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Magnetism: domains, electromagnets and Earth's field
Ferromagnetism and domains, the Curie temperature, why poles come in pairs, electromagnets, and Earth's moving, reversing field. With an electromagnet investigation using current and turns.
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Every magnet has a north and a south pole; like poles repel and unlike poles attract. Magnetic poles cannot be isolated: cut a magnet and each piece has both poles, all the way down to single atoms.[1]
Iron, cobalt, nickel and gadolinium are ferromagnetic. Inside them are domains, small regions that act like tiny bar magnets. In an unmagnetised piece the domains point in random directions; an external field makes them align and grow, and heating and cooling or tapping can lock the alignment in. Above the Curie temperature (1043 K for iron) a ferromagnet cannot be magnetised at all.[2]
Electric current produces magnetic fields; in fact moving charge is the source of all magnetism. A current-carrying coil is an electromagnet: a temporary magnet whose strength rises with current and number of turns, and which an iron core strengthens greatly.[2]
Earth acts like a giant bar magnet. Its magnetic pole near the geographic North Pole is actually a magnetic south pole: that is why the north-seeking end of a compass points to it. The field is generated mostly in the molten iron–nickel outer core, where churning metal and Earth's rotation drive electric currents: the geodynamo.[1][3]
The field is not fixed. Declination, the angle between magnetic north and true north, varies from place to place, and the north magnetic pole currently moves about 55 km per year. The field has also reversed many times, irregularly; the last reversal was about 750,000–780,000 years ago.[4]
The magnetosphere shields the atmosphere from erosion by the solar wind. Auroras are created by energetic electrons that rain down from it into the upper atmosphere.[3]
Try it: What controls an electromagnet's strength?
You need
- An iron rod or large nail as core
- Insulated copper wire (about 3 m)
- A low-voltage DC supply (max 6 V) or battery pack
- An ammeter and a variable resistor (rheostat)
- A pile of identical small paper clips (or a small steel mass and a newton meter)
Safety
Use low voltage only (max 6 V). Coils and resistors get hot: switch off between readings and never leave the circuit on. Do not short-circuit the supply.
What to do
- Wind 20 turns on the core. Connect the coil, ammeter and rheostat in series with the supply.
- Set the current to 0.5 A. Count how many paper clips the core lifts. Switch off between readings.
- Repeat at 1.0 A, 1.5 A and 2.0 A with 20 turns.
- Now keep the current at 1.0 A and test 10, 20, 30 and 40 turns.
- Plot clips lifted against current, and against number of turns. Describe the trends.
- Remove the core and repeat one reading. Compare.
What happens, and why?
The field of a coil increases with both the current and the number of turns, so more clips are lifted. The iron core is magnetised by the coil (its domains align with the field) and adds a much stronger field of its own, which is why removing it weakens the magnet so much. At high currents the core starts to saturate (nearly all domains are aligned), so the graph may level off.[2]
Play: True or false?
Is the statement true or false? Use what you know about domains, poles and Earth's field.
Your best score is kept only on this device. Nothing is sent anywhere.
Quiz
Other ages: Ages 6–7 · Ages 10–11