Permanent & Induced Magnets
The Repulsion Test: To determine if an unknown object is a permanent magnet, it must be tested against a known magnet. If the object is repelled by the known magnet, it must be a permanent magnet itself, as only two like poles can produce a repulsive force.
The Attraction Limitation: Attraction is not a definitive test for a permanent magnet because it occurs in two scenarios: between opposite poles of two permanent magnets, and between a permanent magnet and an unmagnetized magnetic material.
Step-by-Step Identification: First, bring the North pole of a known magnet to the object; if it attracts, flip the known magnet to the South pole. If it still attracts, the object is an induced magnet (magnetic material); if it now repels, the object is a permanent magnet.
| Feature | Magnetically Soft (e.g., Iron) | Magnetically Hard (e.g., Steel) |
|---|---|---|
| Magnetization | Easy to magnetize | Difficult to magnetize |
| Retention | Loses magnetism quickly | Retains magnetism permanently |
| Primary Use | Temporary magnets (Electromagnets) | Permanent magnets (Loudspeakers) |
Soft Materials: These are ideal for applications where the magnetic field needs to be turned on and off, such as in the core of a transformer or an electromagnet. Their ability to lose induction rapidly prevents residual magnetic interference.
Hard Materials: These are used when a constant field is required without an external power source. Once the internal domains are aligned through a strong external field, they remain locked in place, resisting demagnetization.
Identify the Material First: Always check if the material mentioned is magnetic (Iron, Steel, Nickel, Cobalt) or non-magnetic (Aluminum, Copper, Plastic). Non-magnetic materials will experience zero force in a static magnetic field.
Look for Repulsion: In multiple-choice questions asking how to prove an object is a magnet, 'repulsion' is almost always the key word. Never choose 'attraction' as a definitive proof of being a permanent magnet.
Induced Polarity Rule: Remember that the end of the material closest to the magnet will always have the opposite pole. If a North pole is held near an iron nail, the head of the nail becomes a South pole.
The 'All Metals are Magnetic' Myth: A very common error is assuming all metals like aluminum or copper are magnetic. In reality, most metals are non-magnetic; only specific ferromagnetic materials respond to magnets.
Confusing Induced with Permanent: Students often forget that induced magnetism is temporary. If a diagram shows a magnet being moved away, the induced magnet should be described as losing its magnetic properties.
Force Direction: Students sometimes think a magnet can repel an unmagnetized piece of iron. This is impossible; induction always creates an opposite pole at the nearest point, leading strictly to attraction.