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UNIT 9Electrostatics and Currents

Friday March 9th

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Electrostatics and Currents

TODAY’S AGENDA

Electric Charge and Forces

Hw: Practice A (all) p565Practice B (all)

p568Practice C (all)

p570

UPCOMING…

Mon: Electric Field Tue: Problem Quiz #1 (Electric Forces) Wed: Electric Potential Energy Thurs: Problem Quiz #2 (Electric Fields) Fri: NO SCHOOL (Teacher In-Service)

Friday, March 9

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Chapter 16-17

Electrostatics and Current

ConcepTest 21.1a Electric Charge I

1) one is positive, the other

is negative

2) both are positive

3) both are negative

4) both are positive or both

are negative

Two charged balls are repelling each other as they hang from the ceiling. What can you say about their charges?

ConcepTest 21.1a Electric Charge I

The fact that the balls repel each

other only can tell you that they

have the same charge, but you do

not know the sign. So they can

be either both positive or both

negative.

1) one is positive, the other

is negative

2) both are positive

3) both are negative

4) both are positive or both

are negative

Two charged balls are repelling each other as they hang from the ceiling. What can you say about their charges?

Follow-up: What does the picture look like if the two balls are oppositely charged? What about if both balls are neutral?

1) have opposite charges

2) have the same charge

3) all have the same charge

4) one ball must be neutral (no charge)

From the picture, what can you conclude about the charges?

ConcepTest 21.1b Electric Charge II

1) have opposite charges

2) have the same charge

3) all have the same charge

4) one ball must be neutral (no charge)

From the picture, what can you conclude about the charges?

The GREEN and PINK balls must

have the same charge, since they

repel each other. The YELLOW

ball also repels the GREEN, so it

must also have the same charge

as the GREEN (and the PINK).

ConcepTest 21.1b Electric Charge II

ConcepTest 21.2a Conductors I

1) positive

2) negative

3) neutral

4) positive or neutral

5) negative or neutral

A metal ball hangs from the ceiling

by an insulating thread. The ball is

attracted to a positive-charged rod

held near the ball. The charge of

the ball must be:

Clearly, the ball will be attracted if its

charge is negative. However, even if

the ball is neutral, the charges in the

ball can be separated by induction

(polarization), leading to a net

attraction.

1) positive

2) negative

3) neutral

4) positive or neutral

5) negative or neutral

A metal ball hangs from the ceiling

by an insulating thread. The ball is

attracted to a positive-charged rod

held near the ball. The charge of

the ball must be:

remember the ball is a conductor!

ConcepTest 21.2a Conductors I

Follow-up: What happens if the metal ball is replaced by a plastic ball?

Electric Charge

The effects of electric charge were first observed as static electricity:

After being rubbed on a piece of fur, an amber rod acquires a charge and can attract small objects.

Electric Charge

Charging both amber and glass rods shows that there are two types of electric charge; like charges repel and opposites attract.

Electric Charge

All electrons have exactly the same charge; the charge on the proton (in the atomic nucleus) has the same magnitude but the opposite sign:

Electric Charge

The electrons in an atom are in a cloud surrounding the nucleus, and can be separated from the atom with relative ease.

Electric Charge

When an amber rod is rubbed with fur, some of the electrons on the atoms in the fur are transferred to the amber:

Electric Charge

We find that the total electric charge of the universe is a constant:

Electric charge is conserved.

Also, electric charge is quantized in units of e.

The atom that has lost an electron is now positively charged – it is a positive ion

The atom that has gained an electron is now negatively charged – it is a negative ion

Electric Charge

Some materials can become polarized – this means that their atoms rotate in response to an external charge. This is how a charged object can attract a neutral one.

Insulators and Conductors

Conductor: A material whose conduction electrons are free to move throughout. Most metals are conductors.

Insulator: A material whose electrons seldom move from atom to atom. Most insulators are non-metals.

Insulators and Conductors

If a conductor carries excess charge, the excess is distributed over the surface of the conductor.

Charge by InductionA conductor can be charged by induction, if there is a way to ground it.

This allows the like charges to leave the conductor; if the conductor is then isolated before the rod is removed, only the excess charge remains.

Insulators and Conductors

Semiconductors have properties intermediate between conductors and insulators; their properties change with their chemical composition.

Photoconductive materials become conductors when light shines on them.

Coulomb’s LawCoulomb’s law gives the force between two point charges:

The force is along the line connecting the charges, and is attractive if the charges are opposite, and repulsive if the charges are like.

Coulomb’s Law

The forces on the two charges are action-reaction forces.

Coulomb’s Law

If there are multiple point charges, the forces add by superposition.

Coulomb’s Law

Coulomb’s law is stated in terms of point charges, but it is also valid for spherically symmetric charge distributions, as long as the distance is measured from the center of the sphere.

The Electric Field

Definition of the electric field:

Here, q0 is a “test charge” – it serves to allow the electric force to be measured, but is not large enough to create a significant force on any other charges.

The Electric Field

If we know the electric field, we can calculate the force on any charge:

The direction of the force depends on the sign of the charge – in the direction of the field for a positive charge, opposite to it for a negative one.

The Electric Field

The electric field of a point charge points radially away from a positive charge and towards a negative one.

The Electric Field

Just as electric forces can be superposed, electric fields can as well.

Electric Field Lines

Electric field lines are a convenient way of visualizing the electric field.

Electric field lines:

1. Point in the direction of the field vector at every point

2. Start at positive charges or infinity

3. End at negative charges or infinity

4. Are more dense where the field is stronger

Electric Field Lines

The charge on the right is twice the magnitude of the charge on the left (and opposite in sign), so there are twice as many field lines, and they point towards the charge rather than away from it.

Electric Field LinesCombinations of charges. Note that, while the lines are less dense where the field is weaker, the field is not necessarily zero where there are no lines. In fact, there is only one point within the figures below where the field is zero – can you find it?

Electric Field Lines

A parallel-plate capacitor consists of two conducting plates with equal and opposite charges. Here is the electric field:

Shielding and Charge by Induction

Since excess charge on a conductor is free to move, the charges will move so that they are as far apart as possible. This means that excess charge on a conductor resides on its surface, as in the upper diagram.

Shielding and Charge by Induction

When electric charges are at rest, the electric field within a conductor is zero.

Shielding and Charge by Induction

The electric field is always perpendicular to the surface of a conductor – if it weren’t, the charges would move along the surface.

Shielding and Charge by Induction

The electric field is stronger where the surface is more sharply curved.

Shielding and Charge by InductionA conductor can be charged by induction, if there is a way to ground it.

This allows the like charges to leave the conductor; if the conductor is then isolated before the rod is removed, only the excess charge remains.

Electric Flux and Gauss’s LawElectric flux is a measure of the electric field perpendicular to a surface:

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