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# Magnetic field - PowerPoint PPT Presentation

Magnetic field. Chapter 28. Magnetism. Refrigerators are attracted to magnets!. Where is Magnetism Used??. Motors Navigation – Compass Magnetic Tapes Music, Data Television Beam deflection Coil Magnetic Resonance Imaging (MRI) High Energy Physics Research. Cathode. Anode. (28 – 8).

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## PowerPoint Slideshow about ' Magnetic field' - kenyon-bray

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Presentation Transcript

### Magnetic field

Chapter 28

• Refrigerators are attracted to magnets!

• Motors

• Magnetic Tapes

• Music, Data

• Television

• Beam deflection Coil

• Magnetic Resonance Imaging (MRI)

• High Energy Physics Research

Anode

(28 – 8)

S

Consider a Permanent Magnet

The magnetic Field B goes from North to South.

• If the charge is moving, there

• is a force on the charge,

• perpendicularto both v and B.

• F = q vxB

q

A Look at the Physics

There is NO force on

a charge placed into a

magnetic field if the

charge is NOT moving.

There is no force if the charge

moves parallel to the field.

This can be summarized as:

F

or:

v

q

m

B

q is the angle between B and V

Assume all electrons are moving

with the same velocity vd.

L

B out of plane of the paper

.

(28 – 12)

What is force

on the ends??

Loop will tend to rotate due to the torque the field applies to the loop.

F

q

L

S

N

B

I

F

F=BIL

Magnetic Force on a Current Loop

Simplified view:

F=BIL

q

L

d

I

F=BIL

Simplified view:

F=BIL

q

L

d

I

F=BIL

for a current loop

q

L

d

I

F=BIL

Magnetic Force on a Current LoopTorque & Electric Motor

Top view

C

C

(28 – 13)

Magnetic Force on a Current Loop Torque & Magnetic Dipole

By analogy with electric dipoles, for which:

The expression,

implies that a current loop acts as a magnetic dipole!

Here is the magnetic dipole moment,

and

(Torque on a

current loop)

• Define the magnetic

• dipole moment of

• the coil m as:

• =NiA

t=m x B

We can convert this

to a vector with A

as defined as being

normal to the area as

in the previous slide.

L

L

R

L

R

(28 – 15)

B

+ + + +

+ + + +

+ + + +

+ + + +

+ + + +

F

### Trajectory of Charged Particlesin a Magnetic Field

(B field points into plane of paper.)

B

+ + + +

+ + + +

+ + + +

+ + + +

+ + + +

v

F

### Trajectory of Charged Particlesin a Magnetic Field

(B field pointsinto plane of paper.)

v

B

B

+ + + +

+ + + +

+ + + +

+ + + +

+ + + +

+ + + +

+ + + +

+ + + +

+ + + +

+ + + +

v

F

F

Magnetic Force is a centripetal force

s

 = angle,  = angular speed,  = angular acceleration

r

at = r  tangential acceleration

ar = v2 / rradial acceleration

The radial acceleration changes the direction of motion,

while the tangential acceleration changes the speed.

at

ar

Uniform Circular Motion

 = constant  v and ar constant but direction changes

ar

KE = ½ mv2 = ½ mw2r2

ar = v2/r = 2 r

v

F = mar = mv2/r = m2r

Review of Rotational Motion

### Radius of a Charged Particle =  r Orbit in a Magnetic Field

Centripetal Magnetic

Force Force

=

v

B

+ + + +

+ + + +

+ + + +

+ + + +

+ + + +

F

r

v =  r

B

+ + + +

+ + + +

+ + + +

+ + + +

+ + + +

F

r

### Cyclotron Frequency

The time taken to complete one

orbit is:

V cancels !

Smaller Mass =  r

Mass Spectrometer

An Example =  r

A beam of electrons whose kinetic energy is K emerges from a thin-foil “window” at the end of an accelerator tube. There is a metal plate a distance d from this window and perpendicular to the direction of the emerging beam. Show that we can prevent the beam from hitting the plate if we apply a uniform magnetic field B  such that

r =  r

Problem Continued

#14 Chapter 28 =  r

A metal strip 6.50 cm long, 0.850 cm wide, and 0.760 mm thick moves with constant velocity through a uniform magnetic field B= 1.20mTdirected perpendicular to the strip, as shown in the Figure. A potential difference of 3.90 ηV is measured between points x and y across the strip. Calculate the speed v.

• 21.   =  r (a) Find the frequency of revolution of an electron with an energy of 100 eV in a uniform magnetic field of magnitude 35.0 µT . (b) Calculate the radius of the path of this electron if its velocity is perpendicular to the magnetic field.

• 39.   =  r A 13.0 g wire of length L = 62.0 cm is suspended by a pair of flexible leads in a uniform magnetic field of magnitude 0.440 T. What are the (a) magnitude and (b) direction (left or right) of the current required to remove the tension in the supporting leads?