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Magnetism

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Magnetism * * * * * * * * * * * * * * By changing the effective area A in a constant magnetic field B: = B A A Change in Flux Faraday s Law of Induction E ... – PowerPoint PPT presentation

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Title: Magnetism


1
Magnetism
2
Magnetic Dipole
Magnetic Field (B)
3
Magnetic Monopoles
  • Do not exist!
  • They differ from electric dipoles, which can be
    separated into electric monopoles.

4
Magnetic Force on Charged Particle
  • magnitude F qvBsin?
  • q charge in Coulombs
  • v speed in meters/second
  • B magnetic field in Tesla
  • ? angle between v and B
  • direction Right Hand Rule

5
Magnetic Force
Calculate the magnitude and direction of the
magnetic force.
v 300,000 m/s
34o
q 3.0mC
B 200 mT
6
Magnetic Fields
  • Formed by moving charge
  • Affect moving charge

7
Magnetic Field Visualization
8
3D Magnetic Field Visualization
9
3D Magnetic Field Visualization
10
3D Magnetic Field Visualization
11
Magnetic Forces can...
  • accelerate charged particles by changing their
    direction
  • cause charged particles to move in circular or
    helical paths

12
Magnetic Forces cannot...
  • change the speed or kinetic energy of charged
    particles
  • do work on charged particles

13
When v and B are at right angles to each other...
  • qvBsin? mv2/r
  • qB mv/r
  • q/m v/(rB)

x into out
q is
14
When v and B are at right angles to each other...
15
Practice With Directions
What is the direction of the force F on the
charge in each of the examples described below?
negative q
16
Circular Paths in Magnetic Field Fmagnetic
Fcentripetal
17
Electron Beam in Magnetic Field
18
Electric and Magnetic Fields Together
v E/B
19
Magnetic Force on Current-carrying Wire
  • F I L B sin?
  • I current in Amps
  • L length in meters
  • B magnetic field in Tesla
  • ? angle between current and field

20
MAGNETIC FIELD PRODUCED BY CURRENT CARRYING WIRE
A long, straight current carrying wire produces a
magnetic field.
21
Magnetic Field for Long Straight Wire
  • B µoI/(2pr)
  • µo 4p x 10-7Tm/A
  • magnetic permeability of free space
  • I current (A)
  • r radial distance from center of wire (m)

22
Hand Rule
i
  • Curve your fingers
  • Place your thumb in direction of current.
  • Curved fingers represent curve of magnetic field.
  • Field vector at any point is tangent to field
    line.

23
For straight currents
24
Principle of Superposition
When there are two or more currents forming a
magnetic field, calculate B due to each current
separately and then add them together using
vector addition.
25
Force Between Current Carrying Wires
26
Magnetic Field Inside a Solenoid
  • B µonI
  • µo 4p x 10-7Tm/A
  • n number of coils per unit length
  • I current (A)

27
Lab Magnetic Field Map
  • Using a compass, map the magnetic field inside
    and outside your solenoid. Show
  • Current through solenoid
  • Connection to DC outlet
  • Field lines mapped with compass
  • Edge effects (What happens to those field lines
    farther away from the solenoid?)
  • North and South Poles of solenoid

28
Lab Evaluation Magnetic Field Map A) Are field
lines visible in the core of the solenoid? B)
Are the directions of the field lines in the core
consistent with the Right Hand Rule? C) Are the
North and South Poles correctly identified?
29
Magnetic Flux
  • The product of magnetic field and area.
  • Can be thought of as a total magnetic effect on
    a coil of wire of a given area.

30
Magnetic flux
31
Magnetic Flux
  • ?B BAcos?
  • ?B magnetic flux in Webers
  • (Tesla meters2)
  • B magnetic field in Tesla
  • A area in meters2.
  • ? the angle between the area and the magnetic
    field.

32
Magnetic Flux
  • A system will respond so as to oppose changes in
    magnetic flux.
  • Changing the magnetic flux can generate
    electrical current.

33
A Change in Flux
By changing the field strength B going through a
constant loop area A ?F ?B A
34
A Change in Flux
By changing the effective area A in a constant
magnetic field B ?F B ?A
35
Faradays Law of Induction
  • E -N?B/?t
  • E induced potential (V)
  • N loops
  • ?B magnetic flux (Webers, Wb)
  • t time (s)

36
A closer look
  • E -??B/?t
  • E -?(BAcos?)/?t
  • To generate voltage
  • Change B
  • Change A
  • Change ?

37
Lenzs Law
  • The current will flow in a direction so as to
    oppose the change in flux.
  • Use in combination with hand rule to predict
    current direction.

38
(a) As the magnet moves to the right, ?B
increases. (b) To oppose increased ?B , the
induced B must be opposite to that of the bar
magnet. The induced I must be CCW
39
When N of magnet is pushed into a loop the ?B?.
Upward B is induced. As viewed from above, the
induced current must flow CCW
40
When N of magnet is pulled away from loop the
?B?. Downward B is induced. As viewed from
above, the induced current must flow CW
41
Motional emf
  • E B L v
  • E induced potential
  • L length of bar or wire
  • V speed of bar or wire

42
Induced EMF
43
Induced EMF
44
2002B5 p.49
A proton of mass mp and charge e is in a box that
contains an electric field E, and the box is
located in Earth's magnetic field B.The proton
moves with an initial velocity vertically upward
from the surface of Earth. Assume gravity is
negligible.
45
2002B5 p.49
(a) On the diagram, indicate the direction of the
electric field inside the box so that there is no
change in the trajectory of the proton while it
moves upward in the box. Explain your reasoning.
46
2002B5 p.49
(b) Determine the speed v of the proton while in
the box if it continues to move vertically
upward. Express your answer in terms of the
fields and the given quantities.
47
2002B5 p.49
The proton now exits the box through the opening
at the top. (c) On the figure, sketch the path of
the proton after it leaves the box.
48
2002B5 p.49
49
2002B5 p.49
(d) Determine the magnitude of the acceleration a
of the proton just after it leaves the box, in
terms of the given quantities and fundamental
constants.
50
1978B4 p.51
Two parallel conducting rails, separated by a
distance L of 2 m, are connected through a
resistance R of 3 O as shown. A uniform magnetic
field with a magnitude B of 2 T points into the
page. A conducting bar with mass m of 4 kg can
slide without friction across the rails.
51
1978B4 p.51
(a) Determine at what speed the bar must be
moved, and in what direction, to induce a ccw
current I of 2 A as shown.
52
1978B4 p.51
(b) Determine the magnitude and direction of the
external force that must be applied to the bar to
keep it moving at this velocity.
(c) Determine the rate at which heat is being
produced in the resistor and determine the
mechanical power being supplied to the bar.
53
1978B4 p.51
(d) Suppose the external force is suddenly
removed from the bar. Determine the energy in J
dissipated in the resistor before the bar comes
to rest.
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