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Quiz 6

Physics 7C Fall 2008Lecture 6: Field model

Electric Force & Electric Field (review)

PEelectric & Electric Potential

Magnetic Force & Field

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Calendar

Sections 1-4: DLM 17 on TuesdaySection 5: DLM 17 on Weds 8-10:20Section 7 & 10: DLM 17 on Mon 8-10:20 or 2:10-4:30Sections 8, 9, 11: DLM 17 either Monday OR Weds: Ask TA

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Models of Electric Phenomena: Electric Field and Forces

Each source charge Q generates an Electric Field EQ Direction convention shown at right

The net Electric Field is the sum of all the source fields

Charge q, placed in an electric field Etot, experiences a force Felec on q=qEQ. For + test charge, force points in the

same direction as field. For - test charge, force points in opposite

direction of field Field, Forces, Potential Energy, and

Potential

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Models of Electric Phenomena: Field, Forces, Potential Energy,

and Potential PE & Forces

started in 7A Similar relationship to V & E

Potential--started in 7B PE & V have similar

relationship as F & E

PE F

EV

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Reviewing what you’ve previously studied…

Gravitational Potential Energy

1

2

3g

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Relationship between Potential Energy and Force

Pote

ntia

l Energ

y

r

-

0

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Relationship between Potential Energy and Force

1

2

3

Pote

ntia

l Energ

y

r

-

0

Negative means decrease of PE with decreasing r

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Relationship between Potential Energy and Force

1

2

3

Pote

ntia

l Energ

y

r

-

0

More slope closer to earth means F is greater there

F = - PE/r, the - slope

Force increases with greater slope

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Reviewing what you’ve previously studied…

Relationship between Potential Energy and Force.

1

2

3

4

• Magnitude of Force = slope of PE vs. r graph.

•

€

F = −dPE

dr

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Defining a new quantity

Gravitational Potential: How much Potential Energy would a mass m have if placed (x,y)?

1

2

3y

x

g

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Electric Field and Potential: Constant Electric Field

Slope of the potential constant as a function of

distance. negative

Electric field is constant as a function of

distance positive

1 :

ˆ ˆ ˆ3 :

dVD E

dxdV dV dV

D E x y zdx dy dz

→

→

=−

=− − −

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Electric Potential of a point charge:Positive and Negative Charge.

Not all potentials are straight lines!

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Equipotential Surfaces: Lines where V is the same.

Equipotential surfaces for a point charge. Circles are 0.5 V apart. Distance between

circles is NOT uniform! Circles get closer and

closer toward center Potential grows like 1/r

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Putting it all together…

Which quantities depend only on source charge(s)?

a) Electric Field (E)

b) Electric Force (F)

c) Electric Potential Energy (PE)

d) Electric Potential (V)

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Putting it all together…

Which are vector quantities?a) Electric Field (E)

b) Electric Force (F)

c) Electric Potential Energy (PE)

d) Electric Potential (V)

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Putting it all together…

Which quantities are related by slopes (that is, if you take the slope of one, you get the other)

a) Electric Field & Electric Force

b) Electric Potential Energy & Electric Potential

c) Electric Force & Potential Energy

d) Electric Field & Electric Potential

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Field Model:

A source (A) creates a (B) _ field in a direction _. The net (B) _ field is the sum of all the

source fields. A test (A) , placed in a (B) _

field, experiences a (B) _ force Magnitude given by _____ Direction of force: _____

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Field Model: Magnetism

A source ____________ creates a magnetic field in a direction given by _______. The net magnetic field is the sum of all the source

fields. ___________, placed in a magnetic field,

experiences a magnetic force Magnitude given by _____ Direction of force given by _____

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A little background

Compasses or bar magnets, if allowed, will always orient north-south

Why?

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A little background

Compasses orient in the same direction as the magnetic field.

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A little background

Iron fillings also orient in the same direction as the magnetic field.

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Magnetic Field from a wire

If we allow iron fillings freedom to rotate, and put them near a current-carrying wire, this is how they align:

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Magnetic Field from a wire

If we place compasses around a long current-carrying wire, this is how they align

(view is looking down wire)

What does this mean for the magnetic field model??

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Field Model: Magnetism

A source moving charge creates a magnetic field in a direction given by _______. The net magnetic field is the sum of all the source

fields. A test moving charge, placed in a magnetic

field, experiences a magnetic force Magnitude given by _____ Direction of force given by _____

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Phenomenon: Magnet near an electron beam

The beam is composed of electrons--moving charges

Observe the effects of a large magnet on the beam…

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Phenomenon: jumping wires

Two wires Initially no current (observe wires) Connect both wires to a generator, making

current flow. Observe: What happens to the wires? What happens if I reverse the direction of the current

in one wire (compared to first time)? What happens if I reverse the direction of the current

in both wires (compared to the first time)? What would happen if I could put current in only

one wire?

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Field Model: Magnetism

A source moving charge creates a magnetic field in a direction given by RHR1. The net magnetic field is the sum of all the source

fields. A test moving charge, placed in a magnetic

field, experiences a magnetic force Magnitude given by _____ Direction of force given by _____

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Right Hand Rule # 1

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Which direction is the magnetic field at point A?

1) Into the screen

2) Out of the screen

3) Towards the wire

4) Away from the wire

5) Points down

6) Points up

7) Another direction I

B A

C

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Which direction is the magnetic field at point B?

1) Into the screen

2) Out of the screen

3) Towards the wire

4) Away from the wire

5) Points down

6) Points up

7) Another direction I

B A

C

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Which direction is the magnetic field at point C?

1) Into the screen

2) Out of the screen

3) Towards the wire

4) Away from the wire

5) Points down

6) Points up

7) Another direction I

B A

C

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Magnetic Forcev

B

xRHR2 (for positive charge): your thumb points in the direction of the moving charge, B is along your index finger, and F is the middle finger.

Very Bad Finger

F into the screen

F = qvBsin where is the angle between B and v

F

vB

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Magnetic Force

Suppose a large magnetic field points downward at every point in the room. What direction is the force on a positive particle traveling along the chalkboards, to your left? 1) Into the board

2) Out of the board

3) Left (along particle path)

4) Right (opposite path)

5) Down

6) Up

7) No Force

v

B

F = qvBsin where is the angle between B and v

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Magnetic Force

Suppose a large magnetic field points downward at every point in the room. What direction is the force on a positive particle traveling out of the board, to the back of the room? 1) Into the board

2) Out of the board

3) Left (along particle path)

4) Right (opposite path)

5) Down

6) Up

7) No Force

v

B

F = qvBsin where is the angle between B and v

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Magnetic Force

Suppose a large magnetic field points downward at every point in the room. What direction is the force on a positive particle traveling upward, toward the ceiling?

1) Into the board

2) Out of the board

3) Left (along particle path)

4) Right (opposite path)

5) Down

6) Up

7) No Force

v

B

F = qvBsin where is the angle between B and v

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Field Model: Magnetism

A source moving charge creates a magnetic fields in a direction given by RHR1. The net magnetic field is the sum fo all the source

fields. A test moving charge, placed in a magnetic

field, experiences a magnetic force Magnitude given by F=qvBsin Direction of force given by RHR2