E & M Magnetism History

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Alban Mason
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1 E & M magnetism

2 E & M Magnetism History Thales studied lodestone ~590 B.C. Socrates played with lodestone ~750 B.C. ~ 1 A.D. Greek Shepherd Magnus found stones. Magnetite / Lodestone Fe 3 O 4 ~1000 A.C. Chinese Compass: Try hanging magnets

3 E & M Magnetism History 2 Peter de Maricourt 1269 Determines Opposite attract / like repel Has lodestone ground to sphere. How do filings line up? Can NOT make monopole William Gilbert 1600: Earth is big magnet, needles pt to poles, not heaven.

4 E & M Magnetism Field Lines: by tradition, extend out from N, into S Field lines NEVER intersect Can never begin or end on any object other than a field maker. Try Fe filings on transparency

5 E & M Magnetism From U of MN Lab

6 E & M Magnetism Earth: The north pole of Earth is really The south pole of Earth is really

7 E&M Magnetism 1735 Extraordinary Effect of Lightning in Communicating Magnetism. Royal Soc. London. Hans Christian Oersted e m discovery 1820 Remember Circuit Current Detector

8 E & M Magnetism Stuff Creates Act On Mass g field Mass Charges E field Charges Moving Charges B field? E field & B field E&M

9 E & M Magnetism Causes Cause of magnetism must be :. Atomic nuclei cause B 1000 < than e - e - move 2 ways.kinda 1-orbit around nucleus 2-spin on axis Electrons account for B Ferromagnetism: more unpaired spin-dipole e-. Dipole groups align into domains.

10 Compare & Contrast E & B Electric Field Sources The symbol for the electric field is E. Units are Newtons/Coulomb or Volts/meter. An E field is generated by creating a potential difference across parallel plates. Magnetic Field Sources The symbol for the magnetic field is B. Units are Newton/(Coulomb m/se c). This set of units is defined to be Tesla. The units differ from the E field by a factor of velocity units. (A hint!) A B field can be generated by passing current through a coil of wire.

11 Compare & Contrast E & B Electric Field Sources Magnetic Field Sources E d E is found by dividing a voltmeter reading by the gap distance. E = V / d Electric fields come from charges at rest. The electric field lines begin on positive charges and end on negative charges. B is determined by multiplying an ammeter reading by the geometrical properties of the coil. B = o N I / L L is length of coil. N is number of turns around coil. o = 4 E - 7 N/A2 Magnetic fields come from electric charges moving through space. This explains why units of m/s appear in the definition of Tesla. Magnetic field lines do not begin or end.

12 Intro Summary 1. Sources of E are charges at rest while sources of B are charges in motion (current). 2. Units are similar with the exception of m/s in the definition of Tesla. 3. Use a voltmeter to determine E; use an ammeter to calculate B. 4. Field lines can stop and start for E but not for B.

13 E & M Magnetism 1735 Royal Society of London: Of an extraordinary effect of lightning in communicating magnetism. Hans Christian Oersted 1820: Okay class, what the

14 Charges in E Charge in a Pure, Uniform Electric Field Consider four, positively charged particles in a pure E field as shown below. q A q C q B q D The above charges have the following initial conditions: A. q A is at rest. B. q B is initially moving to the right. C. q C is initially moving up the page. D. q D is initially moving up and to the right at some angle from the vertical. A. A will accelerate to the right. B. B will accelerate to the right w/ initial velocity C. C will move along a parabolic path. D. D will move along a parabolic path.

15 E & M Magnetism F B = qvxb =qv B F B must have a component to the v-b plane FB = q v x B = qv B where FB is to the v-b plane Magnetic Force charge q moves at v through B : field will exert a force on the charge faster charge through a magnetic field the greater the force.

16 Charges in B Charge in a Pure, Uniform Magnetic Field Consider four, positively charged particles in a pure B field as shown below. qa qc qb qd The above charges have the following initial conditions: 1. qa is at rest. 2. qb is initially moving to the right. 3. qc is initially moving up the page. 4. qd is initially moving up and to the right at some angle from the vertical. A. Nothing. Boring! B. No Change. Boring, part II! C. Charge will move in a circular. Wrap around the magnetic field lines. Use your right hand. D. v has and ll parts motion is combo of (b) and (c).

17 B and B from q External B field +q B field from moving q

18 New B +q Superposition of both magnetic fields

19 A Few Typical B Values B F qv sin Conventional laboratory magnets G or 2.5 T Superconducting magnets G or 30 T Earth s magnetic field 0.5 G or 5 x 10-5 T

20 Finding the Direction of Magnetic Force Experiments show that the direction of the magnetic force is always perpendicular to both v and B F max occurs when vis perpendicular to B F = 0 when vis parallel to B

Magnetic Fields Permanent Magnets

1 Magnetic Fields Permanent Magnets Magnetic fields are continuous loops leaving a North pole and entering a South pole they point in direction that an isolated North would move Highest strength near poles