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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

Chapter 14: Magnetism

Brent Royuk


Concordia University

  • Magnets are caused by moving charges.
    • Permanent Magnets vs. Electromagnets
  • Magnets always have two poles, north and south.
  • Like poles repel, opposites attract.
  • 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
earth s magnetic field
Earth’s Magnetic Field
  • Probably caused by currents of molten core
  • Drift and reversals
    • Last reversal: 780,000 years ago
magnetic fields
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.
b field lines



B-Field Lines
  • Field line mapping: What defines a field line?
  • The direction of the line is always from N to S.
  • H. C. Oersted, 1820
    • Current-carrying wires exert a force on each other
    • k’ = 1 x 10-7 N/A2
  • The Long, Straight Wire
    • How long is it?

Another Right-Hand Rule:

The Permeability of Free Space:

o = 4 x 10-7 Tm/A


long straight wire
Long Straight Wire
  • What direction is the B-field a) above both wires, b) below both wires, and c) between the wires?
magnetic force on a moving charge
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 
electric field units
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
magnetic force on a moving charge13
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?
direction of the magnetic force
Direction of the Magnetic Force

The Right Hand Rule

  • Wrap or Point from v to B
  • In/Out conventions
  • Positive vs. Negative
force on a wire
Force on a Wire
  • In the picture below, the wire is deflected downward. Which side of the magnet is a north pole?
  • The monstrosity
loops solenoids
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.
application loudspeakers
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.
application the galvanometer
Application: The Galvanometer
  • Torque on a coil of current loops is balanced by a spring.
  • Galvanometers can be configured as voltmeters or ammeters.
  • Problem: How do you run a motor with DC electricity?
    • The commutator

Increase the flux with multiple armatures

faraday s law
Faraday’s Law
  • What does this mean?
  • “What good is a baby?”
  • “One day, sir, you may tax it.”
magnetic flux
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
changing flux
Changing Flux
  • Faraday: 1830, an induced emf is produced by a changing flux in a circuit loop.
  • Demo
changing field strength
Changing Field Strength
  • When is current induced?
  • Demo
  • Dynamic (Induction) Microphone
  • Guitar Pickups
lenz s law
Lenz’s Law
  • Lenz’s Law: An induced current always flows in a direction that opposes the change that caused it.
eddy currents
Eddy Currents
  • Magnet in tube
  • Monstrosity

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

back emf
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
  • 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?
  • 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:
  • 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?
transformer energy loss
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.
the power grid
The Power Grid
  • Edison vs. Westinghouse