Download Electricity and Magnetism

Electricity and Magnetism
Magnets, Magnetism, Magnetic
fields
two poles - north and south
Like repels and unlike attracts
Many similarities
to electric charges
Difference is that
electric charges
can be isolated but
magnetic poles can’t
Magnetic field similar to E field
Shows dir & magnitude of
force on a moving charged
particle, or interaction with
another magnetic field
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)
Ok that’s current carrying wire but what about
regular magnets, i.e. not carrying current?
Magnetic fields in a magnet
All matter has spinning electrons (aha moving
charged particles!)
Electrons revolve around the nucleus and also
rotate!
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!!
What causes the earth’s magnetic
field?
Our theory is: moving charged particles in the
liquid part of the earth’s core
Geographic north pole
is actually magnetic south
Your compass points to
Geo. North and is
attracted to magnetic
south
What determines the strength of a
magnetic field?
Velocity of the charged particles , v
– Similar to current
Charge of the charged particles, q
Distance away from the
charged particles, r
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
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
Magstar
100,000,000,000 Tesla
Getting back to Faraday….
Electricity and magnetism are related in
three major ways:
1. moving charged particles create
magnetic fields around themselves
– Faraday and friends playing with magnets and
current carrying wire, your in-class activity
Magnetic Fields –
Long Straight Wire
A current-carrying 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
Different configurations from inclass Faraday lab
Wire/nail….
Pink loop
Big loop
Small loop
Multiple loops
Do you live near any major current
carrying wires?
Any thoughts?..........
Should you be worried about the magnetic
fields that are produced?
Other stuff Faraday figured out..
2. External magnetic fields exert a force
on a current carrying wire or 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
Demo in class
Make a circuit of 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)
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/experimentinde
x.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=e5434d
DB-7w
http://www.solenoidcity.com/solenoid/man
ual/construction/construction.htm
Magnetic Resonance Imaging
Magnetic resonance imaging (MRI) is a
non-invasive way to take pictures of the
body.
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/learningcen
ter/home/headphonesglossary.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