lect03 handout

Physics 102: Lecture 3, Slide 1

Physics 102: Lecture 3

Electric Potential


Overview for Today’s Lecture

• Electric Potential Energy/ Work– Uniform fields– Point charges

• Electric Potential (like height)– Uniform fields– Point charges


Recall Work from P101

• Work done by the force given by: – W = F d cos()– Positive: Force is in direction moved– Negative: Force is opposite direction moved– Zero: Force is perpendicular to direction moved

• Careful ask WHAT is doing work!– Opposite sign for work done by you!

• Conservative Forces– Potential Energy = W


A B

C

Uniform E

In what direction does the force on a negative charge at point A point?

1) left

2) right

3) up

Preflight 3.1

!!!!ACT!!!!


A B

C

Uniform E

When a negative charge is moved from A to B the ELECTRIC force does

1) positive work.

2) zero work.

3) negative work.

Preflight 3.3

“because the direction of the displacement is 180 degrees from direction of the force”


A B

C

Uniform E

Preflight 3.5

When a negative charge is moved from A to B, the electric potential energy of the charge

1) Increases

2) is constant

3) decreases

E.P.E. = WE field

Electric force did negative work so electric potential energy increased.


AB

+

ACT: Electric Potential Energy

When a negative charge is moved from A to B, the electric potential energy of the charge

(A) increases

(B) is constant

(C) decreases

CAC: W=0

CB: W<0


Work and Potential Energy

• Brick raised yi yf • Charge moved ∞ rf

• FE = kq1q2/r2 (left)

• WE = -kq1q2/rf

• UE= +kq1q2/rf

W = F d cos()Gravity Electric

Fg=mgFg=mgFg=mgFg=mgFg=mgFg=mgFg=mg

yi

yf

Fg=mg

h

• FG = mg (down)

• WG = -mgh

• UG= +mgh

rf


Work done by YOU to assemble 3 charges

• W1 = 0

m

m

m

• W2 = k q1 q2 /r =(9109)(110-6)(210-6)/5 =3.6 mJ• W3 = k q1 q3/r + k q2 q3/r

(9109)(110-6)(310-6)/5 + (9109)(210-6)(310-6)/5

=16.2 mJ

• Wtotal = +19.8 mJ• WE = -19.8 mJ• UE = +19.8 mJ

(watch signs!)


ACT: Work done by YOU to assemble 3 negative charges

m

m

m

A.W = +19.8 mJB.W = 0 mJC.W = -19.8 mJ

How much work would it take YOU to assemble 3 negative charges?

Likes repel, so YOU will still do positive work!


Preflight 3.11

The total work required by you to assemble this set of charges is:

(1) positive

(2) zero

(3) negative

+

+

-m

m

m

1

32


Electric Potential (like height)*

Mount Tabor

Moving to higher potential moving uphill


Electric Potential (like height)*

• Units Joules/Coulomb Volts– Batteries– Outlets – EKG

• Really Potential differences• Equipotential lines at same height • Field lines point down hill• V = k q/r (distance r from charge q)

– V(∞) = 0


Preflight 3.7

The electric potential at point A is _______ at point B

1) greater than

2) equal to

3) less than

Electric field Points from greater potential to lower potential


Preflight 3.9

The electric potential at point A is _______ at point B

1) greater than

2) equal to

3) less than

conductor


Preflight Summary

A→ B Negative

C → B

A → CZero

Path Vfinal - Vinitial U = q V WE field

Negative

Charge

+-

+-

+-

Negative

PositivePositiveNegative


AB

+

ACT: Electric Potential

The electric potential at A is ___________ the electric potential at B.

1) greater than

2) equal to

3) less than

C


Electric Potential due to ProtonWhat is the electric potential a distance r= 0.5310-10 m

from a proton? (Let V()=0)V =U/q= k q/ r =

rf = 0.510-10 m

Volts


Electric Potential due to Proton

What is the electric potential a distance r= 0.5310-10 m from a proton? (Let V()=0)

rf = 0.510-10 m

What is the electric potential energy of an electron a distance r= 0.5310-10 m from a proton?

Hydrogen Balloon E≈ (4.3510-18)(610+23J )=106 J!

V =U/q= k q/ r = (9109)(1.610-19) /0.5310-10= 27.2 Volts

U = Vq = (27.2)(-1.610-19)

JeV


Comparison:Electric Potential Energy vs. Electric Potential

• Electric Potential Energy (U) - the energy of a charge at some location.

• Electric Potential (V) - found for a location only – tells what the EPE would be if a charge were located there (usually talk about potential differences between two locations):

U = Vq• Neither has direction, just value.

Sign matters!


Two Charges

Q=-3.5 CQ=+7.0C

A

6 m

4 m

How much work do you have to do to bring a 2 C charge from far away to point A?

W=U=Vq =(+6.3103V)(2C) =+12.6 mJ

• Calculate electric potential at point A due to charges– Calculate V from +7C charge

– Calculate V from –3.5C charge

– Add (EASY!)

• V = kq/r

V7=(9109)(710-6)/5 = 12.6103V

V3=(9109)(-3.510-6)/5 = -6.3103V

Vtotal = V7+V3 = +6.3103V


ACT: Two Charges

Q=-3.5 CQ=+7.0C

• In the region II (between the two charges) the electric potential is

always positive

positive at some points, negative at others.

always negative

Very close to positive charge potential is positiveVery close to negative charge potential is negative


To Do

• Bring “Problem Solver” to discussion section

• Complete preflight.

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