Physics 7C SS1, Lecture 8:Field Model

Finish Electric

Begin Magnetic

PotentialPotentialEnergy

Field Force

Depend on source and test

Depend on only source

Vector

Scalar

PE=qV

F=qE€

E =dV

dr

€

F =dPE

dr

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 electric force Magnitude given by: q2Etot. Direction of force is:

for +q2: same direction as Etot for -q2: opposite Etot.

-

+

Quiz 3

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

→

→

=−

=− − −

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

Not all potentials are straight lines!

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

Field Model: Magnetic

A source creates a magnetic field B1. Direction is . The net field Btot is the vector sum of all the

source fields. A test , placed in a magnetic field,

experiences an magnetic force Magnitude given by: . Direction of force is: .

A little background Compasses or bar magnets, if allowed,

will always orient north-south

Why?

A little background Compasses

orient in the same direction as the magnetic field.

A little background Iron fillings also orient in the same

direction as the magnetic field.

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:

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)

Right Hand Rule # 1

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 directionI

B A

C

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 directionI

B A

C