electric field

​Electric field

Electric charges are surrounded by electric fields that exert a force on other charged objects.

BIG IDEA:

Electric Fields

MAIN IDEA

An electric field is a property of the space around a charged object that
exerts forces on other charged objects.

Essential Questions

• What is an electric field?

• How are charge, electric field, and forces on charged objects related?

• How can you represent electric fields in diagrams and other models?

Open Ended

state coulombs law

Review Vocabulary:

New Vocabulary

• Electric field
• Electric field line

Electric Fields

Coulomb’s law states that the force between two point charges varies

directly with the product of their charges and inversely with the square of

the distance between them

• Electric force, like gravitational force, varies inversely as the square of

the distance between two point objects. Both forces can act from great distances.

Defining the Electric Field

• How can a force be exerted across what seems to be empty space?

• Object B somehow senses the change in space and experiences a force

due to the properties of the space at its location. We call the changed
property of space an electric field.

• An electric field means that the interaction is not between two distant

objects, but between an object and the field at its location.

Michael Faraday suggested that because an electrically charged object,

A, creates a force on another charged object, B, anywhere in space,

object A must somehow change the properties of space.

• The forces exerted by electric fields can do work, transferring energy from

the field to another charged object.

• This energy is used daily, whether you plug an appliance into an electric

outlet or use a battery-powered, portable device.

Defining the Electric Field

How can you measure an electric field?

Place a small charged object at some location. If there is an electric

force on it, then there is an electric field at that point.

The charge on the object that is used to test the field, called the test

charge, must be small enough that it doesn’t affect other charges.

• The figure illustrates a charged object

with a charge of q.

• Suppose you place the positive test

charge at some point, A, and measure a force, F.

Defining the Electric Field

• According to Coulomb’s law, the force

is directly proportional to the strength of the test charge, q׳.

• That is, if the charge is doubled, so is

the force. Therefore, the ratio of the
force to the charge is a constant.

• If you divide the force, F, by the test charge, q', you obtain a vector

quantity, F/q'.

• This quantity does not depend on the test charge, only on the force, F,

and the location of point A.

• The electric field at point A, the location of q', is represented by the

following equation.

Defining the Electric Field

Electric Field Strength

• The strength of an electric field is equal to the force on a positive test

charge divided by the strength of the test charge.

• The direction of an electric field is the direction of the force on a positive

test charge.

• The magnitude of the electric field strength is measured in newtons per

coulomb, N/C.

• A picture of an electric field can be made

by using arrows to represent the field
vectors at various locations, as shown in the
figure.

• The length of the arrow is used to show the

strength of the field. The direction of the
arrow shows the field direction.

Defining the Electric Field

• To find the field from two charges, the fields from the individual

charges are added vectorially.

• A test charge can be used to map out the field resulting from any

collection of charges.

• An electric field should be measured only by a very small test charge.

• This is because the test charge also exerts a force on q.

• It is important that the force exerted by the test charge does

not cause charge to be redistributed on a conductor, thereby
causing q to move to another location and thus, changing the
force on q' as well as the electric field strength is measured.

• A test charge always should be small enough so that its effect

on q is negligible.

Defining the Electric Field

Open Ended

An electric field is measured using a positive test charge of 3.0×10⁻⁶ C. This test charge experiences a force of 0.12 N at an angle of 15º north of east. What are the magnitude and direction of the electric field strength at the location of the test charge?

Electric Field Strength

An electric field is measured using a positive test charge of
3.0×10−6 C. This test charge experiences a force of 0.12 N at an
angle of 15º north of east. What are the magnitude and direction
of the electric field strength at the location of the test charge?

Step 1: Analyze and Sketch the Problem

• Draw and label the test charge, q׳.

• Show and label the coordinate

system centered on the test charge.

• Diagram and label the force vector

at 15° north of east.

Known:

q׳ = 3.0×10-⁶ C

F = 0.12 N at 15° N of E

Unknown:

E = ?

Identify the known and unknown variables.
Electric Field Strength

Step 2: Solve

Substitute F = 0.12 N, q׳ = 3.0×10−6 C

The force on the test charge and the electric field are in the same direction.

• So far, you have measured an electric field at a single point.

• Now, imagine moving the test charge to another location.

• Measure the force on it again and calculate the electric field.

• Repeat this process again and again until you assign every location in

space a measurement of the vector quantity of the electric field strength
associated with it.

Modeling the Electric Field

• The field is present even if there is no test charge to measure it.

• Any charge placed in an electric field experiences a force on it resulting

from the electric field at that location.

• The strength of the force depends on the magnitude of the field, E, and

the magnitude of the charge, q. Thus, F = Eq.

• The direction of the force depends on the direction of the field and the

sign of the charge.

• A picture of an electric field is shown in the figure.
Modeling the Electric Field

Each of the lines used to represent the actual field in the space around a

charge is called an electric field line.

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• The direction of the field at any point is the tangent drawn to a field line at

that point.

• The strength of the electric field is indicated by the spacing between the

lines.

• The field is strong where the lines are close together. It is weaker where the

lines are spaced farther apart.

• Although only two-dimensional models can be shown here, remember that

electric fields exist in three dimensions.

Modeling the Electric Field

• The direction of the force on a positive test charge

near another positive charge is away from the
other charge.

• Thus, the field lines extend radially outward like

the spokes of a wheel, as shown in the figure.

Multiple Choice

Which of the following statements is false about electric field lines

• electric field lines cannot cross
• electric field lines point away from positive charge
• electric field lines are always straight lines
• electric field lines show how a proton would move in an electric field

Multiple Choice

At which point is the electric field the strongest

• A
• B
• C
• D

Multiple Choice

Consider the electric field lines shown in the diagram below. From the diagram, it is apparent that object A is ____ and object B is ____.

• +,+
• +,-
• -,+
• -,-

Multiple Choice

Based on the arrangement of the electric field lines in the diagram, what are the charges?

• both charges are the same

• the charges are opposite

• they are neutral; no charge

Multiple Choice

Which diagram correctly shows the electric field around a negative charge?

• A

• B

• they are both positive

Multiple Choice

Which diagram correctly shows the field lines between two like charges?

• A

• B

• Both

• Neither

Multiple Choice

The electric field surrounding a positively charged object is

• Away from the charge
• Toward the charge

• Near a negative charge, the direction of the force on

the positive test charge is toward the negative charge,
so the field lines point radially inward, as shown in the
figure.

Modeling the Electric Field

• When there are two or more charges,

the field is the vector sum of the fields
resulting from the individual charges.
The field lines become curved and the
pattern is more complex, as shown in
the figure.

• Note that field lines always leave a positive

charge and enter a negative charge and
that they never cross each other.

• Field lines do not really exist.

• They are simply a means of providing a model of an electric field.

• Electric fields, on the other hand, do exist.

• Although they provide a method of calculating the force on a charged

body, they do not explain why charged bodies exert forces on each other.

Modeling the Electric Field

Open Ended

Search and write how does van de graaf generator work?

• Robert Van de Graaff devised the high-voltage

electrostatic generator in the 1930s.

• Van de Graaff’s machine is a device that

transfers large amounts of charge from one part of the machine to a metal terminal at the top of the device.

Van de Graaff Generators

• Charge is transferred onto a moving belt

at the base of the generator, position A,
and is transferred off the belt at the metal
dome at the top, position B.

• An electric motor does the work needed

to increase the electric potential energy.

• A person touching the terminal of a

Van de Graaff machine is charged
electrically.

• The charges on the person’s hairs repel

each other, causing the hairs to follow
the field lines as shown in the
animation.

Van de Graaff Generators

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Multiple Choice

What is an electric field?

• A.   the change in the properties of the space that surrounds any mass  

• B.   the change in the properties of space that surround any electrically charged object

• C.   the change in the properties of space that surround any conductor

• D.   the change in the properties of space that surround any insulator.

Multiple Choice

An electric field is measured using a positive test charge. This test charge experiences a force at an angle 30° south of east. What is the direction of the electric field at the location of the test charge?

• A.   30° south of east

• B.   60° north of east

• C.   30° north of west

• D.   60° south of west

Multiple Choice

A positive test charge of 4.0×10^-6 C is in an electric field that exerts a force of 1.5×10-4 N on it. What is the magnitude of the electric field at the location of the test charge?  

Multiple Choice

Which of the following electric field diagrams is correct?

•

An electric field exists around any charged object. The field produces
forces on other charged objects.

•

Quantities relating to charge, electric fields, and forces are related and can
be calculated using these formulas:

Measuring Electric Fields

Study Guide

•

Electric field lines provide a pictorial model of the electric field. They
are directed away from positive charges and toward negative charges.
They never cross, and their density is related to the strength of the field.
The direction of the electric field is the direction of the force on a tiny,
positive test charge.

Q3W5CW

Measuring Electric Field

2

Answer only four questions (8 points)

3

Q3W5CW

Measuring Electric Field

Answer only four questions (8 points)

5

Q3W5CW

Measuring Electric Field

Answer only two questions (4 points)

4

Q3W5CW

Measuring Electric Field

Answer only two questions (4 points)