Download Magnetic Fields

Magnetism and
Electromotive Force
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Magnets
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General Properties of Magnets
1. have polarity, north
and south poles
2. poles attract is they
are different and
repel if they are
alike – like electric
charges
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Magnetic Fields
1. Field lines are drawn to represent the
magnetic field.
a. Field lines come out of the North pole
of a magnet (field lines are away from
positive charges)
b. Field lines go into the South pole of a
magnet (field lines are toward negative
charges)
c. Field lines are greatest in number at
the poles
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Electric Field for dipole
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Field of a Permanent Magnet

B
N
S
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This Works For All
Magnets
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Field Lines with iron filings
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The Earth’s Magnetic Field
The northern hemisphere of the Earth contains
the south pole of the “Earth magnet”.
The Earth's magnetic field is similar to that of a
bar magnet tilted 11 degrees from the spin axis
of the earth.
The Earth's core is not magnetic. So how did the
Earth get its magnetic field?
Magnetic fields surround electric currents, so we
surmise that circulating electric currents in the
Earth's molten metallic core are the origin of the
magnetic field.
A current loop gives a field similar to that of the
earth.
More magnetic
properties
3. Permanent magnets
a. do not lose their polarity
b. AlNiCo – aluminum, nickel, cobalt
4. Non-permanent magnets
a. can be magnetized by induction
b. iron, cobalt, nickel alone
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Domains
At the atomic level, we see that magnetism is
produced due to the alignment of the
domains of groups of atoms.
Before magnetism
After magnetism
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. Superconductors repel magnets by
expelling the field lines from their
surfaces. There is no magnetic flux
inside a superconductor.
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Electromagnetism
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Two Electromagnetism
Laws
Ampere’s Law


Electrical currents create magnetic fields
Right Hand Rule
Faraday’s Law


Changing magnetic fields in a conductor
generates electricity
Currents are induced in conductors when the
conductors either…
(a) move in a magnetic field or
(b) are in the presence of a changing magnetic field.
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Electromagnetism
Oersted (Denmark, 1800’s) showed magnetic
effects could be created by moving charges.
(Current carrying wires are surrounded by
magnetic fields.)
Using the first right hand rule, we can see why
current carrying wires will attract if the current
is traveling in the same direction in both wires
and repel if the current is traveling in opposite
directions.
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The First Right Hand Rule
The direction of the magnetic field can be
established by using the first right hand rule.
a. Place your right hand on the wire with
your thumb extended in the direction of
conventional current flow (positive charges
move)
b. Curl you hand around the wire. Your
fingers now point in the direction of the
magnetic field.
c. The magnetic field goes around the wire.
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First Right Hand Rule
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The right-hand rule
gives the direction of
the magnetic field (N
to S).
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Magnetic Field Around a
Coil
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The Second Right Hand Rule
The North pole around a coil can be determined
by using the second right hand rule.
a. Curl the fingers around the coil
pointing in direction of current flow.
b. The direction your thumb points is the
direction of the North pole
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Second Right Hand Rule
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Magnetic Field Near a Coil
Electromagnets can be strengthened by adding
a soft iron core which becomes magnetized
when current is turned on.
A coil of current carrying wire has a magnetic
field making it an electromagnet.
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Wires with Parallel Currents
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Forces Caused by Magnetic
Fields
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Forces Caused by Magnetic
Fields
Faraday discovered that the force on a current
carrying wire in a magnetic field is at right
angles to both the current and the magnetic
field.
The direction of the force on the wire can be
determined using the third right-hand rule.
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The Third Right Hand Rule
Using an open right hand, point the thumb in
the direction of the current flow and the
fingers in the direction of the magnetic field.
The force is visualized as coming out of the
palm of the hand
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Third Right Hand Rule
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The magnitude of the force can be calculated
using the equation:
F = BIL
The strength of the magnetic field (magnetic
induction) is calculated using the equation:
B = F/IL
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Sample Problem
If a wire bearing a current of 10 A lies
perpendicular to a uniform magnetic field and
a force of 0.2 N is found to exist on a section
of that wire 80 cm long, what is the magnitude
of the magnetic field?
F = BIL
B = F/IL
B = 0.2 N/ (10 A x 0.8 m)
B = 0.025 T
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Magnetic Field Strength
Magnetic field strength (actually the density of
the flux lines ) is measured in teslas (T),
equivalent to 1 N/A·m (1 T is very large.)
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Strengths of various magnetic
fields.
1. The strength of the earth’s magnetic
field is about 5 x 10-4 T
2. A lab magnet is about 0.01 T
3. Only the most powerful of
electromagnets have a field strength of
1T
4. Surface of a neutron star 108 T
5. In interstellar space 10-10 T
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The Earth’s Magnetic Field
Earth’s magnetic field
(1) 5 x 10-4 T
(2) points into the earth in the northern
hemisphere
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Devices which incorporate the
effect of magnetic fields on
current carrying wires
Loudspeakers – a coil of current carrying wire is
forced into and out of a magnetic field which
causes a paper cone to vibrate
Galvanometers
a. measure very small amounts of current
b. may measure as little as 50 A
c. can be converted to ammeters or
voltmeters
Electric Motors – use a loop which can rotate 360
as a result of changing the direction of the
current just as the loop reaches vertical
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Force on a Single Charged
Particle
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Force on a Single Charged
Particle
The force of the magnetic field on current
carrying wires is a result of the force exerted
on individual charges.
Cathode ray tubes use this concept to direct
electrons to the inside surface of the screen
where they strike the phosphor producing a
picture.
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This force can be calculated using the formula
F = Bqv
The direction of the force on the particle can be
predicted using the third right hand rule by
extending the thumb in the direction of the
velocity of the positive charge.
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1. The force is perpendicular to the velocity of
the particle and the magnetic field.
2. Charges moving parallel to the magnetic
field experience no force.
3. Maximum force is experienced when the
particle is moving perpendicular to the field
4. The magnitude of the force is directly
proportional to q and to v
5. The direction of the force depends upon the
sign of the charge.
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Sample Problem
A He2+ ion travels at right angles to a
magnetic field of 0.80 T with a velocity of
105 m/s.
Find the magnitude of the magnetic force on
the ion.
F = Bqv
F = (0.80 T)2(1.6 x 10-19 )(105 m/s)
F = 2.56 x 10-14 N
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Sample Problem
A proton moves perpendicularly to a magnetic
field that has a magnitude of 4.2 x 10-2 T.
What is the speed of the particle if the
magnitude of the magnetic force on it is 2.4 x
10-14 N?
F = Bqv so
v = F/Bq
V = (2.4 x 10-14 N)/[(4.2 x 10-2 T)(1.6 x 10-19 C)]
V = 3.57 x 106 m/s
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Electromotive Force
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INDUCTION: Creating Electric
Current from Changing Magnetic
Fields
1. Michael Faraday and Joseph Henry
showed that current could be produced by
moving magnets.
2. Electromagnetic induction is the
production of potential difference by
moving a wire through a magnetic field.
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Faraday’s Law of Induction
A loop of wire is connected to a current
meter…
If a magnet is moved toward or away from the
loop, a current is measured.
• If the magnet is stationary, no current exists.
• If the magnet is stationary and the wire loop is
moved, a current is measured.
\
A current is set up in the loop as long as
there is relative motion between the magnet
and the loop.
Induction
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Electromotive Force
1. EMF is not a force but rather an increase in
potential and is measured in volts.
2. EMF can be calculated using the equation
EMF = BLv
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Devices which incorporate the
concept of magnetic fields producing
current
1. Microphones – movement of a diaphragm
causes a wire to move into and out of a
magnetic field inducing current.
2. Electric generators
a. convert kinetic energy into electrical
energy.
b. mechanical movement of an armature
inside a magnetic field causes current to flow
within the wire.
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Transformers
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Transformers
Change the voltage of the current without
changing the power that is provided with
alternating current.
They allow us to carry high-voltage at low
currents and then convert it to low-voltage,
high current for use.
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How do transformers work?

A changing current through a coil of wire can
create a changing magnetic field.

Currents can be induced in other wires by these
changing magnetic field.

Therefore, the primary coil current must have AC.

The iron core of the transformer is not required but
it does increases the efficiency a great deal.
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Transformer Calculations
3. Voltage changes can be calculated using the
relationship between the voltages and the
number of coils of wire in the transformer.
secondary voltage
primary
voltage
turns on secondary =
turns on primary
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For an Ideal Transformer...
Np
Ep
Is
a


Ns Es Ip
Number
of Turns
Coil
Voltages
Coil
Currents
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Step-up or Step-down?
If a<1, the Es>Ep and we have a
step up transformer.
If a>1, the Es<Ep and we have a
step down transformer.
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Step up transformers
a. increase voltage
b. televisions – 120 V to 25 kV
c. car batteries – 12 V to 103 V
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Step down transformers
a. decrease voltage
b. high voltage lines carry from
4.8 x 105 V to 120 V
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Ratio of the Number of Windings
Matters
Is = Vp = Np
Ip
Vs
Ns
Secondary Voltage is
V2 = (N2/N1) V1
Secondary Current is
I2 = (N1/N2) I1
But Power in =Power
out
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An Example
My laptop computer requires about 12 Volts
AC, which comes from an adapter
(transformer) that is plugged into the wall
socket.
What is the approximate ratio of the number
of turns on this transformer? Which coil has
more turns, the primary or the secondary?
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The Field Concept
Michael Faraday (1791 - ) had the idea that forces
between bodies were cause by Fields that fill all
space and act on the bodies
Electric Field E
due to charge
Faraday discovered the important connection
between Electric Fields & Magnetic Fields:

A moving or changing electric field generates a magnetic
field and a moving or changing magnetic field generates an
electric field.
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axwell - Electricity & Magnetism
James Clerk Maxwell: Treatise on Electricity &
Magnetism (1873) is the last word on classical E&M

Ranks with Newton’s work as one of the great
accomplishments of physics
Maxwell’s Equations
Four equations that completely describe all of
electricity and magnetism
1. Coulomb’s law - relates electric field to charges
 2. Ampere’s law (Generalized)- moving charge or changing
electric field generates a magnetic field
 3. Faraday’s law: changing magnetic field generates an
electric field
 4. Absence of free magnetic “charges” (only pairs of northsouth poles)

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axwell - Electricity & Magnetism
Maxwell’s equations show that electric and magnetic
forces travel at a definite predicted speed -- NOT
instantaneous “action at a distance”
Travel as electromagnetic waves - recall that a
changing electric field generates a magnetic field and
vice versa
Travel in free space (vacuum) at a speed determined
by the constants in Coulomb’s law and Faraday’s law
Using values for the constants measured in the
laboratory, speed predicted to be equal to the speed
of light!
c = 3.0 x 108 m/s
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Electromagnetic
Electromagnetic wave in vacuum (free space)
Wave
Changing electric field generates magnetic field and
vice versa
Direction of
motion
Electric Field
Magnetic Field
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Summary of Classical Physics
Physics as it stood near the end of the 19th Century
Fundamental concepts:






Time flows the same everywhere for all observers
Space is described by 3 dimensions (Euclidean Geometry)
Mass is never created nor destroyed (conserved)
Charge (plus and minus) total is conserved
Energy changes form but is conserved
Momentum is conserved
Fundamental Objects and Laws:


Particles have mass and move according to Newton’s laws
Force originates in interactions between particles of matter


baseballs, rockets, …..
Waves are moving patterns in a medium - e.g. light is
described by Maxwell’s laws

Sound, Light, …..
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