Reading Quiz

The voltage (or electric potential) of a battery determines

• how much work the battery can do on an electric charge.

• how much net electric charge is in the battery.

• how much electric field is around the battery.

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Δ(P.E .) = qΔVWork = qΔV

Electrostatics:

• Force - Coulomb’s Law

A Vector law

• The electric field. E

The electric field is a vector field

• Electric potential.

A scalar -like work

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r F = k q1q2

r2ˆ r

r E = k q1

r2ˆ r

ΔV =r E •

r d

Two positive charges exert equal but oppositely directed forces upon one another, according to Coulomb’s law and Newton’s third law of motion.

Coulomb’s Law:

The electrostatic force between two charged objects is proportional to the quantity of each of the charges and inversely proportional to the square of the distance between the charges.

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F =kq1q2r2

Coulomb’s Law:

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F =kq1q2r2

Note: looks a lot like Newton’s law of Gravitation.

If both charges are negative, the force is also repulsive.

If one is positive and the other negative, the force is attractive.

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F =kq1q2r2

If a system contains many charges, the net force (vector) on any one of them is the (vector) sum of the individual forces from the individual charges (superposition).

Quiz 1

Two equal charges are 1 meter apart. If the distance between them is increased 4 times, what increase in each charge is require to keep the force between them the same.

1. 2 times2. Square root 23. 4 times4. 8 times

Electric Field:

The electric field at a given point in space is the electric force per unit positive charge that would be exerted on a charge if it were placed at that point.

E = F/qo

It is a vector having the same direction as the force on a positive charge.

The direction of the electric field lines around a positive charge can be found by imagining a positive test charge q0 placed at various points around the source charge. The field has the same direction as the force on a positive test charge.

Note: Lines of E point away from positive charges, toward negative charges.

The electric field lines associated with a negative charge are directed inward, as indicated by the force on a positive test charge, q0.

NOTE: We can also write F = qoE, which tells us how much force F is exerted by the field E for any charge qo.

The electric field lines associated with two equal but opposite-sign charges (an electric dipole).

The field can be determined by using the field from one charge, and adding the field from the other charge. This is called “superposition.”

We created this new concept, the electric field, because sometimes it is more convenient to work with.

However, the electric field is real. There actually is energy stored in the field that can be detected by experiment.

Quiz 2Electric field lines 1. Start on a positive charge and

go to infinity2. Start on a negitive charge and

go to infinity3. Start on a positive charge and

end on a negitive charge4. Can start and end on any charge

sign is not important

The Electric Potential

Moving an electric charge through space where electric fields are present can require work, since forces associated with the fields act on the charge.

This work can be described as a change in potential energy. We introduce the new concept of “electric potential” to describe the amount of work needed to move a charge through a region with electric fields.

Two parallel metal plates containing equal but opposite-sign charges produce a uniform electric field in the region between the plates.

(This is a convenient device that allows us to talk about a region where the electric field does not change. This makes the calculations much easier.)

“CAPACITOR”

An external force F, equal in magnitude to the electrostatic force qE, is used to move the charge q a distance d in a uniform field.

The increase in potential energy when a charge q is moved against the electrostatic force is analogous to what happens when a mass m is lifted against the gravitational force.

We call this the electrostatic potential energy (instead of the gravitational potential energy).

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ΔV =ΔPEq

The change in electric potential is equal to the change in electrostatic potential energy per unit of positive test charge:

This is the definition of potential. It is measured in volts.

A positive charge is moved from the bottom plate to the top plate by an external force.

QUIZ 3

A +2 coulomb charge is lifted through the plate how much work is done.

1. 10000 N2. 20000N3. 600N4. 300N

The electric potential (represented by the dashed lines of constant potential) increases as we move closer to a positive charge.

a.b.

Quiz 4What is the potential difference in moving From a to b.1. 15 V2. -15V3. 25 V4. -25V5. Can tell not in a straight line

Quiz 5

a.b.

What is the work done in moving a +2 coulomb charge from a to b?

1. 50 N2. 30 N3. 25 N4. 20 N5. 15 N