Download magnetic field

Electricity and Magnetism
Magnets, Magnetism, Magnetic fields
• Two poles - North and South
• Like repels and opposite attracts
• Sounds a lot like
electricity. What’s
different?
Magnetic field similar to E field
Field lines point from N
pole to S pole outside of
the magnet
Where does a magnetic field come from?
• In general moving charged particles create their own
magnetic fields
• Faraday explored this,
as shown in the movie
(based on Oersted’s work)
So current-carrying wire creates a magnetic field. What
about regular magnets?
Magnetic fields in a magnet
• All matter has spinning electrons ( moving charged
particles)
• Electrons revolve around the nucleus and also rotate
(quantum spin number, s)
• The rotation (more so than the revolution) produces
a magnetic field around each electron.
• A lot of the magnetic fields cancel each other out
(opposite directions, same magnitude)
• But in iron, fields do not cancel entirely
• Each atom is a tiny magnet.
Atoms with unpaired electrons are
paramagnetic
Iron, Nickel, Cobalt—Magnetic Metals
Nuclear Magnetic Resonance
Which is basically the same as MRI
What causes the earth’s magnetic field?
• Our theory is: moving charged particles in the liquid
part of the earth’s core create a magnetic field
• Geographic North pole
is actually magnetic South
• What about a flip?
Northern Lights-charged particles from
sun enter Earth’s magnetic field
Typical strength of magnets and magnetic
fields
• Units are TESLA for mks combination of units;
GAUSS for cgs combination
• Basically it’s a Newton/amp-meter (weird!)
Magnetic field strength
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Typical Values
Here is a list of how strong some magnetic fields can be:
In a magnetically shielded room10^-14 Tesla
Interstellar space
10^-10 Tesla
Earth's magnetic field
0.00005 Tesla
Small bar magnet
0.01 Tesla
Within a sunspot
0.15 Tesla
Small NIB magnet
0.2 Tesla
Big electromagnet
1.5 Tesla
Strong lab magnet
10 Tesla
Surface of neutron star
100,000,000 Tesla
Magnetar
100,000,000,000 Tesla
• (At a distance halfway to the moon, a magnetar could strip information
from the magnetic stripes of all credit cards on Earth.)
What determines the strength of a
magnetic field? Well, that depends…
• In a straight current-carrying wire, B = µ0I/2πr where r is
distance from wire
• In a circular loop, at the center, B = µ0IN/2a where a is radius
of loop.
• Inside a long solenoid, B = µ0IN/L where N is the number of
loops per meter
• µ0 is the permittivity of free space, an electrical constant that
conceptually represents how much a field influences particles
• General Law:
And to further complicate the
issue:
• There are two magnetic fields, H and B. In a vacuum they are
indistinguishable, differing only by a multiplicative constant
that depends on the physical units. Inside a material they
are different. The term magnetic field is historically reserved
for H while using other terms for B. Informally, though, and
formally for some recent textbooks mostly in physics, the
term 'magnetic field' is used to describe B as well as or in
place of H. There are many alternative names for both.
And there’s magnetic flux…
• Sort of how much of the field passes through
the surface.
For charged particles, the force “felt”
due to a magnetic field depends
upon:
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Velocity of the charged particles , v
Charge of the charged particles, q
Distance away from the charged particles, r
Force a magnetic field exerts on a charged
particle, F = qvB sinϴ, where B is the
magnetic field strength (force has max value
when v and B are perpendicular).
Getting back to Faraday….
Electricity and magnetism are related in three
major ways:
1.Moving charged particles create magnetic
fields around themselves
2.External magnetic fields exert a force on a
current carrying wire or stream of charged
particles.
3.Moving magnetic fields induce current.
1. A moving charge induces a
magnetic field.
This means a currentcarrying wire produces
a magnetic field.
The compass needle
deflects in directions
tangent to the circle
– The compass needle
points in the direction of
the magnetic field
produced by the current
The Right Hand Rule
Different configurations from in-class
Faraday lab
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Wire/nail….
Pink loop
Big loop
Small loop
Multiple loops
External magnetic fields exert a force on a current carrying
wire (or a stream of charged particles)
The blue x’s indicate the magnetic
field is directed into the page
– The x represents the tail of the arrow
Blue dots would be used to
represent the field directed out of
the page
– The • represents the head of the
arrow
In this case, there is no current, so
there is no force
Second (or third) right hand rule
Demo in class
• Make a circuit of a voltage source, ammeter
and wires
• Hold section of wire vertically
• Voltage source is OFF
• Bring strong magnet close to vertical wire
• Turn voltage source ON
• Observe…….
• Reverse the current and redo…observe…
B is into the page
The current is up the
page
The force is to the
left
B is into the page
The current is down
the page
The force is to the
right
Electro-magnet Superhero!
• Charged-up superhero at rest has an electric
field.
• Charged-up superhero in motion has E field
AND a magnetic field!! (#1)
• Unfortunately, villains can use powerful
magnets to exert force and control the moving
Superhero! (#2)
• Or Evil Genius
What if the superhero was like a beam
of electrons?
• What if it was a stream of moving charged
particles? like a CRT display in an older tv or
computer screen or oscilloscope?
Let’s look at an oscilloscope
• Link to paer.rutgers.edu/pt3
• http://paer.rutgers.edu/pt3/experimentindex.
php?topicid=10&cycleid=46
• Observe how the electron beam is affected by
the magnet and its magnetic field
Let’s look at the inside of a CRT tube
• Link to ‘how stuff works’
• http://electronics.howstuffworks.com/tv3.ht
m
• What would happen if your tv was on and you
touched the screen with a magnet?
So far…
• 1. Current carrying wires have a magnetic field
• Shape of field depends on configuration of
wire
• 2. EXTERNAL magnetic fields exert a force on a
current carrying wires
• CONCLUSION: magnetic fields interact…
– Can use this info for other purposes…
Finally #3) To induce a voltage, current in a
conducting wire….
• There must be relative motion between the
magnet and the coil of wire
• Faster motion….more voltage, current
• More coils, larger cross-sectional area…more
voltage, current
Actually…the real reason is…
• The relative motion causes a……
• CHANGING MAGNETIC FIELD THROUGH THE
AREA OF THE COIL !!
• Do a demo in class…
• That’s the idea behind a generator!!
Back to #2, If the current carrying wire
is forced to move, doesn’t the wire
then have kinetic energy?
• YESSSSS! That kinetic energy can then be
used to do work!!!!
• THAT is the idea behind…………..
• THE ELECTRIC MOTOR!!
Electric Motor
An electric motor
converts electrical
energy to mechanical
energy
– The mechanical energy
is in the form of rotational
kinetic energy
An electric motor
consists of a rigid
current-carrying loop
that rotates when
placed in a magnetic
field
Generator vs motor
• Motor: electrical energy to
rotational kinetic energy
• Generator: rotational kinetic
energy to electrical energy
To wrap up….
• Principles of electromagnetism can be used
to….
– Design devices to do mechanical work using
electricity
– Design devices to generate electricity by doing
mechanical work
– Many other devices …….
Other cool stuff that uses electricity
and magnetic fields
• A relatively weak magnet can be made stronger by
superimposing the magnetic field from a coil of
current carrying wire…
AN ELECTROMAGNET!
http://hila.webcentre.ca/projects/electromagnet/
A solenoid
• Uses the magnetic field inside a coil of current
carrying wire to force a cylinder of metal
inside it to slide along the inside of the coil
• http://www.youtube.com/watch?v=e5434dDB
-7w
• http://www.solenoidcity.com/solenoid/manua
l/construction/construction.htm
• Unlike x-rays and computed tomographic (CT)
scans, which use radiation, MRI uses
powerful magnets and radio waves. The MRI
scanner contains the magnet. The magnetic
field produced by an MRI is about 10
thousand times greater than the earth's.
• The magnetic field forces hydrogen atoms in the
body to line up in a certain way (similar to how the
needle on a compass moves when you hold it near
a magnet). When radio waves are sent toward the
lined-up hydrogen atoms, they bounce back, and a
computer records the signal. Different types of
tissues send back different signals. For example,
healthy tissue sends back a slightly
different signal than
cancerous tissue.
• Single MRI images are called slices. The
images can be stored on a computer or
printed on film.
• MRIs can be done with or without contrast
dye.
• MRI can easily be performed through
clothing. However, because the magnet is
very, very strong, certain types of metal can
cause significant errors, called artifacts, in the
images.
• It can also attract
other metal objects
that aren’t tied down
Why are these machines noisy?
• The noise is due to the rising electrical current
in the wires of the gradient magnets being
opposed by the main magnetic field. The
stronger the main field, the louder the
gradient noise.
Other devices
• Pick up coil in electric guitar
• Speakers,
Headphones
http://en.wikipedia.org/wiki/Headphones
http://www.crutchfield.com/learn/learningcent
er/home/headphones-glossary.html#driver
Magnetic Resonance Imaging
• You or your body part lies in the bore of a magnetic field
• Strength of MRI magnets ~ 0.5 to 2 Tesla up to 60 T or more
for research
• All metal objects must be removed from MRI room or
secured: can be violently attracted to the machine once it’s
turned on
• Some magnetic fields created by winding of current carrying
wire
• Some fields created by permanent magnets
• Some created by superconducting magnets – like first
situation except resistance of wire is minimized by
supercooling the wire
• The magnetic field aligns your hydrogen atoms along the
direction of the magnetic field (hydrogen has a strong
inclination to do this)
• The machine applies a radio frequency pulse specific to
hydrogen to the part of the body of interest
• This causes those hydrogen atoms to move in the opposite
direction; this is the ‘resonance’ part of the system.
• In addition, smaller magnets are used to create changes in the
overall field
• These smaller magnets are turned on and off in a specific
manner
• And as the RF signal is turned on and off, the hydrogen atoms
return to their original motion and the machine is designed to
detect this and uses computer programs to create a digital
image