Magnetic Field
Magnetic Levitation
These express trains in Japan
are capable of speeds
ranging from 225 ~ 480 km/h
2
Magnetic Resonance Imaging
A “slice” of human brain
3
Nature of Magnets
1. A magnet can be split into two or more
magnets and each of them has N and
S poles which cannot be isolated
2. This tells the nature of a magnet: All
properties of a magnet come from
electric current loops
4
Properties of Magnetic Field
• Magnetic field is a special type of matter
• Magnetic field contains energy
• Interaction between magnetic field and
electric current (electric field)
• Magnetic field strength and direction
5
Magnetic Field Lines
• Magnetic field lines are used to describe
magnetic field
• Magnetic field lines show both direction
and strength of magnetic field
6
Typical Magnetic Field (1)
7
Typical Magnetic Field (2)
8
Similarity of Two Magnetic Fields
9
The Earth’s Magnetic Poles
10
11
Second Right-Hand Rule
N
S
12
Applications of Electromagnet
13
Applications of Electromagnet
14
How to Calibrate the Sensor
• Turn FINE control to mid-rotation position
• Press RUN/20K button, allow a few seconds or the
unit to stabilize
• Zero display using OFFSET knob unless display
shows under 0.05 or so
• Select 2K range and zero display using COARSE
control
• Select 200 range and zero display using FINE
control
15
How to Use the Sensor
• Choose larger range of measurement if no
reading
• Rotate the probe slightly to get peak reading
• Press STOP to turn off the unit
16
Increase Strength of Electromagnet
1. Use iron (steel) core
2. Increase current (voltage)
3. Increase wraps of solenoid
17
Microscopic Picture of Magnets
18
Magnetic Force
19
Measure of Magnetic Field
• Magnetic induction, B, is the identity to
describe a magnetic field
• B is a vector so it has magnitude and direction
• Unit: Tesla or Gauss
1 Tesla = 104 Gauss
20
Third Right-hand Rule
21
Calculate Magnetic Force
F = BIL
F is in Newton, B is in Tesla,
I is in Ampere, and L is in meter
22
Nature of Magnetic Force
F is the resultant force that magnetic field
exerts on all moving charges
F = BIL
=> I = q/t => t = L/v
=> I =q/t = qv/L
=> F = B(qv/L)L = Bqv
23
Steps to Compute Magnetic Force
• Measure distance the
pipe moved
• Compute θ and F in
reference of the FBD
• Measure B and L
• Compute F by F = BIL
• Compare the two Fs
• What makes the two
Fs different
T
θ
F
24
0.2 N
Application of Magnetic Force
• Paper cone attached
to coil
• Sound signal
converted to varying
electric current
25
Galvanometer
26
Electric Motor
27
Key Procedures to Build a Motor
• Make wire about 1 m long
• Remove coating on only ONE and SAME side
of the straight parts of the wire
• Do not set the current greater than 1 amp
28
Electromagnetic
Induction
29
Electromotive Force (EMF)
• EMF should be called electromotive
potential
• Unit of EMF is Volt
• EMF = BLV
(maximum value)
30
Electric Guitar
31
Tape Recorder
32
Electric Generator
33
34
Difference of Generator and Motor
35
Effective Current & Effective Voltage
PAC  12 Pmax  12 PDC
PI R
2
I
2
eff
R I
I eff 
1
2
1
2
2
max
R
I max  0.707 I max
Similarly,
Veff 
1
2
Vmax  0.707Vmax
36
Lenz’s Law
The direction of the induced current is such
that the magnetic field resulting from the
induced current oppose the change in the
field that cause the induced current.
Result opposes cause
37
Lenz’s Law Illustration
38
Transformer
39
How Can EMF Be Induced?
If and only if there is a changing
magnetic field around the conductor
or circuit. Movement of either the
magnetic field or the conductor
(circuit) is not necessary.
40
Ignition System
41
42
Example Problem
A straight wire 0.20 m moves perpendicularly
through a magnetic field of magnetic induction
0.008 T at a speed of 7.0 m/s. What EMF is
induced in the wire?
Solution:
EMF = BLv
= (0.008 T)(0.2 m)(7 m/s)
= 0.11 V
43