Electric fieldsElectric fields

ObjectivesObjectives

• Interpret electric field diagrams.

• Describe and calculate the relationship between electric force and electric field for a point charge.

• Identify examples of electric fields in everyday life.

1. Two charged particles are arranged as shown. Which statement below is true?

AssessmentAssessment

A. Both charges must be positive.

B. Both charges must be negative.

C. The upper charge is positive and the lower charge is negative.

D. The upper charge is negative and the lower charge is positive.

2. The proton in a hydrogen nucleus has a charge of 1.6 × 10-19 C, and the electron, located 5 × 10-11 m away, has a charge of −1.6 × 10-19 C.

AssessmentAssessment

3. If you are caught outside in a lightning storm, would you be safer standing under a tree or getting into a car? Why?

a. Calculate and describe the electric force between them.

b. What is the strength of the electric field at the location of the electron – even if the electron is not there to “feel” it?

Physics termsPhysics terms

• electric force

• electric field

• electric field lines

• Faraday cage

EquationsEquations

electric force

electric field

Where would you be safer in a lightning storm?

in a car?

under a tree?

Why?

ThinkThink

Where would you be safer in a lightning storm?

ThinkThink

The answer to this question lies in an understanding of the nature of electric fields and conductors.

Imagine that a positively-charged sphere is placed in a region of space.

This charged sphere creates an electric field at ALL points in space in this region.

If you bring another charged particle into this region, it will feel force of attraction or repulsion due to this electric field.

What is the electric field?What is the electric field?

+ ++ + + + +

Electric field diagramsElectric field diagramsHow can we represent the electric field if it exists at ALL points in space?

The electric field can be represented with electric field lines, similar to magnetic field lines.

Electric field lines trace the direction of the force on a positive test charge.

Electric field diagramsElectric field diagrams

Electric field lines . . .

Electric field linesElectric field lines

• always point away from positive charges

• always point toward negative charges

• are close together where the force is strong

• are far apart where the force is weak

• never cross or tangle

Electric vs. gravitational fieldsElectric vs. gravitational fields

Gravitational force field diagram for Earth

The force of gravity is always attractive.

Electric force field diagrams: Positive charge

+ test charge is repelled

Electric vs. gravitational fieldsElectric vs. gravitational fields

Gravitational force field diagram for Earth

The force of gravity is always attractive.

Electric force field diagrams: Positive charge Negative charge

+ test charge + test charge is repelled is attracted

Gravitational force field diagram for Earth

The force of gravity is always attractive.

Electric vs. gravitational fieldsElectric vs. gravitational fields

Electric force field diagrams: Positive charge Negative charge

+ test charge + test charge is repelled is attracted

Electric fieldsElectric fields

At every point in space, the

electric field has both a

magnitude and a direction.

Electric field of two chargesElectric field of two charges

When the electric field is created by two or more charges, then

E is the vector sum of the electric fields from each charge.

Electric field of two chargesElectric field of two charges

two positive charges two negative charges two opposite charges

A positive test charge will feel a force from each charge creating the field.

When the electric field is created by two or more charges, then

E is the vector sum of the electric fields from each charge.

A positive (+) test charge experiencesequal and opposite forces which sum to

zero here.

Like chargesLike charges

A positive (+) test charge is repelled by positive charge and attracted by negative

charge here.

Unlike chargesUnlike charges

Unlike chargesUnlike charges

The field is weak where lines are far apart.

The field is strong where lines are close.

A positive (+) test charge is repelled by positive charge and attracted by negative

charge here.

Like charges repel one another.

If a conductor has excess charge, those charges will move as far apart as possible – to the surface of the conductor.

Electric charge on a conductorElectric charge on a conductor

Like charges repel one another.

If a conductor has excess charge, those charges will move as far apart as possible – to the surface of the conductor.

This is true regardless of the shape of the conductor.

Electric charge on a conductorElectric charge on a conductor

The electric field inside a conductor in electrostatic equilibrium is always zero . . .

E = 0

Electric field of a conductorElectric field of a conductor

Electric field of a conductorElectric field of a conductor

The electric field inside a conductor in electrostatic equilibrium is always zero . . .

even if the conductor is placed in an external electric field!

English scientist Michael Faraday invented the Faraday cage in 1836.

If a cage is made of metal,

anything inside the cage is shielded from outside electricity and electric fields.

The Faraday cageThe Faraday cage

The Faraday cageThe Faraday cage

In a lightning storm, the metal frame of a car acts as a Faraday cage.

The Faraday cageThe Faraday cage

If lightning strikes the outside of the car, electric charges will redistribute themselves to keep the electric field inside the car at zero.

In a lightning storm, you would be much safer inside a car than under a tree!

Shielding effect of conductorsShielding effect of conductors

Some electrical appliances create electric fields that can interfere with the operation of other sensitive devices.

Some electrical appliances create electric fields that can interfere with the operation of other sensitive devices.

One way to prevent interferenceis to enclose the device in a conducting material to blockoutside electric fields.

Shielding effect of conductorsShielding effect of conductors

Standard RJ45 cable has conducting metal mesh on the outside to

shield the signal from interference.

The relationship between the electric force and electric field is:

Calculating force and fieldCalculating force and fieldElectric field is a vector quantity, so it has both magnitude and direction.

The relationship between the electric force and electric field is:

Calculating force and fieldCalculating force and fieldElectric field is a vector quantity, so it has both magnitude and direction.

The electric field is a property of a point in space.

It tells you the force that will be exerted—per coulomb of charge placed at that point.

The force of gravity near Earth’s surface is:

g is the force per unit mass. It can be measured in N/kg!

A useful analogyA useful analogy

The force of gravity near Earth’s surface is:

g is the force per unit mass. It can be measured in N/kg!

The electric force in an electric field is:

E is the force per unitcharge, measured in N/C.

A useful analogyA useful analogy

P

Imagine that the electric field at some point P near this charged sphere has a strength of 500 N/C.

•What is the force on a 1 C charge placed at that point?

•What is the force on a 0.5 C charge placed that that point?

+ ++ + + + +

The electric field has units of newtons per coulomb.

Applying the conceptApplying the concept

Imagine that the electric field at some point P near this charged sphere has a strength of 500 N/C.

•What is the force on a 1 C charge placed at that point? 500 N

•What is the force on a 0.5 C charge placed that that point? 250 N

+ ++ + + + +

The electric field has units of newtons per coulomb. P

Applying the conceptApplying the concept

Electric field of a point chargeElectric field of a point charge

Two charges, q and q2, exert

repulsive forces on each other:

The electric field created by q at the location of q2 is:

Electric field of a point chargeElectric field of a point charge

Two charges, q and q2, exert

repulsive forces on each other:

Electric field of a point chargeElectric field of a point charge

E has this value at this point in space even if q2 is not there

to “feel” it.

Two charges, q and q2, exert

repulsive forces on each other:

The electric field created by q at the location of q2 is:

Test your knowledgeTest your knowledge

What is the strength of the electric field 1.00 meter away from a point charge of q = +2.0 x 10-6 C?

What is the strength of the electric field 1.00 meter away from a point charge of q = +2.0 x 10-6 C?

Test your knowledgeTest your knowledge

AssessmentAssessment1. Two charged particles are arranged as shown. Which statement below is true?

A. Both charges must be positive.

B. Both charges must be negative.

C. The upper charge is positive and the lower charge is negative.

D. The upper charge is negative and the lower charge is positive.

1. Two charged particles are arranged as shown. Which statement below is true?

AssessmentAssessment

A. Both charges must be positive.

B. Both charges must be negative.

C. The upper charge is positive and the lower charge is negative.

D. The upper charge is negative and the lower charge is positive.

AssessmentAssessment

a. Calculate and describe the electric force between them.

2. The proton in a hydrogen nucleus has a charge of 1.6 × 10-19 C, and the electron, located 5 × 10-11 m away, has a charge of −1.6 × 10-19 C.

a. Calculate and describe the electric force between them.

b. What is the strength of the electric field at the location of the electron – even if the electron is not there to “feel” it?

AssessmentAssessment

attractive

2. The proton in a hydrogen nucleus has a charge of 1.6 × 10-19 C, and the electron, located 5 × 10-11 m away, has a charge of −1.6 × 10-19 C.

a. Calculate and describe the electric force between them.

b. What is the strength of the electric field at the location of the electron – even if the electron is not there to “feel” it?

AssessmentAssessment

attractive

2. The proton in a hydrogen nucleus has a charge of 1.6 × 10-19 C, and the electron, located 5 × 10-11 m away, has a charge of −1.6 × 10-19 C.

3. If you are caught outside in a lightning storm, would you be safer standing under a tree or getting into a car? Why?

AssessmentAssessment

AssessmentAssessment3. If you are caught outside in a lightning storm, would you be

safer standing under a tree or getting into a car? Why?

Answer: you would be safer in the car. The car acts as a Faraday cage, shielding you from outside electricity and electric fields.

Point charges create electric fields that are non-uniform. These fields are strong close to the charges and weak far from the charges.

Question: Is it possible to create a uniform electric field— a field that has the same strength at all points?

Uniform fields (advanced)Uniform fields (advanced)

Capacitors are electrical components that CAN create a uniform electric field.

A capacitor consists of two metal plates separated by an insulating material called a dielectric.

Capacitors (advanced)Capacitors (advanced)

When the capacitor is connected to a voltage source, the two plates become oppositely charged.

A uniform electric field is created in the region between the two plates.

Capacitors (advanced)Capacitors (advanced)

Capacitors are one of the most common circuit elements, and have many uses.

One of these uses is to store and quickly release energy, so they are used in flash photography and in defibrillators.

Uses of capacitors (advanced)Uses of capacitors (advanced)

Capacitors can be combined with resistors into RC circuits, which are useful as timing devices.

Capacitors can be combined with resistors and inductors to create LRC circuits, which can be tuned to specific frequencies, such as for radio reception.

Uses of capacitors (advanced)Uses of capacitors (advanced)