Teacher sheet · Ages 15–16 · about 45–50 minutes
Magnetism: domains, electromagnets and Earth's field
Lesson page: echoesof.science/school/magnetism/ages-15-16/ · Pupil worksheet: echoesof.science/school/magnetism/ages-15-16/print/
Learning aims
- Explain ferromagnetism with domains and the Curie temperature.
- Know that magnetic poles come in pairs and that current produces magnetic fields.
- Investigate the factors affecting an electromagnet's strength.
- Describe Earth's field: geodynamo, declination, pole movement, reversals and the magnetosphere.
Lesson plan
| Time | Activity |
|---|---|
| 5 min | Starter: stroke a nail with a magnet, then drop it on the floor several times. Test it each time. |
| 10 min | Read the text; discuss domains. |
| 25 min | Electromagnet investigation in pairs. |
| 5 min | Share graphs; discuss saturation. |
| 5 min | Quiz, or True or false? as homework. |
Materials for the experiment
- 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.
Common misconceptions
| Pupils may think… | Actually… |
|---|---|
| Geographic North is a magnetic north pole. | It is a magnetic south pole: it attracts the compass's north end. |
| Earth's field is caused by a giant permanent magnet inside. | The core is far above the Curie temperature; the field is produced by currents in the molten outer core. |
| More current always gives a proportionally stronger electromagnet. | With a core, saturation makes the increase level off. |
Worksheet answers
- Domains (tiny magnetic regions) start out random; an external field aligns them so their fields add up. Heating gives the atoms energy to randomise the domains again; above the Curie temperature they cannot stay aligned.
- More current and more turns both lifted more clips; at high current the increase may slow as the core saturates.
- Because unlike poles attract: the north-seeking end of a compass is pulled towards it, so it must be a magnetic south pole.
- The magnetic pole moves (about 55 km per year at the moment), so declination at a place changes over time.
Quiz answers
- Above the Curie temperature, iron… cannot be magnetised. Heat randomises the domains; for iron the Curie temperature is 1043 K.[2]
- A compass's north-seeking pole points to Earth's geographic North. Magnetically, that pole of the Earth is… a south pole. Unlike poles attract.[1]
- What is the source of all magnetism, according to the textbook? Moving electric charge (current). Even permanent magnets come from the motion of charges in atoms.[2]
- What is magnetic declination? The angle between magnetic north and true north. It differs from place to place.[4]
- When did Earth's magnetic field last reverse? About 750,000–780,000 years ago. Reversals happen irregularly.[4]
- Why does an iron core make an electromagnet much stronger? Its domains align with the coil's field and add their own field. The core becomes a magnet itself.[2]
Going further
Use a plotting compass to draw the field around one bar magnet, then around two magnets with like poles facing and with unlike poles facing. Where is there a neutral point?
Sources used in this lesson
- [1] College Physics 2e, 22.1 Magnets (OpenStax, Rice University): reliability High
- [2] College Physics 2e, 22.2 Ferromagnets and Electromagnets (OpenStax, Rice University): reliability High
- [3] Earth's Magnetosphere: Protecting Our Planet from Harmful Space Energy (NASA Science): reliability High
- [4] Geomagnetism Frequently Asked Questions (NOAA National Centers for Environmental Information): reliability High
Full list with links and a note on each source's reliability: echoesof.science/school/magnetism/sources/
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