# Magnetic Fields Notes - jflaherty1@kleinisd.net

Ηλεκτρονική - Συσκευές

18 Οκτ 2013 (πριν από 4 χρόνια και 7 μήνες)

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Created by Jim Hammons

jhammons@kleinisd.net

1

Magnetic Fields Notes

I Magnets

A.

Properties of magnets

1.

Magnet has polarity. The two ends are called North pole and South Pole. Note: North
pole is North seeking.

2.

Two like poles repel and two unlike poles attract.

3.

Being brought near a magnet can pol
arize some metals. They become temporary
magnets.

B.

Magnetic fields and Permanent Magnets

1.

Magnetic field
-

Space around a magnet throughout which magnetic force exists.

2.

Iron filings around a magnet can represent magnetic field lines.

3.

Magnetic flux
-

The

number of magnetic field lines passing through a surface.

a.

The flux per unit area is proportional to the strength of the magnetic field.

b.

The flux lines are most concentrated at the poles where the magnetic field is the
greatest.

c.

The direction of the mag
netic field lines is defined as the direction to which North
pole of a compass points when it is placed in the magnetic field.

d.

The field lines come out of the magnet at its North pole and enters the magnet at
its South pole.

N S

C.

Electromagnetism

Christian Oersted discovered that electric current in a wire creates a magnetic field.

First Right Hand Rule
-

Grasp the wire with your right hand
. Keep your thumb in the
direction of the positive current. The fingers of your hand circle the wire and point in the
direction of the magnetic field.

Created by Jim Hammons

jhammons@kleinisd.net

2

D.

Magnetic filed near a coil

1.

Magnetic field lines about a current carrying wire crowd up
when the wire is bent into
a loop.

2.

If the wire is bent into another loop the magnetic field lines inside the double loop is
twice as much as in the single loop. Thus, for many loops a very strong magnetic field
can be obtained.

3.

If a piece of iron is place
d in a current carrying coil of wire, the magnetic domains in
the iron are induced into alignment. This will further increase the magnetic field.

4.

Electromagnet
-

A device that uses an electric current to produce a concentrated
magnetic field.

Second R
ight Hand Rule
-

Grasp the coil with the right hand curl your fingers around
the loops in the direction of the positive current flow. Then your thumb points toward the
North pole of the electromagnet.

II.

Forces caused by magnetic fields

A.

Forces on currents

in magnetic fields.

F = BIL

where B

Magnetic field

I

Current in the wire

L

Length of wire

B.

Third Right Hand Rule
-

Right hand fingers point in the direction of the magnetic f
ield.
Point your thumb in the direction of the positive current in a wire. The palm of your hand
then faces in the direction of the force acting on the wire.

C.

Notation for the direction of magnetic field

B field

B field into page B field out of page

X X X X X X . . . . . . . . . . .

X X X X X X

. . . . . . . . . . .

X X X X X X . . . . . . . . . . .

X X X X X X

. . . . . . . . . . .

X X X X X X . . . . . . . . . . .

Created by Jim Hammons

jhammons@kleinisd.net

3

D.

Typical magnetic fields

1. Strong laboratory magnet 10 T

2. small bar magnet .01 T

3. Earth’s magnetic field 5 x 10
-
5

T

E.

Magnetic fields from current bearing wires

1.

Two current bearing wires are attracted when the currents are in the same direction.

F F

I I

2.

Two current bearing wires are repelled when the currents are in opposite directions.

F

F

I I

F.

Calculating the force on a single charged particle

F = Bqv where F

Force (N)

B

Magnetic field (T)

q

v
-

velocity of the p
article (m/s)

Created by Jim Hammons

jhammons@kleinisd.net

4

Example 1. A wire .1 m long carries a current 5 A. The wire is at right angle to a uniform

magnetic field. The force on the wire is 2 N. What is the magnitude of the magnetic

field, B?

Example 2. A beam of electrons travel at 3 x 10
6

m/s through a .04 T uniform magnetic field.

a. The beam is at right angles to the magnetic field. What is the magnitude of the

force acting on eac
h electron?

b.

Compare the force acting on a proton moving at the same speed and in the same
direction to the force acting on the electron in part a.

Example 3. A cathode ray beam (m = 9.11 x 10
-
31

kg, q =
-
1.6 x 10
-
19

C) is bent
in a circle of

radius 3.5 cm by a uniform field with B = 2.5 x 10
-
3

T. What is the speed of the

electrons?

Example 4. An electron is accelerated from rest through
a potential difference of 5000V. It enters
a region where B =6 x10
-
3

T perpendicular to its velocity. Calculate the radius of the path it will
follow.

Created by Jim Hammons

jhammons@kleinisd.net

5

Example 5 Use the right hand rule to draw the magnetic field lines.

a.

N N

b.

I

Example 6. A proton enters a magnetic flux density of 5T wi
th a velocity of 4.5 x10
7

m/s at an
angle of 30° with the field. Compute the force on the proton.

Example 7. The magnetic field of a loud speaker is .25 T. The wire consists of 400 turns wound
on a 2.5 cm cylindrical form. The resistance o
f the wire is 10
Ω. Find the force exerted on the
wire when 20 V is placed across the wires.