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12.1.2 Magnetic materials; permanent and induced magnets

Magnetic materials

Definition

Magnetic material

A magnetic material is a material that is attracted by a magnet and that can itself be magnetised, such as iron, steel, nickel and cobalt.

  1. Only a small number of materials are magnetic. The four you are expected to name are iron, steel, nickel and cobalt.
  2. Steel is an alloy of iron with a small amount of carbon, so it behaves magnetically because of the iron it contains.
  3. Being a metal is not the same as being magnetic. Copper, aluminium, brass, gold and silver are all metals, yet a magnet does not attract any of them.
  4. Non-metals such as wood, plastic, glass, rubber and card are never magnetic.
  5. This difference is put to work in recycling. A rotating magnetic drum lifts steel food tins out of a moving stream of waste while aluminium drinks cans travel past untouched.
  6. A magnetic material can do two separate things: it is attracted by a magnet, and it can itself be magnetised so that it becomes a magnet.
  7. A piece of magnetic material can be magnetised by stroking it repeatedly in one direction with one pole of a permanent magnet, or by placing it inside a coil carrying a current.
  8. A magnet can be demagnetised by heating it strongly, by hammering it, or by placing it inside a coil carrying an alternating current that is then slowly withdrawn.

Permanent and induced magnets

Definition

Permanent magnet

A permanent magnet is a magnet that produces its own magnetic field all of the time and keeps that field when it is taken away from any other magnetic field.

Definition

Induced magnet

An induced magnet is a magnetic material that becomes a magnet only while it is placed inside another magnetic field.

  1. A permanent magnet needs nothing done to it. It has its own north pole and south pole, it produces a magnetic field at all times, and that field is still there when every other magnet is taken away.
  2. An induced magnet borrows its magnetism. A piece of magnetic material becomes a magnet only while it sits inside the field of another magnet, and it loses most or all of that magnetism when it is taken out of the field.
  3. The chain of reasoning for induced magnetism runs in four steps:
    1. The magnetic material is placed inside the magnetic field of a permanent magnet.
    2. The field magnetises the material, so the material becomes an induced magnet with its own poles.
    3. The end of the material nearest the permanent magnet always becomes the opposite pole to the pole it is facing.
    4. Unlike poles attract, so the material is pulled towards the permanent magnet.
  4. Because the nearest end always takes the opposite pole, induction always produces attraction. A permanent magnet cannot repel an unmagnetised piece of iron whichever pole is offered to it.
  5. This is why a chain of paper clips can hang from a single magnet. The first clip becomes an induced magnet, its far end becomes a free pole, and that pole induces magnetism in the next clip down the chain.
  6. Take the permanent magnet away and the chain falls apart, because each clip loses the induced magnetism that was holding the next one on.
  7. How much magnetism is kept afterwards depends on the material. Iron keeps almost none, while steel can keep a great deal and may end up as a weak permanent magnet.
Example

Working out the induced poles on a nail

  • The north pole of a permanent bar magnet is brought close to the head of an unmagnetised iron nail, with the point of the nail furthest away.
  • The nail is iron, which is a magnetic material, so the magnet's field magnetises it and the nail becomes an induced magnet.
  • The nearest end takes the opposite pole, so the head of the nail becomes an induced south pole and the point becomes an induced north pole.
  • A north pole now faces an induced south pole across the gap. These are unlike poles, so the nail is attracted and moves towards the magnet.
  • If the bar magnet is turned round so that its south pole faces the head, the induced poles swap over and the head becomes an induced north pole. The nail is still attracted.

Choosing the right magnetic material

Definition

Temporary magnetic material

A temporary magnetic material is a magnetic material that is easily magnetised but loses almost all of its magnetism as soon as the magnetising field is removed.

  1. Magnetic materials are sorted by how well they hold on to magnetism once the magnetising field has gone.
  2. Permanent magnetic materials are harder to magnetise in the first place but keep their magnetism well. Steel is the standard school example, and alloys containing nickel and cobalt make the strongest permanent magnets used in industry.
  3. Temporary magnetic materials magnetise very easily but lose that magnetism just as easily. Iron is the standard example.
  4. Use a permanent magnetic material whenever the object has to stay magnetic without any power supply:
    1. compass needles, which must go on pointing along the Earth's field for years
    2. fridge magnets and the catches that hold cupboard doors shut
    3. the fixed magnets inside loudspeakers, headphones and electric motors
    4. magnetic tool holders and screwdriver tips that keep hold of steel screws
  5. Use a temporary magnetic material whenever the magnetism has to be switched on and off:
    1. the iron core of a scrapyard crane magnet, so a car body can be lifted and then dropped
    2. the iron core inside an electric bell, which must release the hammer between strikes
    3. the iron core in a relay, so a small current can open and close a separate switch
    4. magnetic door locks that release the moment the current is cut, which is what makes them safe in a fire
  6. Putting a steel core in a crane magnet would be a design fault. The steel would stay magnetised after the current was switched off, so the load would not drop.
Practical

Testing materials for magnetism and induced magnetism

  • Aim: to sort a set of materials into magnetic and non-magnetic, and to compare how much magnetism an iron nail and a steel nail keep after they are removed from a magnetic field.
  • Apparatus: a strong bar magnet, an iron nail, a steel nail, samples of copper, aluminium, brass, wood and plastic, a box of small steel paper clips, a plotting compass, a wooden ruler and a wooden or plastic tray.
  • Variables: the material under test is the independent variable, the number of paper clips picked up is the dependent variable, and the magnet used, the pole used, the contact time and the size of the paper clips are all kept the same.
  • Method, sorting the materials:
    • Lay the samples out on a wooden tray so that they are not touching each other.
    • Bring the north pole of the bar magnet slowly up to each sample and record whether it is attracted, then repeat with the south pole.
    • A magnetic material is attracted by both poles. Nothing in the set should be repelled, because none of the samples starts out as a magnet.
    • Record the results in a table with columns for the material, the result with the north pole, the result with the south pole and the conclusion.
  • Method, comparing iron and steel:
    • Check first that both nails start unmagnetised by holding each one near the plotting compass. An unmagnetised nail attracts the needle whichever end is used, so it must not push the needle away.
    • Hold the iron nail against one pole of the bar magnet and lower its free end into the pile of paper clips. Count how many clips hang from it while it is still touching the magnet.
    • Pull the nail away from the bar magnet without shaking it, and count how many clips are still hanging from it. This second count measures the magnetism the nail has kept.
    • Stroke the nail thirty times in the same direction with one pole of the magnet, lifting the magnet clear at the end of every stroke, then repeat the paper-clip count.
    • Repeat the whole sequence with the steel nail, using the same pole, the same number of strokes and the same pile of clips.
    • Carry out three trials for each nail and calculate a mean number of clips, since the clips tangle and a single count is unreliable.
  • Expected pattern: iron, steel, nickel and cobalt are attracted while copper, aluminium, brass, wood and plastic are not. Both nails hold clips while touching the magnet, but the iron nail drops almost all of them once the magnet is removed, whereas the steel nail keeps a noticeable number and can be shown with the compass to have become a weak permanent magnet.
  • Watch out: nails that were already magnetised from an earlier lesson, stroking back and forth instead of in one direction, dragging the magnet back across the nail on the return stroke, using clips of different sizes between trials, and holding the nail so close to the pile that clips are picked up by the bar magnet itself.
  • Safety: nail points are sharp, so handle them by the head, keep the magnets well away from phones and bank cards, and collect spilt paper clips off the floor straight away.
Exam technique

Naming a material and its property

  • Questions that ask you to choose a material carry two marks: one for the material and one for the property. Naming steel without saying that it keeps its magnetism scores half.
  • Write the property as a full reason, for example that iron suits an electromagnet because it is easily magnetised and easily demagnetised.
  • For an explanation of attraction by induction, give the three linked stages in order: the material becomes an induced magnet, the nearest end becomes the opposite pole, and unlike poles attract.
  • The mark most often dropped is the middle one. Say which pole is induced at which end rather than writing only that the nail becomes magnetic.
  • If a question asks whether an object is a permanent magnet or an induced magnet, look for what happens after the other magnet is removed. Magnetism that disappears means induced.
Common Mistake
  • Do not write that all metals are magnetic. Only iron, steel, nickel and cobalt are magnetic among the metals you meet.
  • Do not say that an induced magnet can be repelled by a permanent magnet. The nearest end always takes the opposite pole, so induction gives attraction every time.
  • Do not confuse a magnetic material with a magnet. An iron nail is a magnetic material at all times, but it is only a magnet while it is magnetised.
  • Do not mix up the two properties of steel and iron. Steel is the harder one to magnetise and the better one at keeping magnetism.
Self review
  • Name four magnetic materials and two metals that are not magnetic.
  • State the difference between a permanent magnet and an induced magnet.
  • Explain why an unmagnetised steel paper clip is attracted to either pole of a bar magnet.
  • Give the material used for the core of a scrapyard crane magnet and the property that makes it suitable.
  • Describe how you would show experimentally that steel keeps more magnetism than iron.
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12.1.2 Magnetic materials; permanent and induced magnets Revision Guide

  1. GCSE
  2. /Physics
  3. /12.1.2 Magnetic materials; permanent and induced magnets

Revision notes for Edexcel GCSE Physics 12.1.2 Magnetic materials; permanent and induced magnets: explanations and worked examples.

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