AP Physics 2 - Magnetism

Magnetism

• The ability of a substance to attract or repel and create a magnetic field

History

• Greeks

  • 80 B.C.E. in Magnesia

  • Discovered that certain stones (magnetite) attract pieces of iron

• Chinese

  • 200 B.C.E

  • Were using certain stones to indicate direction

    • Natural magnets (Lodestones) will align themselves with the magnetic field of the Earth

• 18th Century

  • Charles Coulomb studied the forces between lodestones

  • 1819 Hans Christian Oersted

    • Found relationship between Magnetism and Electricity

  • 1831 Michael faraday

    • Found current could be induced by magnetic fields

What Makes a Material Magnetic

• The Domain Theory of Magnetism

  • magnetism is a property of a charge (electron) in motion

  • An electron revolving about the nucleus of an atom imparts a magnetic property to the atom structure

  • An electron spinning on its axis acts as a tiny bar magnet

• Domains

  • microscopic magnetic regions atoms are grouped into

• If atoms line up, the material is magnetic

Types of Materials

• Ferromagnetic

  • Favorably attracted

  • Iron, Nickel, Cobalt

• Paramagnetic

  • Slightly attracted

  • Aluminum, Platinum

• Diamagnetic

  • Slightly repelled

  • Zinc, Gold, Mercury

Magnetic Field Near a Coil

• The strength of the magnetic field around a current carrying wire can be enhanced by forming a loop

• One side of the loop resembles a north pole and the other resembles a south pole

  • Resembles the field around a bar magnet

• Several closely spaced loops is called a coil or solenoid

  • Often called an electromagnet because it only acts as a magnet when the current is flowing

  • Proportional to the current

  • Proportional to the number of loops

• B = μ₀ * n * I

  • n = N / L

    • N = number of loops

    • L = length of coil

Electromagnet

• Direction is found using the coil right hand rule

  • grasp the wire with your right hand

  • curl your fingers in the direction of conventional current

  • you thumb points in the direction of the magnetic north

Magnetic Force

• Magnetic fields have the ability to exert a force

• Moving charges create magnetic fields of their own

• Moving charge will experience a force when in a magnetic field

  • Max force is when field (B) and velocity (V) are 90°

    • Min when 0°

  • Direction is determined using right hand rule

  • F = q * vel * B * sin(θ)

Magnetic Torque

• Torque is a force that creates rotation

• Applications:

  • Galvanometer

    • Takes advantage of the torque on a current loop to measure current

  • Motors

    • An electric motor also takes advantage of the torque on a current loop, to change electrical energy to mechanical energy

  • Loudspeakers

Induced EMF

• Almost 200 years ago, Faraday looked for evidence that a magnetic field would induce an electric current

  • He found no evidence when the current through the left-hand loop was steady, but did see a current induced in the right-hand loop when the switch was turned on or off

• We observe that, when a magnet is moved near a conducting loop, a current is induced

  • When the motion stops, the current stops

• Faraday’s Law / Lenz’s Law

  • We conclude that current is induced only when the magnetic field through the loop changes

  • An induced emf is produced in a loop or complete circuit whenever the number of magnetic field lines passing through the plane of the loop or circuit changes

• Faraday’s Law

  • Φ = B A cos θ

    • Measured in webers (Wb)

    • Angle is between field and normal

  • EMF = -N (ΔΦ / Δt)

• Lenz’s Law

  • An induced emf in a wire loop or coil has a direction such that the current it creates produces its own magnetic field that opposes the change in magnetic flux through that loop or coil

    • if the magnetic field is increasing, the induced current will produce a field in the opposite direction, tending to decrease the field