Electromagnetism: Magnetic Fields, Permeability, and Forces between Wires, Slides of Physics

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Electromagnet

Wire Field

A moving charge generates amagnetic field.

•^

Symmetry with experiencingforce

-^

Perpendicular to direction ofmotion

-^

Circles around path

Outside a straight wire themagnetic field is cylindrical.

•^

Decreases inversely withdistance

I

B

I r

B

^  2 

Mutual Attraction

Two parallel wires with currentwill exert a force on eachother.

•^

Wire length

l

•^

Separation

d

Currents in the same directionattract.

Currents in the oppositedirection repel.

I d

B

^ ^2

1 0

12

I^1

d I^2

d

l I I

lB I

F

^2

2 1 0

12 2

12

l 

Wire Force

Find the force per unit lengthexperienced by each of twolong parallel wires in avacuumseparated by 1.0 m carryingcurrents of 1.0 A.

This is the official definition ofthe SI unit of the ampere (A).

•^

1 A = 1 C/s

Convert the force on the wireinto a force per unit length.

•^

F/l

^

x 10

-^

Tm/A)(1.0 A)2 /

(1.0 m)

-^

F/l

= 2 x 10

-^

N/m

II d

F l

^2

2 1 0

12

The ampere is that constant current which, if maintained in two straightparallel conductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in vacuum, would produce betweenthese conductors a force equal to 2 x 10

-^

newton per meter of length.

Solenoid



Multiple parallel loops form a solenoid.

-^

Each of N loops contributes a share of the field

-^

Solenoids create very uniform magnetic fields

R

I N

B

 2

Tight,^  short coil

Close Loops

Field in a solenoid varies at theedges from each loop of wire.

Closer loops in a solenoidmake a stronger field.

•^

Turns per length

n = N/l

•^

Formula changes for a longcoil

A split coil would have half thefield.

•^

Equivalent to saying half thefield leaks out between loops.

B

NI l

nI

B

 