Permanent magnets
Definition
Permanent magnet
A magnet that produces its own magnetic field at all times and stays magnetised without another magnet nearby.
Definition
Magnetic material
A material, such as iron, steel, nickel or cobalt, that is attracted to a magnet and can be magnetised.
- A permanent magnet produces its own magnetic field all the time, without needing anything else nearby.
- Bar magnets, horseshoe magnets and fridge magnets are all permanent magnets.
- A permanent magnet has its own north pole and south pole, so it can either attract or repel another permanent magnet, depending on which poles face each other.
- Only a few materials are magnetic materials, and the ones you need to know are iron, steel, nickel and cobalt.
- Steel is magnetic because it is made mostly of iron.
- Most other materials, including copper, aluminium, brass, wood, glass and plastic, are not magnetic, so a magnet has no noticeable effect on them.
- A permanent magnet attracts any piece of magnetic material, even one that is not a magnet itself.
- Permanent magnets are usually made from steel or special alloys, because once these are magnetised they keep their magnetism for a very long time.
Induced magnets
Definition
Induced magnet
A piece of magnetic material that becomes a magnet while it is in a magnetic field and loses most or all of its magnetism when it is taken out of the field.
- When a piece of magnetic material, such as an iron nail, is placed in the magnetic field of a permanent magnet, it becomes a magnet itself.
- This magnetism is called induced magnetism, because it is caused by the field the material is sitting in.
- The end of the nail nearest the permanent magnet always becomes the opposite pole to the pole it is facing.
- For example, if the north pole of a bar magnet is brought near an iron nail, the near end of the nail becomes a south pole and the far end becomes a north pole.
- The nearest end is always an unlike pole, so an induced magnet is always attracted to the magnet that induced it and is never repelled.
- This is why a bar magnet picks up an unmagnetised paper clip whichever of its poles is used.
- An induced magnet can pass its magnetism on, so a magnet can hold a chain of paper clips, with each clip magnetised by the one above it.
- Induced magnetism is usually temporary, so when the permanent magnet is taken away, the induced magnet loses most or all of its magnetism.
- Iron loses its induced magnetism almost straight away, while steel keeps some of it, which is how a steel needle can be turned into a weak permanent magnet.
Practical
Comparing induced magnetism in iron and steel
- Aim: to show that magnetic materials become induced magnets in a magnetic field, and to compare how well iron and steel keep their magnetism once the field is removed.
- Apparatus: a strong bar magnet, an iron rod and a steel rod of the same size, a tray of identical small steel paper clips, and a clamp stand and clamp.
- Method:
- Clamp the bar magnet vertically with one pole pointing downwards, and place the tray of paper clips underneath.
- Touch the top of the iron rod against the lower pole of the magnet so that the rod hangs below it.
- Lower the bottom of the iron rod into the paper clips, lift it out slowly, and count how many clips cling to it.
- Hold the iron rod firmly at its top and slide the magnet away from it, then count how many clips are still attached after five seconds.
- Return all the clips to the tray and repeat the test with the steel rod, using the same magnet, the same pole and the same clips.
- Carry out the whole test three times for each rod and work out the mean number of clips in each case.
- Variables: the material of the rod is the independent variable, the number of clips held is the dependent variable, and the magnet, the pole used, the rod size and the clips are kept the same.
- Results: both rods pick up a cluster of clips while they touch the magnet, but when the magnet is removed almost all the clips drop from the iron rod while several stay on the steel rod.
- Conclusion: both materials become induced magnets in the field, iron loses its magnetism almost at once, and steel keeps some, which is why steel is used for permanent magnets.
- Watch out: shaking the rod knocks clips off and makes the counts unreliable, and a rod that has been stroked with a magnet earlier may already be slightly magnetised.
- Safety: keep the strong magnet away from phones and bank cards, and lift the rods over the tray so that falling clips are caught.
Comparing permanent and induced magnets
- The two types of magnet differ in three ways.
- Where the field comes from
- A permanent magnet produces its own field, whereas an induced magnet only has a field while it is inside another magnet's field.
- How long the magnetism lasts
- A permanent magnet stays magnetised, whereas an induced magnet loses most or all of its magnetism when the other field is removed.
- The forces it can produce
- A permanent magnet can attract or repel another magnet, whereas an induced magnet is only ever attracted to the magnet that induced it.
- Attraction is not a reliable test for a magnet, because a magnet attracts plain pieces of magnetic material as well as other magnets.
- Repulsion is the only sure test, because an object can only be repelled by a magnet if it is a permanent magnet itself.
- A second test is to remove the original magnet, because a permanent magnet still picks up paper clips on its own while an induced magnet drops them.
Example
Finding the magnet among three metal bars
- Three bars that look identical, labelled A, B and C, are tested two at a time by bringing their ends together.
- A attracts B at both of its ends, and A attracts C at both of its ends.
- B and C repel when one particular pair of ends is brought together.
- Repulsion only happens between two permanent magnets, so B and C are both permanent magnets.
- A is attracted by both magnets but never repels either of them, so A is a plain piece of magnetic material that becomes an induced magnet.
Example
Explaining a chain of paper clips
- A steel paper clip hangs from the south pole of a bar magnet, and a second clip hangs from the bottom of the first.
- The magnet's field magnetises the top clip, making its upper end a north pole and its lower end a south pole.
- The top clip is now an induced magnet, so its own field magnetises the second clip in the same way.
- When the bar magnet is pulled away, the top clip loses most of its induced magnetism, so it can no longer hold the second clip and the chain falls apart.
Exam technique
Comparing the two types of magnet
- Answer a question on the difference between the two types with a direct comparison, using whereas to link both types in one sentence.
- Cover where the magnetism comes from and what happens when the other magnet is removed, since giving only one of these is a common way to drop a mark.
- When asked how to test whether a bar is a magnet, say that you would look for repulsion and explain that attraction alone could be caused by induced magnetism.
Common Mistake
- Do not say that an object attracted to a magnet must be a magnet, because unmagnetised iron, steel, nickel and cobalt are attracted too.
- Do not say that an induced magnet can repel the magnet that induced it, because its nearest end always becomes an unlike pole.
- Do not describe all metals as magnetic, because copper, aluminium and brass are not attracted to magnets.
Self review
- Name three magnetic materials.
- State one difference between a permanent magnet and an induced magnet.
- Explain why an induced magnet is always attracted to the magnet that induced it.
- What happens to an iron induced magnet when the permanent magnet is removed?
- What is the only sure test for showing that an object is a permanent magnet?
