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Magnets are mysterious things. Somematerials是磁性的,而另一些则不是。磁铁可以在tract or repel each other, and magnets attract some things that don't seem to be magnetic themselves! In this article, we look at the different types of magnetic materials: permanent and induced magnets.
Let's first look at what permanent and induced magnetism mean.
First things first: what is a magnet?
A磁铁is a material or object that produces its own magnetic field.
Every magnet has anorth-seeking poleornorth poleand asouth-seeking poleorsouth pole, and they will be equally strong. These magnetic poles are defined such that the north pole will be pulled to theEarth'sgeographical north pole, and the south pole to the geographical south pole, as a result of the Earth's magnetic field.
Note that this means that at the location of the North Pole of the Earth (in Arctica), there is a magnetic south-seeking pole, i.e. a magnetic south pole. In illustrations, and sometimes on magnets themselves, the north pole of a magnet is often coloured in red. A north pole always repels another north pole and always attracts a south pole, and a south pole always repels another south pole.
Alright, and what does it mean for a material to be magnetic?
A material is called磁铁icif it is attracted or repelled by a magnet (i.e. it feels anyforceas a result of a magnetic field).
What do we mean by permanent and induced magnets? The names already give away their definitions.
Apermanent磁铁is another name for a magnet: it is a material that produces its own magnetic field.
Aninduced magnetis a material that is magnetic only when it is placed in a magnetic field.
Induced magnets are always attracted (and never repelled) to permanent magnets. We see from all the definitions that both permanent and induced magnets are magneticmaterials.
Now we know what permanent and induced magnets are, we can look at some examples.
A permanent magnet is a material that has to be magnetised at some point, so permanent magnets are specific things.
Acompassis a bar magnet that can spin freely so that its magnetic north pole always points to the earth's geographical north. The magnet in a compass is a permanent magnet.
A compass including the needle with the magnetic north pole (red side) and magnetic south pole (silver side), Wikimedia Commons Public Domain.
The Earth can be regarded as a permanent magnet: it has a magnetic south pole (in Arctica) and a magnetic north pole (in Antarctica), and although its magnetic field is quite weak, compasses can use it to orient themselves. Some animals can use the Earth's magnetic field to orientate themselves and navigate as well!
Permanent magnets can be found in lots of things: some watch straps use it, there exist magnetic blocks and balls to play with, whiteboard erasers are magnetic such that they stick to the whiteboard, etc.
In contrast with permanent magnets, induced磁铁ismis a property ofmaterials: some materials are induced magnets, and others are not. Most magnetic materials are metals, but only some metals are magnetic. The most common metals that experience induced magnetism are iron, nickel, and cobalt. Most materials containing these metals (like steel) are also induced magnets.
Anything made from iron, steel, nickel, or cobalt is an induced magnet, i.e. is attracted to magnets. Think of nails, whiteboards, fridges, paperclips, cars, etc.
You can find out which things in your house experience induced magnetism by grabbing a permanent magnet and seeing what objects are attracted to it. Remember: a permanent magnet is, by definition, not an induced magnet.
We can tell that the fridge is an induced magnet because the fridge magnets (which are permanent magnets) stick to it and are never repelled by it, Wikimedia Commons CC BY-SA 3.0.
Permanent magnets are always magnetic, but induced magnets are only magnetic as long as they are in a magnetic field. Thus, induced magnets only experience so-calledtemporary磁铁ism: they are magnetic for a finite amount of time before becoming non-magnetic again. The difference between temporary磁铁ismand permanent磁铁ismis that permanent磁铁ismis - you guessed it - permanent.
For GCSE Physics, it is not necessary to know the causes of permanent and induced magnetism. Nevertheless, this section gives some insight into how to think about permanent and induced magnets, which can be very handy.
得到一个直观的图片磁性是如何工作的, imagine theparticlesin permanent and induced magnets to be little (permanent) magnets themselves that normally point in random directions.
让我们来钉作为诱导增效的一个例子et to explain the causes of induced magnetism. The little magnets inside it all point in random directions, so there is nothing special about this nail. However, once we put the nail inside a magnetic field, the little magnets line up according to this magnetic field, and they all point in the same direction. Now, our nail has become magnetic, because at the edges we have a north pole and a south pole, and in the middle, the head-to-tail character of the little magnets will annihilate the little north and south poles. This alignment explains why induced magnets are always attracted to permanent magnets: their magnetisation aligns according to the magnetic field they are in.
Once we pull the nail out of the magnetic field, the little magnets return to their original positions (much like a folded piece of paper wants to unfold itself once you let go of it), and the nail becomes non-magnetic again.
Let's stick with our nail. How do we make it a permanent magnet? The trick is to heat it up until the individual little magnets each have so much energy that they can turn in any direction they want without a problem (this is called the Curie temperature). We put it in a magnetic field such that the preferred direction of the little magnets is along the magnetic field, and after this we let it cool down again. The standard position of most of the little magnets is now that they line up, which makes the nail permanently magnetised: it is now a permanent magnet.
Note that this image of magnetisedmaterialsalso explains why magnets always have two poles, even after you cut them in half.
Visualisation of the little magnets insidematerials. Material (a) is not magnetised, and material (b) is magnetised, StudySmarter.
A permanent magnet is a material that produces its own magnetic field regardless of circumstances. An induced magnet is a material that is magnetic only when it is placed in a magnetic field.
Permanent magnetism is found everywhere on Earth, because the Earth itself is a permanent magnet. Try searching for examples of permanent magnets to see where to find permanent magnetism besides the Earth itself.
There are different ways to turn a material into a permanent magnet. One way is to heat it up, place it in a magnetic field, and let it cool down again.
Permanent magnets have many uses. They are used in compass needles for navigation, in electric motors for transportation, in whiteboard erasers to stick to whiteboards, etc.
Typical horseshoe magnets or magnets for on your fridge are permanent magnets. Other examples are compass needles, some watch straps, and whiteboard erasers.
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