Magnetism

1
Magnetism

• Force of Mystery
• demo

2
Magnetism Standards

• Students know magnetic materials and electric
  currents (moving electric charges) are sources of
  magnetic fields and are subject to forces arising
  from the magnetic fields of other sources. (Ch
  36)
• Students know how to determine the direction of a
  magnetic field produced by a current flowing in a
  straight wire or in a coil. (Ch 36)
• Students know changing magnetic fields produce
  electric fields, thereby inducing currents in
  nearby conductors. (Ch 37)

3
Famous 19th Century Quote

• The nation that controls magnetism controls the
  Earth

4
Magnetic Poles

• North and South
• Like poles repel N-N S-S
• Unlike poles attract N-S

5
Magnetic Poles Are Not Charges

• Single poles cannot be isolated
• Magnetic Monopoles do not exist in nature
• Break a magnet
• Get two smaller ones

N
S
S N
N
S
6
Ferromagnetic Materials

• Show strong magnetic effects
• Iron
• Cobalt
• Nickel
• Gadolinium
• Neodymium

7
Permanent Magnets
Hi tech Neodymium iron boron magnets
8
Magnetic Field

• Earth has field
• Lines go from North to South

9
Units of Magnetic Field B

• Tesla (SI Unit)
• Gauss (cgs unit)
• 1 Tesla 104 Gauss
• Earth magnetic field about 0.5 gauss

10
Direction of Magnetic Field
The direction the north pole of a compass would
point when placed at that location
11
Ferromagnetism

• Magnet made of domains
• 1 mm length
• Each acts like tiny magnet
• Normally domain cancel
• External field aligns domains
• Strong magnet can make other ferromagnetic
  materials into permanent magnets

12
Electrons Have Spin

• Even permanent magnets owe strength to currents
• No way to divide a current and get N or S pole
• Magnetism is electrical in origin

13
Earths Magnetic Field

• Very weak
• Like bar magnet
• North magnetic pole
• South magnetic pole

14
Electric Currents Produce Magnetism

• Magnetic field around long straight wire

Right hand rule determines direction of magnetic
field
I
15
Right Hand Rule(s)

• Long Straight Wire (Rule 1)
• Point thumb in direction of current
• Fingers wrapped around wire point in direction of
  magnetic field
• Circular loop of Wire (Rule 2)
• Curl fingers around wire with tips in field
  direction
• Thumb points in direction of current

16
Alternate (preferred) version of Second RHR

• Put curled fingers in current direction around
  loop or loops thumb points in field direction
  INSIDE loop or coil.

17
Force on Current Carrying Wire

• F BIL sinQ
• is angle between
• field and wire

I
q
Force is perpendicular to both current and field
direction
18
Third Right Hand Rule

• Long straight fingers in (positive) current
  direction (or direction of moving charged
  particle). Curled fingers in magnetic field
  direction, thumb points in direction of force on
  current carrying wire or positive charged
  particle
• If particle is negative, change answer

19
Force on Moving Charged Particle in Uniform
Magnetic Field

• F Bqvsinq
• This force is perpendicular to the magnetic field
  and particle velocity vector

20
Charge Particle Path in Uniform Magnetic Field

• Circle or helix
• F ma
• qvB mv2/r (centripetal acceleration)
• r mv/qB
• Direction follows right hand rule

21
How can F BIL sinQ be Used to measure a Field?

• Hint use a rectangular loop of wire

22
Force on a Charged particle in a Magnetic Field

• Demo
• F qvB sinQ
• Force perpendicular to both particle direction
  and field

23
Magnetic Field Due to Straight Wire

• B m0I/2pr
• F BIL
• m0 permeability of free space
• 4 p x 10-7

I
24
Force Between Parallel Wires

• F/l (mo/2p) I1I2/L
• Force per unit length of wire
• L is distance between wires
• Parallel currents attract
• Antiparallel currents repel

25
Electrical vs. Magnetic Forces

• Similarities
• Both involve attracting and repelling
• Both decrease with distance
• Differences
• Isolated poles do not exist
• Only electrical forces can be produced by
  stationary charges
• Only moving charged particles experience magnetic
  force
• Only electrical forces can do work
• Magnetic forces on charged particles are
  perpendicular to field direction but electrical
  forces are in or opposite to electric field
  direction