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P7 C) Electromagnetism
P7 C) Electromagnetism
A magnetic field is created when a current flows through a wire. The magnetic field is made up of concentric circles that are perpendicular to the wire (concentric circles means circles that have the same centre and the centre for these circles is the wire that the current is flowing through). Below is a diagram of the circular magnetic field that is created by a current flowing through a wire.
We are able to use what is known as the ‘right-hand thumb rule’ to work out the direction that the magnetic field is travelling in. We use this rule by having our right hand in the thumbs up position. We then point our thumb in the direction of the current through the wire – for the above exam, the current was travelling upwards so we have our thumb pointing upwards. The direction of the curls on the fingers of our right hand will be the direction of the circular magnetic field.
If the direction of the current changes, the direction of the magnetic field also changes. I am now going to have the current travelling downwards through the wire. The current travelling downwards means that out thumb on our right hand will be pointing downwards. The direction of the curls on our fingers is the direction of the magnetic field produced. I have shown the direction on the diagram below.
We are able to prove that a magnetic field is produced when a current flows through a wire by using a piece of card and a compass. We do this by placing the compass in the magnetic field produced by the wire and tracing the magnet field lines that are produced; we do this in a similar way to how we found the magnetic field that a bar magnet produced. The set up for the experiment and the outcome when the current is travelling upwards is shown below.
We can then change the direction of the current and see what effect this has. The outcome of the experiment when the current is travelling downwards is shown below.
The Strength of the Magnetic Field
The strength of the magnetic field produced depends on two different factors. These are:
The strength of the magnetic field produced depends on two different factors. These are:
- Distance from the wire – the closer that you are to the wire, the stronger the magnetic field will be. The further you are away from the wire, the smaller the magnetic field will be.
- Size of current – a stronger current passing through the wire will result in a stronger magnetic field. A smaller current passing through the wire will result in a smaller magnetic field.
Solenoid
We are able to increase the strength of the magnetic field produced from a wire by coiling the wire/ creating lots of loops for the wire. A diagram of a solenoid is shown below.
We are able to increase the strength of the magnetic field produced from a wire by coiling the wire/ creating lots of loops for the wire. A diagram of a solenoid is shown below.
The field lines from each of the loops of wire in the coil line up. This results in all of the field line inside the solenoid travelling in the same direction. The magnetic field inside the solenoid is very strong and uniform. The magnetic field at the end of the solenoid are very similar to the magnetic field lines produced by a bar magnet. This means that one end of the solenoid behaves like the north pole of a magnet and the other end behaves like the south pole of a magnet. A solenoid with its field lines on are shown below
We are able to find the direction that the field lines from the solenoid are going by using the ‘right-hand thumb rule’ for both the top and bottom of the solenoid. For the top of the solenoid, the current is coming out of the page, which means that the field lines produced by each of the coils will be anticlockwise. The field lines from all of these coils interreacting with one another together will result in the field lines going rightwards. For the bottom of the solenoid, the current is going into the page, which means that the field lines for each coil will be clockwise. The field lines from all of these coils interreacting with one another together will result in the field lines going rightwards.
The magnet field from a solenoid will only be present when a current is flowing through the current-carrying wire. This means that as soon as the current is switch off, the magnetic field produced by the solenoid is lost.
Increasing the Strength of the Solenoid
We can increase the strength of the magnetic field produced by the solenoid by increasing the number of coils in the wire. A greater number of coils will result in a stronger magnetic field.
Another way that we can increase the strength of the magnetic field is to increase the current that is passing through the wire. A greater current means a greater magnetic field is produced by the wire.
We are able to increase the strength of the magnetic field produced by placing an iron core in the middle of the solenoid. A solenoid with an iron core is known as an electromagnet. When a current is flowing through the solenoid wire, the iron core inside the coiled wire will become an induced magnet. We use an iron core because iron is a magnetically soft material, which means that it loses its magnetism quite quickly as soon as the current is turned off. If we used a magnetically hard material like steel, the magnetically hard core would keep its magnetism for a while after the current has been switched off. This would reduce the effectiveness of the electromagnet.
The magnet field from a solenoid will only be present when a current is flowing through the current-carrying wire. This means that as soon as the current is switch off, the magnetic field produced by the solenoid is lost.
Increasing the Strength of the Solenoid
We can increase the strength of the magnetic field produced by the solenoid by increasing the number of coils in the wire. A greater number of coils will result in a stronger magnetic field.
Another way that we can increase the strength of the magnetic field is to increase the current that is passing through the wire. A greater current means a greater magnetic field is produced by the wire.
We are able to increase the strength of the magnetic field produced by placing an iron core in the middle of the solenoid. A solenoid with an iron core is known as an electromagnet. When a current is flowing through the solenoid wire, the iron core inside the coiled wire will become an induced magnet. We use an iron core because iron is a magnetically soft material, which means that it loses its magnetism quite quickly as soon as the current is turned off. If we used a magnetically hard material like steel, the magnetically hard core would keep its magnetism for a while after the current has been switched off. This would reduce the effectiveness of the electromagnet.
Usefulness of Electromagnets
Electromagnets are extremely useful because they allow us turn on and turn off a magnetic field.
Electromagnets are used to pick out magnet materials (like cans) out of rubbish when the rubbish is passed along a conveyer belt with an electromagnet above it. The magnetic materials will be attracted to the electromagnet and leave the conveyer belt. When the rest of the rubbish that is not magnetic has passed along the conveyer belt, we can turn the electromagnet off (turn the current off), which will result in the electromagnet losing its magnetism, which causes the magnetic materials to drop back onto the conveyer belt. If we were unable to turn the magnet off, we would be left with a situation where the magnetic materials would be stuck to the magnet and this is why electromagnets are very useful.
Electromagnets are extremely useful because they allow us turn on and turn off a magnetic field.
Electromagnets are used to pick out magnet materials (like cans) out of rubbish when the rubbish is passed along a conveyer belt with an electromagnet above it. The magnetic materials will be attracted to the electromagnet and leave the conveyer belt. When the rest of the rubbish that is not magnetic has passed along the conveyer belt, we can turn the electromagnet off (turn the current off), which will result in the electromagnet losing its magnetism, which causes the magnetic materials to drop back onto the conveyer belt. If we were unable to turn the magnet off, we would be left with a situation where the magnetic materials would be stuck to the magnet and this is why electromagnets are very useful.
Another example of where electromagnets are used is at a car scrap yard to move cars. When we want to move a car, we turn the current on which creates a magnetic field from the electromagnetic. The car is made out of a magnetic material, which means that it will be attracted to the electromagnet, which means that we can be moved from one place to another. When we have finished moving the car, we can turn the current off which means that the magnetic field from the electromagnet disappears. The car will no longer be attracted to the electromagnet, and will therefore be dropped.