Measuring Electric Fields Lesson 3

Measuring Electric Fields

Lesson 3

Focus Question

How can we explain how electric forces
work at a distance?

New Vocabulary

electric field

electric field line

Review Vocabulary

Coulomb’s law: states that the force between
two point charges varies directly with the
product of their charge and inversely with the
square of the distance between them

Defining the Electric Field

• Michael Faraday suggested the idea of a force field

to explain how the electric force can act at a
distance.

• An electric field is a property of the space around a

charged object that exerts forces on other charged
objects.

• You cannot see an electric field, but there are ways

to detect that an electric field is present.

Multiple Choice

What is an electric field?

• A property of the space around a charged object that exerts forces on other charged objects.

• The force between two point charges.

• The direction of the force on a positive test charge.

• The spacing between electric field lines.

Defining the Electric Field

• You can envision the force from an electric field

by modeling its effects on a small charged
object—a test charge (q’)—at some location.

• If there is an electrostatic force on the object,

then there is an electric field at that point.

• According to Coulomb’s law, the force is directly

proportional to the strength of the test charge (q’),
so the ratio of the force to the strength of the
test charge is a constant.

Multiple Choice

How can you detect the presence of an electric field?

• By observing the electric field lines.

• By measuring the force on a test charge.

• By calculating the magnitude of the electric field.

• By measuring the distance between charges.

Defining the Electric Field

• The electric field at point A, the location of q’, is

represented by the following equation.

• The direction of an electric field is the direction of

the force on a positive test charge.

• The magnitude of the electric field is newtons per

coulomb (N/C).

Electric Field Strength

Multiple Choice

What is the direction of an electric field?

• The direction of the force on a positive test charge.

• The direction of the force on a negative test charge.

• The direction of the electric field lines.

• The direction of the test charge.

Multiple Choice

Which type of charge should be used as a test charge for determining the effect of an electric field?

• A positive charge.

• A negative charge.

• A neutral charge.

• A large charge.

Defining the Electric Field

• You can make a model of

an electric field by
using arrows to represent
the field vectors at
various locations.
• The arrow’s length

represents the field
strength.

• The direction of the

arrow represents
the field direction.

Defining the Electric Field

• You can use a test charge to map the electric field

resulting from any collection of test charges.

• A test charge should be small enough so that its

effect on the charge you are testing (q) is
negligible.

• The test charge exerts forces back on the charges

that produce the electric field. It is important that
these forces do not redistribute the charges that
you are trying to measure, thereby affecting the
electric field that you are trying to map.

Defining the Electric Field

If, and only if, the charge q is a point charge or
a uniformly charged sphere, you can calculate
its electric field from Coulomb’s law.

Multiple Choice

What is the correct equation for the electric field at a point?

• E = F/q

• E = Kq/r²

• E = q/F

• E = r²/Kq

Modeling the Electric Field

• You can represent electric fields with electric field

lines. An electric field line indicates the direction of
the force due to the electric field on a positive test
charge.

• The direction of the electric field at any point is the

tangent drawn to a field line at that point.

• The spacing between the lines indicates the electric

field’s strength. The field is stronger where the lines
are spaced more closely.

Multiple Choice

How can you represent an electric field?

• With electric field lines.

• With arrows representing field vectors.

• With the spacing between electric field lines.

• With the magnitude of the electric field.

Modeling the Electric Field

• Electric field lines are directed toward negative

charges and away from positive charges.

• Electric field lines are only a way of representing

electric fields. They are not the electric field itself.

Multiple Choice

What happens to electric field lines near positive charges?

• They are directed towards the positive charges.

• They are directed away from the positive charges.

• They become more closely spaced.

• They become less closely spaced.

Modeling the Electric Field

When more than one electric charge is present, the
electric fields add together.

Multiple Choice

How do electric fields add together when multiple charges are present?

• They cancel each other out.

• They combine to form a stronger field.

• They repel each other.

• They become perpendicular to each other.

Van de Graaff Generators

• In a Van de Graaff generator, a belt is driven by two

rollers or pulleys.

• When the roller attached to the base rotates, the

belt moves.

• The roller builds up a strong negative net charge,

and the belt builds a weaker positive net charge.

• The roller’s electric field repels electrons in the tips

of a nearby brush assembly, causing the tips of the
brushes to become positively charged.

Multiple Choice

How does a Van de Graaff generator work?

• By building up a strong positive net charge on a roller.

• By attracting positive ions from the air.

• By repelling electrons in a brush assembly.

• By aligning the electric field around a person's head.

Van de Graaff Generators

• At the same time, the roller’s electric field strips

electrons from nearby molecules in the air. These
electrons are attracted to the positively charged
tips of the brush.

• The positive ions from the air move toward the

negatively charged roller but collect on the belt
and are transported to the metal dome.

Van de Graaff Generators

• When a person touches the metal dome, the

positively charged dome attracts the electrons in
the person’s body, causing the person’s hair to
become positively charged. The individual hairs
repel each other and align with the electric field
around the person’s head.

Multiple Choice

What happens when a person touches the metal dome of a Van de Graaff generator?

• The dome becomes positively charged.

• The person's hair becomes positively charged.

• The person's body attracts electrons from the dome.

• The person's hair aligns with the electric field.