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Physics 7C SS1, Lecture 7: Field Model

Electric (and gravitational analogies)

Magnetic?

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Quiz 1 grades 142 students, average of 8.4 (low B) 34 in the As (12 A+, 9 A, 15 A-) 59 in the Bs (11 B+, 19 B , 29 B-) 44 in the Cs (23 C+, 15 C, 6 C-) 5 Ds, 0 Fs

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Tesla Coil Powered by electricity Observe the top--what do you

see? Observe as a fluorescent

lamp is brought near--what do you see?

Image: http://tommcmahon.typepad.com/photos/uncategorized/2007/08/16/teslacoil2.jpg

Takeaway message: the device is generating something electrical throughout space in the vicinity of the coil.

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New Models: Electric Field and Forces Field, Forces, Potential Energy, and Potential

Magnetic Field and Forces

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Field Model: Generic A source (A) creates a (B) field.

Direction given by _______. The net (B) field is the vector sum of all

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

experiences a (B)force Magnitude given by _____ Direction of force given by _____

What goes blanks for gravitation? Electricity? Magnetism?

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PotentialPotentialEnergy

Field Force

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Field Model: Gravitation

A source mass m1 creates a gravitational field gm1.

Direction is toward mass m1.

The net field gtot is the vector sum of all the source fields.

A test mass m2, placed in a gravitationalfield, experiences a gravitationalforce

Magnitude given by: m2gm1

Direction of force is: attractive, along gm1.

m1

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Field Model: Electric

A source charge q1 creates an electric field Em1.

Direction is toward negative charge OR away from positive charge.

The net field Etot is the vector sum of all the source fields.

A test charge q2, placed in an electric field, experiences an electricforce

Magnitude given by: q2Etot. Direction of force is:

for +q2: same direction as Etot

for -q2: opposite Etot.

-

+

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Do Electric Field and Electric Force differ?

What is the minimum number of object(s) required for each to exist?

a) Both require just one object

b) Field requires 1 object; force requires 2

c) Force requires 1 object; field requires 2

d) Both require at least two objects

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Phenomenon: van de Graaff generator

Observe what happens as the generator builds charge

What happens when a neutral conductor is brought near the generator?

a) No affect

b) Pulled toward

c) Pulled away

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Phenomenon: van de Graaff generator

Next: Touch the conducting sphere to the generator… What will happen now?

a) No affect

b) Pulled toward

c) Pulled away

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Superposition of Fields

The + and - charges have the same magnitude. Which direction does the net Electric Field point?

-

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Superposition of Fields

-

7 Etot = 08 Not enough

info

1

2

3

4

5

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The + and - charges have the same magnitude. Which direction does the net Electric Field point?

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Checking Understanding

-

7 Etot = 08 Not enough

info

1

2

3

4

5

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A charge is placed at the marked location. What direction is the force on that charge?

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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

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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 differ by 0.5 V. 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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PotentialPotentialEnergy

Field Force

Depend on source and test

Depend on only source

Vector

Scalar

PE=qV

F=qE€

E =dV

dr

€

F =dPE

dr