Physics · Ch 3 — Magnetism and Magnetic Effects of Electric Current
SUMMARY
SUMMARY
- A vertical plane through the geographic axis is the geographic meridian; through the magnetic axis, the magnetic meridian. Declination is the angle between them; dip (inclination) is the angle the Earth's total field makes with the horizontal in the magnetic meridian.
- Magnetic dipole moment p_m = pole strength times magnetic length (vector, S to N pole, unit A m^2). Magnetic field B is the force on a unit north pole (unit N per A per m). Magnetic flux is the number of field lines through an area (scalar, SI unit weber).
- Coulomb's law of magnetism: force between two poles proportional to q_mA q_mB / r^2. A dipole in a uniform field feels a torque tau = p_m x B and stores potential energy U = -p_m . B.
- Tangent galvanometer: a moving-magnet instrument based on the tangent law, B = B_H tan(theta).
- Magnetising field H (unit A per m) magnetises a sample. Permeability measures how well field lines pass through a material. Intensity of magnetisation is net moment per unit volume. Susceptibility chi_m = M/H. Materials are diamagnetic, paramagnetic, or ferromagnetic. Hysteresis is B lagging behind the cyclic variation of H.
- Right hand thumb rule: thumb along current, curled fingers give the field. Maxwell's cork screw rule: screw advances along the current, its turning sense gives the field.
- Ampere's circuital law: closed line integral of B.dl = mu0 times I_enclosed. Field inside a solenoid or a toroid is B = mu0 n I (n = turns per unit length).
- Lorentz force: F = qE + q(v x B). A charged particle in a uniform field moves in a circle.
- Fleming's Left Hand Rule: forefinger = field, middle finger = current, thumb = force.
- Definition of the ampere: the current that makes two infinite parallel wires, 1 m apart in vacuum, pull on each other with 2x10^-7 N per metre of length. …