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. 2 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 3 What’s Wrong With This Picture? 4 Earth’s Magnetic Field • Probably caused by currents of molten core • Drift and reversals – Last reversal: 780,000 years ago 5 Magnetic Fields • The magnetic field B surrounds magnets analogously to the electric field • Is there an analog to Coulomb’s Law? No, the Bfield is more complicated. 6 B-Field Lines • Field line mapping: What defines a field line? • The direction of the line is always from N to S. N S 7 Electromagnetism • H. C. Oersted, 1820 – Current-carrying wires exert a force on each other F 2 kI1I 2  l r – k’ = 1 x 10-7 N/A2  QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. 8 Electromagnets • The Long, Straight Wire – How long is it?  oI B 2 r The Permeability of Free Space: Another RightHand Rule: o = 4 x 10-7 Tm/A Demo 9 Long Straight Wire • What direction is the B-field a) above both wires, b) below both wires, and c) between the wires? 10 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  11 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 12 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? 13 Direction of the Magnetic Force The Right Hand Rule – – – Wrap or Point from v to B In/Out conventions Positive vs. Negative 14 What’s Wrong With This Picture? 15 Force on a Wire • In the picture below, the wire is deflected downward. Which side of the magnet is a north pole? • The monstrosity 16 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 currentcarrying wires. 17 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. 18 Application: The Galvanometer • Torque on a coil of current loops is balanced by a spring. • Galvanometers can be configured as voltmeters or ammeters. 19 Motors • Problem: How do you run a motor with DC electricity? – The commutator 20 Motors Increase the flux with multiple armatures 21 Faraday’s Law • What does this mean? E  t • “What good is a baby?” • “One day, sir, you may tax it.”  24 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 25 Changing Flux • Faraday: 1830, an induced emf is produced by a changing flux in a circuit loop. • Demo 26 Changing Field Strength • • When is current induced? Demo 27 Applications • Dynamic (Induction) Microphone 28 Applications • Guitar Pickups 29 Lenz’s Law • Lenz’s Law: An induced current always flows in a direction that opposes the change that caused it. 30 Eddy Currents • • Magnet in tube Monstrosity 31 Eddy Currents 32 Generators What’s the difference between a motor and a generator? 33 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 34 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? 35 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: N1 V1 I2   N 2 V2 I1  36 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? 37 Transformer Energy Loss • Losses can come from flux leaks, selfinduction, resistive heating. – • Mechanical losses: Transformer hum Eddy currents can be minimized with laminated cores. 38 The Power Grid • Edison vs. Westinghouse 39 The Power Grid QuickTi me™ and a TIFF (Uncompressed) decompressor are needed to see this picture. 40 The Power Grid 41