Physics Notes
Magnetism
Concepts:
(A detailed list of terms and
objectives is available.)
- Magnetism
- Magnetism Concept Summary (PowerPoint)
- Magnetic Poles
- Like poles repel, unlike poles attract
- called north & south
- Unlike charges, poles never exist alone (monopoles)
- Broken magnets retain N & S poles
- Magnetic Fields
- Magnetic field lines
- From N to S poles
- Unbroken loops
- Produced by the motion of an electric charge
- Electrons revolve about nucleus and spin on their
axis
- Spin generally predominates
- electron spins align or are opposite in atom
- Spins cancel in most meterials
- Spins do not always cancel in some materials
(iron)
- Spinning electron is a tiny magnet
- Current produces a magnetic field
- Field "circulates" around the wire
- Electromagnets
- Many coils (turns) of wire increase field inside the
coil
- Iron core intensifies magnetic field inside the
coil
- Superconducting Electromagnets
- Iron core can "saturate"
- Superconducting core generates powerful field with
little or no power
- Magnetic domains
- Large clusters of atoms (iron) align
- Domains randomly oriented in unmagnetized iron
- External magnetic field causes:
- Magnetic domains aligned with the field to grow
- Domains rotate to align with the field
- Creating Permanent magnets
- Place Iron or iron alloy in strong magnetic field
- Soft iron is easier to magnetize
- Tapping the iron can help align the domains
- Stroke iron or iron alloy with a magnet
- Destroying a Permanent Magnet
- Magnetic Force on a Particle
- Only on moving charges
- Force perpendicular to charge's velocity
- Force perpendicular to the magnetic field
- F=qvBsinQ
- B measured in Tesla
- Tesla = field that produces a maximum force of 1 N on
a charge of 1 C moving at 1 m/s
- Old unit is Gauss = 10-4 Tesla
- Earth's magnetic force at surface about 1 Gauss =
10^-4 Tesla
- Right hand rule
- Maximum force when v perpendicular to B
- Zero force when v parallel to B
- Magnetic Force on a Wire
- Reversing current reverses force
- Due to forces on moving charges in the wire
- Meters
- Current through loop generates magnetic force
- Ammeter
- measures current
- low resistance
- Galvanometer
- Voltmeter
- Measures potential difference
- needs high resistance
- Motors
- Similar to galvanometer in design
- Permanent magnet turns current-carrying coil
- Coil current direction reversed every half turn
- Earth's Magnetic Field
- Earth's magnetic poles not aligned to geographic poles
- Cause not entirely known
- May be due to circulation of iron core
- Very small velocity would account for the field
- Not stable
- More than 20 reversals in past 5 million years most
recent 700 000 years ago
Links:
last update April 10, 2007 by JL
Stanbrough