PAGE EDITOR(S): Isabella Norcini, Carla Pacini, Daniela Reyes-Capo, Stephanie Rice

Natland Note (04/26/10): There is no "Blv" stuff yet. A sample problem would be nice. Also, some of the pictures are not showing up.

A charged object will attract any/all objects, while a magnet only attracts certain objects/materials
Magnetic fields are created by moving charges
2 Main "Types" of Magnets:
  • Induced/Permanent Magnet:
    • Magnetic fields are created by electrons in motion around atoms and by varying their spin
    • Most substances will not have a net magnetic effect because of the orientation of atoms/ magnetic domains are randomized
  • Electromagnet:
    • The moving charge in a wire can create a magnetic field around it
Magnetic Field (B):
  • units: Tesla (T)
external image flux-images.jpg
Magnetic force experienced by a charged particle moving in a magnetic field:
  • 2 conditions must be met:
    • The charge must be moving to experience a force (A magnetic force will not be applied to a stationary charge in a magnetic field)
    • The velocity of a moving charge must have a component perpendicular to the direction of the magnetic field to experience a force
  • F(sub B) = |q| v x B -> F(sub B) = |q| v (perpendicular to) B = |q| v B sin (-)
    • |q|: charge moving in magnetic field
    • v: velocity
    • B: magnetic field
    • (-): angle between velocity and magnetic field
Magnetic Field Lines:
  • Always point from north to south!
  • If current and magnetic field point in same direction, then no force experienced!

(easy to remember ouT, doT...both end in "T")

Into Page (crosses or X's)

Right Hand Rule (RHR) #1
  • Determines the direction of the magnetic force
  • Refers to the force experienced by a positive charge
  • If you have a negative charge use RHR and reverse the direction of the force experienced (aka the force experienced by the charge will be in the opposite direction)
    • velocity (V)= pointer
    • B-field (B)= middle
    • magnetic force (Fb)=thumb

Figure 1
Figure 1
has a distinct advantage in that the formula F = qvB follows the same exact order as the rule. Thumb = Force, Index Finger = qv, and Middle Finger = B.

Book's Right Hand Rule
In the book's right hand rule, the direction of the thumb represents the direction of the current, the direction of the magnetic field is the direction of the four fingers, and the direction of the magnetic force is the the direction out of the palm of the hand. Refer to Figure 2 for much better clarification.

Figure 2
Figure 2
Right Hand Rule (RHR) #2
  • typically used on a current carrying wire
    • current direction (I)= pointer
    • B-field (B)= middle
    • magnetic force (Fb)= thumb
  • Equation
    • Fb= |q| x V x B x sin (-) x (t/t)
    • Fb= BIL sin(-)

"The Curl" (a variation on RHR)
  • used to determine the direction of a B-field aruond current carrying wire

external image gmr-magwiresmall.jpg
    • current direction (I)=thumb
    • B-field (B)=direction of the curl of your fingers

external image right_hand_grip_rule_1.jpg

Force of One Wire on Another
external image Image119.gif
  • The magnetic forces (Fb) point towards eachother. The reason that the magnetic forces must point towards eachother is because if not there will be a net force, which is not the case.

Motional Induced Voltages and Rail Guns
- consider a conducting rod of length "L" moving with a speed "v" through a uniform B-field "B"
- the more mobile electrons in the conductor will experience a force causing them to move within the conductor of force Fb= |q| x V x B
- net result:
  • electrons will experience a force downwards
  • rod will become polarized
  • electric field will point downwards
  • the polarization of the rod creates an electric field within the rod, and equilibrium will quickly be established
- the two ends of the rod will now have a potential difference (V) between the two ends

  • V= EL= BLv

Magnetic Flux
  • Deals with the magnetic field and the surface through which the B-field lines pass
    • the "number of magnetic field lines" through a given area

  • Equation:
    • displaystyle Phi_m = BA cos theta,
      displaystyle Phi_m = BA cos theta,
    • B = strength of the B-field (tesla)
    • A = area of interest/the loop (meters)
    • theta = the angle btwn the B-field and the area vector
    • units: Wb (Weber)

For a link to seeing differences in magnetic flux click below:

  • Why is flux important?
    • If the loop is a loop of wire, a conductor, a change in flux will induce a current in the wire
    • The current induced in the wire will induce a magnetic field (Bind) to counteract the change in flux
    • The current is induced in the wire because a voltage is induced in the loop acting kind of like a temporary battery during the period of time while the flux is changing.
      • induced voltage = induced emf
Faraday's Law

 |mathcal{E}| =  N left| {{dPhi_B} over dt} right|
|mathcal{E}| = N left| {{dPhi_B} over dt} right|

  • this is equal to the induced voltage as well
  • N = the number of loops of wire
  • dΦB = change in flux
  • dt = the time interval that flux is changing
  • overset{ mathcal{E}}{}
    overset{ mathcal{E}}{}
    = induced voltage in the loop
*nature abhors change

Fun Facts:
  • "Physicists in Japan have discovered that the melting point of water increases slightly in a strong magnetic field."
  • The Earth's magnetic field is similar to that of a bar magnet, with each of the bar's ends at its poles.

    external image images?id=296858