancels its effect in determining strength. For example, if the test charge were doubled

in strength, then Coulomb’s law states that the force would be twice as large, but after

dividing by the doubled charge, the field strength is the same. Electric fields are

measured in newtons per coulomb; there is no special name for this combination of

units.

The stronger the field at a given point, the greater the force it will exert on any charge at

that point. You can perhaps envision this by considering the nature of the gravitational

field surrounding the Earth. The gravitational field is stronger near the surface of the

Earth than it is at locations farther away. An object with a particular mass will

experience more gravitational force closer to the Earth, where the field is stronger, than

farther away, where the field is weaker. The gravitational field is stronger on the surface

of Jupiter than it is on the Earth’s surface. The stronger field means you weigh more on

Jupiter because there is more gravitational force pulling on you.

In this section, we implicitly focused on fields produced by a single electric charge. This

is an important case, and provides a concrete example of how fields arise. However, as

you advance in your studies, you will often study fields and their effects by themselves,

with less focus on their sources.

In the industrialized world, you are constantly surrounded by electric fields. You may

experience electric fields ranging in strength from nearly zero up to 10 N/C due to

electrical appliances and wiring as you walk around your house. Outside your house

you may experience significantly greater electric fields. The field at ground level directly

beneath a power transmission line is about 2000 N/C. This is enough to cause a

fluorescent tube to light up if you hold it vertically in the field. Within atoms, electric

fields are extremely large. The electric field due to a proton at the distance typical of an

electron in a hydrogen atom is 5×10^11 N/C.

Electric fields

Charged object exerts force at a

distance

Field surrounds charged object

Field equals force per unit charge

Electric field

E = F/qtest

E = strength of electric field

F = force on test charge

qtest = positive test charge

Direction of field: same as force

Units: newtons per coulomb (N/C)

What is the electric field at the

location of the test charge?

E = F/qtest

E = (1.2 N) / (1.5×10í^6 C)

E = 8.0×10^5 N/C

(same direction as force)

(^420) Copyright 2007 Kinetic Books Co. Chapter 23