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Chapter 14: Magnetism. Brent Royuk Phys-110 Concordia University. Magnets. Magnets are caused by moving charges. Permanent Magnets vs. Electromagnets Magnets always have two poles, north and south. Like poles repel, opposites attract. Magnets.

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Chapter 14 magnetism l.jpg

Chapter 14: Magnetism

Brent Royuk


Concordia University

Magnets l.jpg

  • Magnets are caused by moving charges.

    • Permanent Magnets vs. Electromagnets

  • Magnets always have two poles, north and south.

  • Like poles repel, opposites attract.

Magnets3 l.jpg

  • North means north-seeking, so Earth’s north pole is what kind of pole?

  • But any pole attracts metal: Why?

  • Bar magnets are dipoles. Can there be a monopole?

  • History: lodestones and magnetic compasses. Remember Magnesia?

  • Permanent magnets vs. electromagnets: More later

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Earth’s Magnetic Field

  • Probably caused by currents of molten core

  • Drift and reversals

    • Last reversal: 780,000 years ago

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Magnetic Fields

  • The magnetic field B surrounds magnets analogously to the electric field

  • Is there an analog to Coulomb’s Law? No, the B-field is more complicated.

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B-Field Lines

  • Field line mapping: What defines a field line?

  • The direction of the line is always from N to S.

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  • H. C. Oersted, 1820

    • Current-carrying wires exert a force on each other

    • k’ = 1 x 10-7 N/A2

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  • The Long, Straight Wire

    • How long is it?

Another Right-Hand Rule:

The Permeability of Free Space:

o = 4 x 10-7 Tm/A


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Long Straight Wire

  • What direction is the B-field a) above both wires, b) below both wires, and c) between the wires?

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Magnetic Force on a Moving Charge

  • A moving charge moving in a perpendicular direction through a B-field experiences a force perpendicular to its motion

  • Qualitative: FqvB sin 

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Electric Field Units

  • [B] = [F/qv]

  • 1 N/Am  1 tesla (T)

    • Neutron star: 108

    • Big magnet: 1.5

    • Small bar magnet: .01

    • Earth’s magnetic field: 5 x 10-5

    • Interstellar space: 10-10

    • Magnetically shielded room: 10-13

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Magnetic Force on a Moving Charge

  • F=qvB sin 

    • Note that the force is maximum when perpendicular, minimum at parallel. Weird.

  • What is the significance of a field line for a moving charge?

  • Example: An electron moves at right angles to a magnetic field of 0.12 T. What is its speed if the force exerted on it is 8.9 x 10-15 N?

  • Have you ever brought a magnet near a CRT screen?

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Direction of the Magnetic Force

The Right Hand Rule

  • Wrap or Point from v to B

  • In/Out conventions

  • Positive vs. Negative

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Force on a Wire

  • In the picture below, the wire is deflected downward. Which side of the magnet is a north pole?

  • The monstrosity

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Loops & Solenoids

  • Loops concentrate magnetic effects.

  • What is the direction of the B-field in the vicinity of a current-carrying loop?

  • Solenoids contain multiple loops.

    • Solenoids increase magnetic fields.

    • Solenoids increase the force felt by current-carrying wires.

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Application: Loudspeakers

  • A modulated current is sent to a voice coil, which experiences a force from a magnet that is transmitted onto a speaker cone.

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Application: The Galvanometer

  • Torque on a coil of current loops is balanced by a spring.

  • Galvanometers can be configured as voltmeters or ammeters.

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  • Problem: How do you run a motor with DC electricity?

    • The commutator

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Increase the flux with multiple armatures

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Faraday’s Law

  • What does this mean?

  • “What good is a baby?”

  • “One day, sir, you may tax it.”

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Magnetic Flux

  • Water pipe analogy

    • Flow through a butterfly net

  •  = BA

  • Can be visualized as the number of field lines passing through a current loop

    • Orientation matters too.

  • Unit: 1 weber (Wb) = 1 T m2

  • Ways flux can change

    • Relative motion, Changing field strength, Changing orientation, Changing area of loop

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Changing Flux

  • Faraday: 1830, an induced emf is produced by a changing flux in a circuit loop.

  • Demo

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Changing Field Strength

  • When is current induced?

  • Demo

Applications l.jpg

  • Dynamic (Induction) Microphone

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  • Guitar Pickups

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Lenz’s Law

  • Lenz’s Law: An induced current always flows in a direction that opposes the change that caused it.

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Eddy Currents

  • Magnet in tube

  • Monstrosity

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What’s the difference between a motor and a generator?

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Back EMF

  • There is a “braking effect” caused by a generator that is a voltage that resists the changing current, and it’s called Back EMF.

  • When motors are spun by electricity, they generate a back EMF

    • Maximum current occurs during the startup of an electric motor.

    • “Cold-cranking amps.”

  • Generators have a counter torque.

    • Hand-crank generator

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  • Place two solenoids side-by-side.

  • How can a DC voltage in one produce a voltage in the other?

  • How can an AC voltage in one produce a voltage in the other?

Transformers34 l.jpg

  • Get two coils to share the same changing flux and their voltages will differ by the number of turns in the coils.

  • The transformer relations:

Transformers35 l.jpg

  • Step-Up vs. Step-Down

  • Isolation Transformers

  • Suppose that our neon transformer draws 4 A of current. How much current does it supply to the discharge tube?

    • Neon transformers have an inductor in series with the transformer. Why?

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Transformer Energy Loss

  • Losses can come from flux leaks, self-induction, resistive heating.

    • Mechanical losses: Transformer hum

  • Eddy currents can be minimized with laminated cores.

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The Power Grid

  • Edison vs. Westinghouse