Magnets and their poles
Cut a bar magnet in half and you get two magnets, each with its own north and south pole.
North-seeking and south-seeking
Hang a bar magnet from a thread so it can turn freely, and one end swings round to point roughly north. That end is called the north pole of the magnet, or more exactly the north-seeking pole; the other end is the south-seeking pole. Every magnet has both, whatever its shape: bars, horseshoes, rings, the flat sheets on a fridge door. The poles are where the magnet's pull is strongest, which is why paper clips gather at the ends of a bar magnet and hardly at all in the middle.
Bring two magnets together and the rule is short: like poles repel and unlike poles attract. North pushes north away, south pushes south away, and north and south pull together. Electric charges follow a similar rule, but there is one big difference, which the next section is about.
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You cannot get a pole on its own
Positive and negative charges can be pulled apart: rub a balloon and it carries extra negative charge on its own. Magnetic poles never come apart. Cut a bar magnet in the middle, hoping to separate north from south, and each half becomes a complete magnet with one new pole at its cut face. Keep cutting and the same thing happens, all the way down to a single iron atom, which still has a north and a south. On the Sun, sunspots often occur in pairs with opposite magnetic polarities.
A single isolated pole would be called a magnetic monopole. None has ever been found, and physicists still search for one. Because poles always come in pairs, the modern view is that all magnetism comes from moving electric charge: electrons spinning and circling inside atoms, or current flowing in a wire. Unit 4 builds a magnet out of a current.
Why only some metals stick
A fridge magnet sticks to a steel door but not to an aluminium pan or a copper coin. Only a few materials respond strongly to magnets. They are called ferromagnetic, from the Latin ferrum, iron: iron itself, cobalt, nickel and gadolinium, plus alloys such as steel. Some alloys of rare earth elements, neodymium among them, make the strongest permanent magnets in common use. Other materials do react, but so weakly that only sensitive instruments notice.
Inside a piece of iron are small regions called domains. In each domain the atoms, each a tiny magnet, point the same way, but the domains themselves point every which way, so their effects cancel. Bring a magnet near and the domains that line up with it grow, sometimes to millimetre size, while the others shrink. The iron becomes a magnet with its unlike pole facing yours, so it is pulled in. Heat or hard knocks can scramble the domains again. Above a temperature called the Curie temperature, 770 °C for iron (1043 K), iron loses its ferromagnetism and becomes paramagnetic.
| North pole of a magnet | The north-seeking end of a freely hanging magnet |
| Rule for poles | Like poles repel, unlike poles attract |
| Ferromagnetic | Strongly magnetic: iron, cobalt, nickel, gadolinium |
| Domain | Region where atomic magnets point the same way |
| Curie temperature of iron | 770 °C (1043 K) |
| Magnetic monopole | A single isolated pole; never observed |
- Every magnet has a north-seeking and a south-seeking pole.
- Like poles repel; unlike poles attract.
- Poles cannot be separated: a cut magnet makes two magnets.
- Iron, cobalt, nickel and gadolinium are ferromagnetic; their domains line up near a magnet.
- Above the Curie temperature (770 °C for iron) iron loses its ferromagnetism.
- Two north poles are brought close. They:
- Repel
- Attract
- Do nothing
- Turn into south poles
- A north pole and a south pole are brought close. They:
- Attract
- Repel
- Do nothing
- Swap places
- You cut a bar magnet in half. What do you get?
- Two magnets, each with N and S
- A lone N pole and a lone S pole
- Two pieces of non-magnetic metal
- One magnet and one piece of plain iron
- Which metal is NOT attracted strongly by a magnet?
- Aluminium
- Iron
- Nickel
- Cobalt
- Ferromagnetic takes its name from the Latin word for:
- Iron
- Stone
- North
- Magnet
- The Curie temperature of iron is 1043 K. What is it in °C? (one decimal, use 273.15)
- Why does an unmagnetised iron nail stick to a magnet?
- Its domains line up, making it a magnet with an unlike pole facing yours
- It already has a permanent N pole
- Iron is positively charged
- The magnet heats it
- What has never been observed?
- A magnetic monopole
- A south pole
- A horseshoe magnet
- A magnetic domain
- Heating a magnet strongly will:
- Scramble its domains and weaken it
- Make it stronger
- Swap its poles
- Turn it into copper
- In which year did Gilbert publish De Magnete?
- 1600
- 1492
- 1687
- 1820
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