Magnetic Fields
What causes material to be magnetic?
Does just spinning make a magnet?
Why can some material become magnetic?
It is the alignment of the spin that causes material to be
magnetized.
Polarization: opposite spin or different direction of the
domain.
1. If the alignment is fixed or constant then the material is a
permanent magnet.
a. Permanent magnets are made of Al, Ni, and Co .
Ferromagnetic material
2. If the alignment is temporary, then the material will make a
temporary magnet. (Iron)
3. If the alignment is not obtainable, then the material is nonmagnetic. (Plastic)
Magnetic Field Lines
Description of the magnetic flux.
Magnetic flux acts in the direction of a positive charge.
Magnetic field lines go out of north poles and to south
poles.
The closer together the field lines are, the stronger the
field.
North attracts to the South and repel to North.
Magnetic Field
Electric Field Lines
1.
2.
3.
4.
5.
6.
Magnetic Field Lines
Vector force
Denser lines = stronger
Lines cannot cross
Created at (+) pole
Destroyed at (-) pole
Charge does not move
through the battery
7. Charges begin and end
1. Vector force
2. Denser lines = stronger
3. Lines cannot cross
4. Begins at North pole
5. Moves to the South pole
6. Lines move through the
magnet, continuous flux
7. Never ends. Always form
closed loops
Magnetic Interaction
North to South pole attract.
North to North or South to South repel.
http://brightstorm.com/science
TLS: Page 665 # 59-62
Electromagnetism
Han Christian Oersted: first to relate an electric current to
magnetism.
Findings:
1. Magnetic force act perpendicular to the direction of the
current.
2. The direction of the magnetic field line depends on the
direction of the current.
3. The strength of the magnetic field lines is inversely
proportional to the distance from the conductor.
4. The strength of the field is proportional to the strength of
the current. Increase current = increases strength
5. The number of turns of the conductor (solenoid) is
proportional to the strength of the field. Increase
turning = increase strength
6. The direction of the field can be changed with a
change in the direction to the current.
7. Better conductors = better strength
Strength of an Electromagnet
1. Increase current (increase force ‘volts’)
2. Decrease resistance to the current
3. Increase the number of coils (increases the number of
lines of force).
4. Better conductors (allows for better alignment of the
domain)
5. Decrease temperature (decreases resistance)
Electromagnet Force
First Right Hand Rule: determines the direction of a
magnetic field relative to the direction of the current.
Electromagnetic Force
Second Right Hand Rule: determines the direction of
magnetic field relative to the direction of the current in
an electromagnet.
Calculating Electromagnetic Force
Third Right hand Rule: Thumb is the direction of current, the
figures show the direction of the magnetic field lines and the
palm show the direction of the force created.
F = ILB
I = current measured in ampere, A. (C/s)
L (l)= length of the current carrying
conductor (m).
B = Strength of the magnetic field, Talsa (T)
T = N/A m
Electromagnetic Sampler
1. A 45 cm conductive wire carries a current of 7.5 A at a right
angle to a 0.025 T magnetic field. How strong is the force
acting on the conductive wire?
Electromagnetic Sampler
2. What is the strength of a magnetic field when a 120 cm
conductive wire carrying 6 A exerts a force of 2.3 N on the
wire?
TLS: #16-20, p 654
Problems Set One.
Calculate the Electromagnetic Force of
a Moving Particle
The force on a moving particle in a magnetic field is proportional
to the charge, the velocity of the particle at a right angle to
the field and the magnetic field strength.
F = qvB
Unit for the force is Newton, N
q+ use the right hand rule
q- use the left hand rule
v velocity in m/s
B electric field intensity, Tasla, T
Electromagnetic Sampler
1. What is the force acting on an electron ( 1.6 x 10-19 C )
moving from left to right in a magnetic field of 0.23 T at a
velocity perpendicular velocity of 5.2 x 106 m/s?
2. What direction is the force acting?
Electromagnetic Sampler
2. The force acting on a charge, q+, is 5.2 x 10-14 N. If the charge is
of a magnitude equal to 1.55 x 10-17 C and is traveling at 2.5 x
106 m/s, what is the strength of the magnetic field?
What direction is the force acting?
TLS: #21-25, p 658
Problem Set Two
Electromagnetic Induction
EM Induction: the generation of a magnetic field through an
electric current.
Changing one current induces a change in another current.
Michael Farraday: moving a conductive wire through a
magnetic field would generate an electric current in the wire.
That generated current will cause change in another current.
Fourth Right Hand Rule:
Thumb  in the direction of the motion of the wire
Finger  in the direction of the magnetic filed N S
Palm  in the direction of the force acting on the wire.
The Force = EMF
Electromotive Force
EMF: a ‘Force’ applied by the magnetic flux on a charge carried in a
conductive wire that is introduced into the magnetic field.
EMF = potential difference = voltage
EMF is the influence that causes a current to flow form low to high
potential.
EMF = BLv
B = magnitude of the magnetic field (T)
L = length of the conductive wire (m)
v = velocity perpendicular to the field (m/s)
Sampler One EMF
A conductive wire that is 150 cm long is moved at a constant speed of
4.5 m/s perpendicular to a magnetic field measured at 0.0075 T.
What is the EMF?
At a resistance of 0.25Ω, what is the current?
Sampler #2 EMF
A conductive wire that is 0.45 m in length moves through a magnetic
field at 15 m/s. If the wire carries a current of 0.33A and has an
internal resistance of the wire is 1.52 Ω, what is the magnitude of
the magnetic flux?
Electric Generators
Converts mechanical energy to electric energy.
Diagram:
Strength (generated HP)
1. Increase magnet
2. Increase current (decrease resistance)
3. Increase the number of turnings.
Page 675 # 1-4, and page 692 #60-63, 67, 68.
Effective Current/Voltage
Effective Current: The amount of current generated by a moving
conductor within a magnetic field.
Ieff = √2 /2 x Imax
Note page 677
Ieff = 0.707 x Imax
Effective Voltage: The amount of voltage generated at the effective
current.
Veff = √2 /2 x Vmax
Note page 678
Veff = 0.707 x Vmax
Reinforcement
Ohm’s law
I = V/R
Power
P = I2R or P = V2/R or PAC = ½ PAC max
Page 678, #5-8
Transformers
Device used to increase or decrease AC voltage.
Components:
Iron core
primary coil
secondary coil
conductive turnings
Type of Transformers
1.
Step-Up Transformer: increase the voltage by increasing the
number of turnings.
a. Primary coil has fewer turnings than the secondary coil.
2. Step-Down Transformer: decreases the voltage by decreasing the
number of turnings.
a. Primary coil has more turnings than the secondary coil.
Transformers
The increase or decrease in the voltage is proportional to the
number of turnings.
From 2 turns  20 turns, there is a 10X increase in the
number of turns and therefore a 10 fold increase in voltage. (step
up)
From 100 turns  50 turns, there is a 2X decrease in the
number of turns and therefore a ½ decrease in the voltage (step
down).
Transformer Calculation
Vs / Vp = Ns /Np
The current is inversely proportional to the voltage and therefore to
the number of turnings
Increase voltage by increasing turning and the results is a decrease
in the current.
Is / Ip = Vp / Vs = Np / Ns
Transformer Sampler
1. A transformer has 200 turns on the primary coil and 3000 turns of
the secondary coil. The primary coil is supplied with 90 V.
a. What type of transformer is this?
b. What is the voltage in the secondary coil?
c. The current in the secondary coil is measured at 2A. What is the
current in the primary coil?
Transformer Sample
2.
In a transformer the primary coil is supplied with 360 V. All
you know is that the turnings has been decrease 3X.
a. If the primary coil has 30 turnings, how many turning are
in the secondary coil?
b. What is the voltage in the secondary coil?
c. If the current in the primary coil is 0.5 A, what is the
current in the secondary coil?