ConcepTest 16.1ConcepTest 16.1 Electric Charge IElectric Charge I

The fact that the balls repel each

other only can tell you that they

have the same chargesame 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 Two charged balls are

repelling each other as repelling each other as

they hang from the ceiling. they hang from the ceiling.

What can you say about What can you say about

their charges?their charges?

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

Clearly, the ball will be attracted if its

charge is negativenegative. However, even if

the ball is neutralneutral, the charges in the

ball can be separated by inductioninduction

(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 16.2ConcepTest 16.2 Conductors IConductors I

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

While the conductors are connected, positive positive

charge will flow from the blue to the green charge will flow from the blue to the green

ball due to polarizationball due to polarization. Once disconnected,

the charges will remain on the separate charges will remain on the separate

conductorsconductors even when the rod is removed.

Two neutral conductors are connected

by a wire and a charged rod is brought

near, but does not touch. The wire is

taken away, and then the charged rod

is removed. What are the charges on

the conductors?

ConcepTest 16.3ConcepTest 16.3 Conductors IIConductors II1) 0 0

2) + –

3) – +

4) + +

5) – –

0 0

? ?Follow-up:Follow-up: What will happen when the What will happen when the

conductors are reconnected with a wire?conductors are reconnected with a wire?

The force F2 must have the same magnitudesame magnitude as F1. This is

due to the fact that the form of Coulomb’s Law is totally

symmetric with respect to the two charges involved. The

force of one on the other of a pair is the same as the reverseforce of one on the other of a pair is the same as the reverse.

Note that this sounds suspiciously like Newton’s 3rd Law!!Note that this sounds suspiciously like Newton’s 3rd Law!!

QQ QQF1 = 3N F2 = ?

1) 1.0 N

2) 1.5 N

3) 2.0 N

4) 3.0 N

5) 6.0 N

What is the magnitude What is the magnitude

of the force of the force FF22??

ConcepTest 16.4ConcepTest 16.4 Coulomb’s Law ICoulomb’s Law I

Originally we had:

Fbefore = k(Q)(Q)/d2 = F

Now we have:

Fafter = k(Q)(Q)/(3d)2 = 1/9 F

1) 9 F

2) 3 F

3) F

4) 1/3 F

5) 1/9 F

The force between two charges

separated by a distance d is F. If

the charges are pulled apart to a

distance 3d, what is the force on

each charge?

QQFF

QFF

dd

QQ??

QQ??

33dd

ConcepTest 16.5ConcepTest 16.5 Coulomb’s Law IIICoulomb’s Law III

Follow-up:Follow-up: What is the force if the original distance is halved? What is the force if the original distance is halved?

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17.1 Electric Potential Energy and Potential Difference

Review of work and energy

The electrostatic force isconservative.

W = Fd = qEd

PEb – PEa = -W

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17.1 Electric Potential Energy and Potential Difference The electric potential is the electric

potential energyper unit charge.

Voltage

q

PEPEV abba

q

PEV aa

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17.2 Electric Potential and Electric Field

If we have uniform electric field, then W = -qVba and W = qEd

Therefore Vba = -Ed or

In general, the electric field is the rate at which theelectric potential decreases over a distance.

d

VE ba

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17.3 Equipotential Lines

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17.3 Equipotential Lines No work is required to move along an

equipotentialline and E is perpendicular to the line.

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17.4 The Electron Volt

The energy acquired by a charge (± e) after passing through a potential difference of 1 volt.

1 eV = 1.6 x 10-19 J

Lichtenberg Figuresand

Lightning

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17.5 Electric Potential due to Point Charges

If V = 0 at r = ∞ then

The principle of superpositionallows us to determine the potential due to multiple

charges

r

QkV

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17.5 Electric Potential due to Point Charges