Overview

History
 Magnesia in Asia Minor (~Turkey today)
 Certain rock types (lodestones) attracted each other and were called “magnets”
 The properties of these rocks have been known for over 2000 years
 For a long time they were thought to be magical!

Magnetic Material
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Only a few elements on the periodic table actually have any magnetic properties strong enough to be worth mentioning.
These elements are known as a group as ferromagnetic elements. The name comes from the Latin name for iron... ferrum. The main ferromagnetic elements are:
1. Iron
2. Cobalt
3. Nickel

Magnets
 Similar to electrostatic charges with poles (N and S)
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N-seeking end points to N pole and S-seeking end points to S pole
 Unlike e/static charges that can exist independently, poles always exist as pairs, called dipoles .
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Ex. If a bar magnet is cut in half, there will be 2 smaller N-S magnets

Magnetic Fields
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Magnetic fields surround magnets as electrical fields surround e-static charges
Magnetic field lines point away from N and toward S
Imagine a compass being placed in the field
– the compass would point in the direction of the field lines.

Magnetic Fields
 Big Rule
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Like poles repel, unlike poles attract
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Thus, N repels N, S repels S, N attracts S

Magnetic Fields - Visually
 If iron filings were sprinkled over a bar magnet, they would arrange themselves as shown. Each thin iron filing becomes a small compass needle or magnet.

Magnetic Fields
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Symbol for magnetic field strength
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B
Units
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Tesla (T) - vector
Examples
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Earth’s = 5 x 10 -5 T
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Small Fridge Magnet = 0.01 T
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Magnet in school lab = 2-10 T

Domain Theory
 Microscopic examination reveals that a magnet is actually made up of tiny regions known as domains .
 Domains are at most about 1 mm in length or width.
 Each domain behaves like a miniature magnet with its own north and a south pole.
 The domains are created by the motion of electrons in the metal

Magnetic Domains
 In an unmagnetized ferromagnetic object (like a bar of iron) these domains are arranged randomly so that their magnetic effects cancel each other out. This means it is not a magnet.
 In a magnet , the domains are basically lined up in one direction so that they create an overall uniform magnetic field.

Force from charge moving in a magnetic field
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The force experienced by a charge (q) moving in a magnetic field (B) with a speed (v) at an angle ( θ) to the field lines is:
F = qvB (sin θ),
 (Note: F is max when angle is perpendicular)
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Ex. TV tube

Practice
 A proton is speeding @ 3 x 10 7 m/s and experiences a magnetic field of 4 T. What is the magnetic force pulling on the proton?
 Solve F = qvB for F
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F = 1.6 x 10 -19 x 3 x 10 7 x 4
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F = 1.9 x 10 -11 N

 Consider a straight wire, length L, carrying current (I) passing through a magnetic field
(B), at right angles, then
 Force (F) = B x I x L

 A 0.1 m (L) wire carries a current (I) of
5A at right angles to a uniform magnetic field. If the force (F) experienced by the wire is 0.2 N, what is the magnitude of the magnetic field (T)?
 Solve F = B*I*L for B
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B = F/IL
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B = 0.2/(5*.1) = 0.4
T

Right/Left Hand Rules

Magnetic Field Strength
 Magnetic flux
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The number of magnetic field lines passing through an area – think “density” of magnetic field.
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Symbol for flux
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(phi)
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Formula for total flux
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= area x magnetic field strength
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= A x B
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Units
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Weber

Flux - magnitude is larger when more lines of magnetic field pass through a unit area – see examples below:

Practice - flux
 Billy is pedaling his bike down a street that is perpendicular to the earth’s magnetic field of 5 x 10 -5 T. What is the flux through the metal rim of his bike wheel, if the wheel has an area of 1.13m
2 ?
 Solve = AB for
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= 1.13 x 5 x 10 -5 = 6.2 x 10 -5 Wb

 If a conductor moves through a magnetic field, or the magnetic field moves past a conductor, a current flow is induced in the conductor.
 It is the relative motion of wire and magnetic field that produces the current.
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Faraday’s Law
Induced Voltage = (number of turns)(change in flux)/elapsed time)
V = N Δ /Δt

Practice – induced voltage
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If the bicycle takes 2 seconds to make a 90 turn into a northbound street, what is the induced voltage in 1 metal rim of the bicycle?
Solve V =
N Δ /Δt for V
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V = 1 x 6.2 x 10 -5 /2
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V = 3.1 x 10 -5 V

 Transformers rely on the property of mutual induction , whereby:
• the change in current in one coil induces a potential difference in another coil.
 Using the concept of mutual induction , a transformer converts a current of one voltage to a current of another voltage

 The coil with the applied current is called the primary coil, and the coil with the induced voltage is called the secondary coil.
Secondary
Primary

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An AC voltage in a primary winding (Vp) induces an AC voltage in the secondary winding (Vs), according to the ratio of the # of turns of wire
(Np/Ns), per
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Vp/Vs = Np/Ns (for constant power)
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Vp/Vs = Is/Ip (from a current view)
 2 types of transformer
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Step-up – Vp < Vs
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Step-down – Vp > Vs

Practice - transformer
 If the primary voltage in a transformer is
6000V and is stepped down to 240V for households using a secondary coil of
100 turns, how many turns are there in the primary coil?
 Solve Vp/Vs = Np/Ns for Np
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Np = VpNs/Vs = 6000 x 100/240
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Np = 2500 turns

Summary
 Big rule – like poles repel, unlike poles…
 F = qvB (charge moving in mag field)
 F = BIL (current in wire in mag field)
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= BA (flux density of mag field)
V = N / t (Faraday’s law)
 Vp/Vs = Np/Ns = Is/Ip (transformers)