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### CHAPTER 19Magnetism

e-

proton

There is a relationship between electricity and magnetism.

A moving electric charge produces a magnetic field (B-field).

Question:

All materials contain moving electric charges (electrons). So why are not all materials magnetic?

Answer:

Their atoms are randomly orientated and the

B-fields (vectors) cancel each other out.

(No Magnetism)

Domains Roughly Aligned

(Some Magnetism)

Question:

Why are there any magnetic materials?

Answer:

Some materials contain “domains” ( 10-4m across) where the motion of electrons is roughly aligned. When these domains are aligned, the material becomes magnetic.

Magnetic Materials (have domains)

Iron

Nickel

Cobalt

Magnets have “north” and “south” poles.

Opposites attract. Like poles repel.

The pointed end of the compass needle is a north pole and it points to a south pole (magnetic) in the picture and when used to find your way home.

Magnetic Field Lines

A model to help visualize the invisible.

Exit north pole of a magnet.

Enter south pole of a magnet.

Never cross.

o I

2r

B

I

r

=

o

2

B =

I

r

B =

Magnetic FieldsCreated by Long Current Carrying Wireso = permeability of free space

o = 4x10-7 Tm/A

o I

2r

I

B =

What about the B-field direction?

Use another Right-Hand-Rule

Grab wire with thumb pointing in direction of current.

Fingers wrapping around wire point in direction of B-field

v

q+

N

S

[ ]

[B] =

FB = q v B

N

C m/s

[ ]

[B] =

Wb = Weber

Wb

m2

Magnetic Forces Acting on Moving ChargesA magnetic force, FB, is exerted on a charged particle moving through a magnetic field.

FB = q v B sin

q = electric charge of the particle (C)

v = velocity of particle (m/s)

B = strength of magnetic field (T) T = Teslas

= angle between velocity and B- field vectors

FB = FB,max where =90; = 90 (often in AP Physics)

[B] = [T]

Example #2

B

FB

v

v

q-

q+

B

B

What is the direction of B-field?

What is the direction of FB?

Note “ ” demotes tip of arrow pointed out of paper.

FB = q v B = magnitude of the force

But what is its direction?

- 2nd Right Hand Rule (where v B)
- With right hand flat:
- Outstretched fingers point in direction of B-field.
- Thumb points in direction of velocity of a positively charged particle. [Use opposite direction or left hand for velocity of a negatively charged particle]
- Palm points in direction of magnetic force acting on the moving charge.

FBX

Note “x” denotes tail of arrow pointed into paper

mv2

r

mv2

r

FC =

= q v B

r =

m v

q B

Motion of a Charged Particle Moving in a Magnetic FieldWhat do we call this type of force?

Answer:

Centripetal (FC)

FC = FB

FBis the centripetal force

From Right Hand Rule we see that FB is always directed towards the center of circular path.

Often used formula easily derived

l

L

t

L

t

v =

q

t

FB = q B

= I

Answer:

FB

Magnetic Forces Acting on Current Carrying WiresFB = q v B

l = length of wire in the magnetic field

FB = I L B

Test Yourself

What will be the direction of the FB acting on the wire?

I

Forces on Parallel Current Carrying WiresA second current carrying wire is placed in this B-field.

What is the direction of FB on this second wire?

Use RHR.

FB on right wire points toward left wire.

Parallel wires with same direction current are attracted to each other.

Prove left wire is attracted to right wire.

Prove if current are opposite directions, wires repel each other.

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