Magnets are mysterious things. Some materials are magnetic, while others are not. Magnets can attract 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.
Definition of permanent and induced magnetism
Let's first look at what permanent and induced magnetism mean.
Magnetism
First things first: what is a magnet?
A magnet is a material or object that produces its own magnetic field.
Every magnet has a north-seeking pole or north pole and a south-seeking pole or south pole, and they will be equally strong. These magnetic poles are defined such that the north pole will be pulled to the Earth's geographical 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 magnetic if it is attracted or repelled by a magnet (i.e. it feels any force as a result of a magnetic field).
Induced and permanent magnets
What do we mean by permanent and induced magnets? The names already give away their definitions.
A permanent magnet is another name for a magnet: it is a material that produces its own magnetic field.
An induced magnet is 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 magnetic materials.
Examples of permanent and induced magnetism
Now we know what permanent and induced magnets are, we can look at some examples.
Examples of permanent magnets
A permanent magnet is a material that has to be magnetised at some point, so permanent magnets are specific things.
A compass is 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.
Examples of induced magnets
In contrast with permanent magnets, induced magnetism is a property of materials: 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.
Difference between permanent and temporary magnetism
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-called temporary magnetism: they are magnetic for a finite amount of time before becoming non-magnetic again. The difference between temporary magnetism and permanent magnetism is that permanent magnetism is - you guessed it - permanent.
Causes of permanent and induced magnetism
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.
To get an intuitive picture of how magnetism works, imagine the particles in permanent and induced magnets to be little (permanent) magnets themselves that normally point in random directions.
Causes of induced magnetism
Let's take a nail as an example of an induced magnet 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.
Causes of permanent magnetism
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 magnetised materials also explains why magnets always have two poles, even after you cut them in half.
Visualisation of the little magnets inside materials. Material (a) is not magnetised, and material (b) is magnetised, StudySmarter.
Permanent and Induced Magnetism - Key takeaways
- A (permanent) magnet is a material or object that produces its own magnetic field. Every magnet will have a north pole (red) and a south pole (white). A north pole always repels another north pole and always attracts a south pole.
- An induced magnet is a material that is magnetic only when it is placed in a magnetic field. Induced magnets are always attracted to (and never repelled by) permanent magnets.
- A material is called magnetic if it is attracted or repelled by a magnet.
- Examples of permanent magnets are compasses, the Earth, and whiteboard erasers.
- Examples of induced magnets are nails, whiteboards, fridges, paperclips, and cars.
- Temporary magnetism is magnetism that is not permanent. Induced magnets are temporary magnets.
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