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Home›Physics

Magnetism

Magnets and their poles, field lines and compasses, Earth's own magnetic field, electromagnets, and the motors and generators built from them. The energy lab draws field lines you can probe with a compass, and a coil whose turns and current you can change.

Progress

About this course

In 1600 the English physician William Gilbert published De Magnete, a book of his own experiments with magnets. He carved a lodestone into a ball, carried small compass needles over its surface, and argued that Earth itself behaves as a great magnet. Earlier writers had suspected some link between Earth and the compass, but Gilbert's book is the first to state it plainly. The idea still explains why a compass works: the magnetic pole near the geographic North Pole is, in magnet terms, a south pole, and that is why a compass's north end points to it.

The five units start with magnets and their poles. Cut a bar magnet in half and you get two magnets, each with its own north and south pole. Magnetic fields and field lines come next, with Earth's field at the surface about 25 to 65 microtesla and the field inside a hospital MRI solenoid tens of thousands of times stronger. Earth's own field has a unit that follows Gilbert. Electromagnets show that a coil with 200 turns in 10 cm carrying 2 A makes about 5 mT inside, about 100 times Earth's. The last unit covers motors and generators: a 5 cm wire carrying 20 A across a 1.5 T field is pushed with a force of 1.5 N, and a loop of that idea is a motor. The energy lab draws field lines you can probe with a compass and Earth as a tilted magnet. You change the turns and current of a 10 cm coil, and reversing the current swaps its poles.

Before you start

The first three units on poles, field lines and Earth's field need nothing beforehand. Electromagnets, motors and generators depend on current, so working through Electricity first will make the second half much easier to follow.

When you finish

Once done, you can predict whether two poles attract or repel, sketch field lines and explain why they never cross, and say why a compass's north end points to what is, in magnet terms, a south pole. You can estimate the field inside a coil, work out the force on a current-carrying wire in a field, and explain how that force runs a motor.

  1. 1Magnets and their polesCut a bar magnet in half and you get two magnets, each with its own north and south pole.
  2. 2Magnetic fields and field linesEarth's field at the surface is about 25 to 65 microtesla; the field inside a hospital MRI solenoid is tens of thousands of times stronger.
  3. 3Earth's magnetic fieldThe magnetic pole near the geographic North Pole is, in magnet terms, a south pole; that is why a compass's north end points to it.
  4. 4ElectromagnetsA coil with 200 turns in 10 cm carrying 2 A makes a field of about 5 mT inside, about 100 times Earth's.
  5. 5Motors and generatorsA wire 5 cm long carrying 20 A across a 1.5 T field is pushed with a force of 1.5 N; spin that idea into a loop and you have a motor.

Related courses

  • Motion
    Position, speed, velocity and acceleration, then falling and flying objects. The motion lab launches a ball on Earth, the Moon or Mars and works out the time, height and range as you watch.
  • Forces
    Pushes, pulls, friction and gravity, and the three laws Newton used to connect force with motion. The motion lab swings a pendulum on Earth, the Moon or Mars and times it against the formula.
  • Energy
    Work, kinetic energy, potential energy, the conservation of energy and power, with the numbers worked out. The energy lab runs a roller coaster with energy bars and a friction slider, and lets you push or lift a load to see the work and power.
  • Waves and sound
    What a wave is, how wavelength, frequency and speed fit together, how sound travels, how waves add up, why strings and pipes ring at set notes, and the electromagnetic spectrum. The wave lab draws ropes, springs, sound in a tube, ripple tanks and standing waves from the same formulas used in the lessons.
Sources for this course (16)
  • OpenStax College Physics 2e (openstax.org/books/college-physics-2e): 22.1 Magnets (all magnets have N and S poles, north-seeking and south-seeking, like poles repel and unlike attract, poles cannot be separated, the magnetic pole near the geographic North Pole is a south magnetic pole, sunspots come in N/S pairs)
  • 22.2 Ferromagnets and electromagnets (iron, cobalt, nickel, gadolinium; neodymium alloys; domains grow to millimetre size; Curie temperature of iron 1043 K (770 °C); Oersted saw a compass needle deflected by a current; electromagnets in scrapyard cranes, particle accelerators and MRI; electric current is the source of all magnetism; no monopoles observed)
  • 22.3 Magnetic fields and field lines (direction = where a compass north end points; four rules: tangent, density = strength, never cross, closed loops from N to S outside)
  • 22.7 Magnetic force on a current-carrying conductor (F = I l B sin θ, 1 T = 1 N/(A·m), example 20.0 A, 5.00 cm, 1.50 T gives 1.50 N; motors)
  • 22.9 Magnetic fields produced by currents (Oersted 1820, right hand rule 2, B = μ0 I ÷ (2π r), μ0 = 4π × 10⁻⁷ T·m/A, Earth's field about 5.0 × 10⁻⁵ T, 25 A at 5.0 cm gives twice that; loop centre B = N μ0 I ÷ 2R; solenoid B = μ0 n I, 2000 loops over 2.00 m with 1600 A gives 2.01 T, MRI; ferromagnetic cores)
  • 23.1 Induced emf and magnetic flux (Faraday's iron-ring apparatus, emf only while the field changes, magnet pushed into a coil, faster motion gives more emf, generators, Φ = B A cos θ)
  • NIST CODATA 2022 vacuum magnetic permeability μ0 = 1.25663706127(20) × 10⁻⁶ N/A² (physics.nist.gov/cgi-bin/cuu/Value?mu0)
  • NOAA NCEI Geomagnetism Frequently Asked Questions (ncei.noaa.gov/products/geomagnetism-frequently-asked-questions): field like a dipole at Earth's centre, over 90% from the outer core, intensity roughly 25,000 to 65,000 nT (0.25 to 0.65 gauss)
  • 1 gauss = 100,000 nT, declination = angle between magnetic north and true north (east positive)
  • inclination, dip poles, dipole axis 9.41° from the rotation axis (WMM2020, 2020.0)
  • north magnetic pole moving about 55 km per year north-northwest (2001 and 2007 surveys)
  • last reversal about 750,000 to 780,000 years ago, reversals spaced 5 thousand to 50 million years, a reversal would take several thousand years, models renewed every five years, blackout zones where H < 2000 nT, compass balance weights differ by hemisphere, true bearing = magnetic bearing + declination
  • NOAA NCEI news "World Magnetic Model 2025 Released" (17 December 2024)
  • NOAA NCEI "December 2025 State of the Geomagnetic Field" (north dip pole 36 km/yr and south dip pole 9 km/yr since 1 January 2025)
  • USGS, "Pre-gilbertian conceptions of terrestrial magnetism" (pubs.usgs.gov/publication/70009843): Gilbert's De Magnete (1600) first suggested that Earth behaves as a great magnet. Royal Institution, Michael Faraday's ring-coil apparatus (rigb.org): made August 1831, success on 29 August 1831, the first electric transformer
  • APS News "This Month in Physics History" (Aug 2001): Faraday's electromagnetic rotation experiments of September 1821, the principle behind the electric motor.

Checked: 2026-10-17