# Electric and Magnetic Fields

Electronics - Devices

Oct 18, 2013 (4 years and 6 months ago)

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Electric and Magnetic Fields
Electric Currents Produce Magnetic Fields
Experiment shows that an electric current
produces a magnetic field.
Electric and Magnetic Fields
Pattern caused by an electric charge
Pattern caused by an electric current
Magnets and Magnetic Fields
Magnetic fields can be visualized using
magnetic field lines, which are always closed
loops.
Electric and Magnetic Fields
Pattern caused by two charges
Pattern caused by two poles
Electric Currents Produce Magnetic Fields
The direction of the
field is given by a
right-hand rule.
Solenoids and Electromagnets
A solenoid is a long coil of wire. If it is tightly
wrapped, the magnetic field in its interior is
almost uniform:
Electric Currents Produce Magnetic Fields
Some objects can be
picked up by a magnet.
Iron and nickel are common magnetic
materials
Aluminum, plastic, and glass are not
Electric Currents Produce Magnetic Fields
Electric Currents Produce Magnetic Fields
Magnets and Magnetic Fields
The Earth’s magnetic field is similar to that of a
bar magnet.
Note that the Earth’s
“North Pole”
is really
a south magnetic
pole, as the north
ends of magnets are
attracted to it.
Force on Electric Charge Moving in a
Magnetic Field
The force on a moving charge is related to
the force on a current:
Force on an Electric Current in a Magnetic
Field
A magnet exerts a force on a current-
carrying wire. The direction of the force is
given by a right-hand rule.
Force on an Electric Current in a Magnetic
Field
The force on the wire depends on the
current, the length of the wire, the magnetic
field, and its orientation.
Force between Two Parallel Wires
The magnetic field produced
at the position of wire 2 due to
the current in wire 1 is:
The force this field exerts on
a length l2
of wire 2 is:
Force between Two Parallel Wires
Parallel currents attract; antiparallel
currents
repel.
Force on Electric Charge Moving in a
Magnetic Field
If a charged particle is
moving perpendicular
to a uniform magnetic
field, its path will be a
circle.
Force on Electric Charge Moving in a
Magnetic Field
mv2/r = qvB
=> r = mv/qB
Force on Electric Charge Moving in a
Magnetic Field
Force on Electric Charge Moving in a
Magnetic Field
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Induced EMF
Almost 200 years ago, Faraday looked for
evidence that a magnetic field would induce
an electric current with this apparatus:
Induced EMF
He found no evidence when the current was
steady, but did see a current induced when the
switch was turned on or off.
Faraday’s Law of Induction; Lenz’s Law
The induced emf
in a wire loop is proportional
to the rate of change of magnetic flux through
the loop.
Magnetic flux:
Unit of magnetic flux: weber, Wb.
1 Wb
= 1
T∙m2
Faraday’s Law of Induction; Lenz’s Law
Magnetic flux will change if the area of the
loop changes:
Faraday’s Law of Induction; Lenz’s Law
Magnetic flux will change if the angle between
the loop and the field changes:
Faraday’s Law of Induction; Lenz’s Law
In terms of internal forces . . .
What will happen to an electric
charge in this conductor?
Faraday’s Law of Induction; Lenz’s Law
In terms of internal forces . . .
The electrons will continue to be pushed
by the magnetic force until they pile up
causing a net electric charge.
Faraday’s Law of Induction; Lenz’s Law
In terms of internal forces . . .
This continues until the electric force caused
by the field induced by the separated
charges exactly balances the magnetic force
Faraday’s Law of Induction; Lenz’s Law
In terms of internal forces . . .
FB
= qvBsinθ
= qvB
= FE
= qE
=> E = ∆V/∆L = vB
Faraday’s Law of Induction; Lenz’s Law
In terms of internal forces . . .
E = ∆V/∆L = vB
=> ∆V = vLB
= ∆AB
Electric Generators
A generator is the opposite of a motor –
it
transforms mechanical energy into electrical
energy. This is an ac generator:
The axle is rotated by an
external force such as
falling water or steam.
The brushes are in
constant electrical
contact with the slip
rings.
Production of Electromagnetic Waves
Since a changing electric field produces a
magnetic field, and a changing magnetic field
produces an electric field, once sinusoidal fields
are created they can propagate on their own.
These propagating fields are called
electromagnetic waves.
Production of Electromagnetic Waves
Oscillating charges will produce
electromagnetic waves:
Production of Electromagnetic Waves
Far from the source, the waves are plane
waves:
Production of Electromagnetic Waves
The strength of the field is found decrease as
1/r, but the energy carried by the wave:
So, the energy decreases as 1/r2
Production of Electromagnetic Waves
The electric and magnetic waves are
perpendicular to each other, and to the
direction of propagation.
Production of Electromagnetic Waves
So Electromagnetic Waves are transverse
waves. Since its fields that are oscillating, not
matter, they can propagate through empty
space.
Production of Electromagnetic Waves
When Maxwell calculated the speed of
propagation of electromagnetic waves, he
found:
This is the speed of light in a vacuum.
Light as an Electromagnetic Wave and the
Electromagnetic Spectrum
Light was known to be a wave; after producing
electromagnetic waves of other frequencies, it
was known to be an electromagnetic wave as
well.
The frequency of an electromagnetic wave is
related to its wavelength: