1. Electricity and Magnetism

2. 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

3. Magnetic field similar to E field
Shows dir & magnitude of
force on a moving charged
particle, or interaction with
another magnetic field

4. 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?

5. 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!!

6. 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

7. 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

8. 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!)

9. 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 Tesla 
Earth's magnetic field Tesla 
Small bar magnet Tesla
Within a sunspot Tesla
Small NIB magnet Tesla
Big electromagnet Tesla 
Strong lab magnet Tesla
Surface of neutron star ,000,000 Tesla
Magstar ,000,000,000 Tesla 

10. 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

11. 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

12. Different configurations from in-class Faraday lab
Wire/nail….
Pink loop
Big loop
Small loop
Multiple loops

13. Do you live near any major current carrying wires?
Any thoughts?
Should you be worried about the magnetic fields that are produced?

14. 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

15. 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…

16. 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

17. 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)

18. 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?

19. Let’s look at an oscilloscope
Link to paer.rutgers.edu/pt3
Observe how the electron beam is affected by the magnet and its magnetic field

20. Let’s look at the inside of a CRT tube
Link to ‘how stuff works’
What would happen if your tv was on and you touched the screen with a magnet?

21. 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…

22. 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

23. 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!!

24. 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!!

25. 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

26. Generator vs motor Motor: electrical energy to
rotational kinetic energy
Generator: rotational kinetic
energy to electrical energy

27. 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 …….

28. 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!

29. 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

30. Magnetic Resonance Imaging
Magnetic resonance imaging (MRI) is a non-invasive way to take pictures of the body.

31. 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.

32. 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.

33. Single MRI images are called slices
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.

34. MRI can easily be performed through clothing
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

35. 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.

36. Other devices Pick up coil in electric guitar Speakers, Headphones

37. 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

38. 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