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 • 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 • Probably caused by currents of molten core • Drift and reversals • Last reversal: 780,000 years ago
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.
S N B-Field Lines • Field line mapping: What defines a field line? • The direction of the line is always from N to S.
Electromagnetism • H. C. Oersted, 1820 • Current-carrying wires exert a force on each other • k’ = 1 x 10-7 N/A2
Electromagnets • The Long, Straight Wire • How long is it? Another Right-Hand Rule: The Permeability of Free Space: o = 4 x 10-7 Tm/A Demo
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 • A moving charge moving in a perpendicular direction through a B-field experiences a force perpendicular to its motion • Qualitative: FqvB sin
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 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 The Right Hand Rule • Wrap or Point from v to B • In/Out conventions • Positive vs. Negative
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 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 • 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 • Torque on a coil of current loops is balanced by a spring. • Galvanometers can be configured as voltmeters or ammeters.
Motors • Problem: How do you run a motor with DC electricity? • The commutator
Motors Increase the flux with multiple armatures
Faraday’s Law • What does this mean? • “What good is a baby?” • “One day, sir, you may tax it.”
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 • Faraday: 1830, an induced emf is produced by a changing flux in a circuit loop. • Demo
Changing Field Strength • When is current induced? • Demo
Applications • Dynamic (Induction) Microphone
Applications • Guitar Pickups
Lenz’s Law • Lenz’s Law: An induced current always flows in a direction that opposes the change that caused it.
Eddy Currents • Magnet in tube • Monstrosity
Generators What’s the difference between a motor and a generator?
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
Transformers • 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?
Transformers • 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:
Transformers • 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 • 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 • Edison vs. Westinghouse