5-Magnetism

Slideshow: Magnetism

Textbook: Chapters 17 & 18

Right Hand Rule Practice: http://www.cstephenmurray.com

Objectives:

• Magnet structure
• explain magnetic domains in ferromagnets (permanent) compared to paramagnetic materials (weak & temporary) in terms of number of domains aligned and amount of alignment.
• dipole is backwards from field line direction.
• why no monopoles?
• Magnets in fields
• explain dipole alignment of a magnet in a field (example: compasses & earth as a dipole)
• Explain metal filings & paper clips as induced dipoles
• magnetic force on a charged particles/objects
• Use the right hand rule and F = qvB to solve and explain problems
• including 0º, 90º and 180º degree angles of v to field, but discuss effect at other angles
• magnetic force created by a current carrying wire
• Use grabbing right hand rule to find B fields tangential to concentric circles at specific points
• compare strength of field at distances from the wire
• Use B=(u/2pi)(I/r)
• Electromagnetic Induction
• Magnetic flux is kind of like magnetic field density (#lines passing through and area), but is mathematically B(cosθ)A
• Changing flux induces an electric field
• if there are charges (like in a wire), the E field created by the changing flux cancause a current. according to ɛ= –ΔB/t, where the negative sign indicates the EMF opposes the change in flux.
• conservation of energy applies to magnetic forces
• summary: 4 ways to cause a magnetic force
• 2 magnets
• 1 magnet with charged particle(s) moving nearby
• 1 magnet moving near charged particle(s)
• multiple charged moving objects (ex. 2 wires carrying current, straight or in loops)

Common Misconceptions:

Fields

• Misconception: North and south magnetic poles are the same as positive and negative charges.
• Correct Principle: Magnetism and electric charge interactions are separate (although they can influence each other). Magnets are caused by a magnetic dipole based on spin , which will always have a N and S side, where electric charge interactions are based on the intrinsic properties of a particle. You can have single charges isolated from their opposite, but not single magnetic poles.
• Misconception: Poles can be isolated.
• Misconception: Flux is the same as field lines.
• Misconception: Flux is actually the flow of the magnetic field.
• Misconception: Magnetic fields are the same as electric fields.
• Misconception: Charges at rest can experience magnetic forces.
• Misconception: Magnetic fields from magnets are not caused by moving charges.
• Misconception: Magnetic fields are not 3-dimensional.
• Misconception: Magnetic field lines hold you on the Earth.
• Misconception: Charges, when released, will move toward the poles of a magnet.
• Misconception: Magnetic field lines start at one pole and end at the other.

Induction

• Misconception: Generating electricity requires no work.
• Misconception: When generating electricity only the magnet can move.
• Misconception: Voltage can only be induced in a closed circuit.
• Misconception: Magnetic flux, rather than change of magnetic flux, causes an induced emf.
• Misconception: All electric fields must start on (+) and end on (-) charges.
• Misconception: Water in dams causes electricity.