Magnetism

What we are learning: (component knowledge)

Types of magnets and forces:
There are two types of magnets, firstly there is the permanent magnet which produces its own magnetic field because all of the domains inside are aligned. The second is called an induced magnet, here it is simply a material that becomes a magnet when it is placed inside a magnetic field. This induced magnet causes a force of attraction but as soon as the magnetic field is removed, it stops having magnetic characteristics.
Ensure that you remember that like poles repel and opposite poles attract. In magnetism, there are North and South poles. Try not to get confused but understand that if the North side of a compass points to the North Pole of Earth, then one of those must actually have been a South Pole otherwise they would have repelled. You may come across the terms North-seeking and South-seeking for this reason.

Magnetic materials and magnetic strength:
Iron, steel, cobalt and nickel are the magnetic materials that you need to remember.
The closer that a magnetic material is to a magnet, the stronger the force will be and around a magnet, the strongest force is felt at the poles (this is where the field is strongest). Both the attraction and repulsion felt are excellent examples of non-contact forces. The area (well, volume as it is a 3D region) around the magnet in which a force can be felt is called the magnetic field. Just like with gravity, you will experience the force (weight) pulling you towards Earth but the further away you travel (into space) the weaker it becomes. This is why even the strongest magnets you might have in your home can stick to the fridge door but they will not cause paper clips in the next room to come flying towards it unless you move them much closer together. Remember that as a paperclip will become an induced magnet when it is inside the permanent magnet's field, the force is always a force of attraction. Magnets always attract magnetic materials and only repel the same pole of a different magnet.

Magnetic fields:
As mentioned above, this is the region (3D remember) around a magnet in which it can have influence over other magnets or magnetic materials. The field lines are found using a small plotting compass around a magnet and the lines are always drawn to come out of the North and enter the South.
A magnetic field can be generated using a direct current flowing through a wire. Using the right hand thumb rule, the flow of current goes from the palm of your right hand up and out of the end of your thumb, the magnetic field flows at 90° around the wire, this is shown by your 4 fingers that are curved around. If you are looking down a wire and the current is flowing directly towards your eye, then the magnetic field line will be going anti-clockwise around the wire. As the current increases, so does the strength of the magnetic field and, as previously discussed, the closer the magnetic material gets to the wire, the stronger the force it will feel.

Fleming’s left had rule and electromagnetism:
When applying Fleming's Left Hand Rule, your first finger is the field, second finger is the current and the thumb is the movement direction. Try it, it does work. This tells us that if we pass a DC current through a wire in a magnetic field, a force is applied. What we have to picture is our right hand thumb rule. There is a field generated by the current in the wire and this will interact with the existing field from the original magnets. The wire will try to align itself and this will cause is to move.
This also explains electromagnetism. If we wrap a coil of wire around an iron core and pass a DC current through it, it will generate a magnetic field. The iron core will become an induced magnet as a result. The electromagnet can be made more powerful by increasing the number of turns of wire that are wrapped around it and/or by increasing the size of the current flowing through the wire. Make sure that you are confident to explain why it would only work with DC current, if we used AC, then the North and South poles would be swapping 50 times per second so would cancel themselves out.

The motor effect and forces:
Building on from Fleming's left hand rule, we can describe the motor effect. We know that a static field acting on a wire with a DC current will cause a force on the wire. In a motor, the wires are wrapped around a central block on an axle. When a current is passed through, then the wires are forced to move up or down (one side is forced up and the other forced down) which generates rotational movement. This is the motor effect. The motor can be made to spin faster by increasing the DC current flowing through it. A motor may be made to be more powerful by adding more turns of wire and having stronger magnets to produce a stronger field.
These forces that are being felt by the conductor can be quantified using the formula:
Force (N) = Magnetic flux density (T) x Current (A) x length (m)
F=B I l We haven't met the units of Magnetic Flux Density yet, T is tesla.
These all contribute to the above statement about increasing the power of an electric motor. Remember BIl, increase the strength of the magnets (B), Increase the current (I) and more coils/increase the length (l).

Key words/terms for this topic

Attraction      Electromagnet      Fleming's left hand rule      Induced      Magnetic field      Magnetic Flux Density      Motor effect      Non-contact Force      Permanent      Poles      Repulsion      Solenoid      Split-ring commutator     

Curriculum Health Check:

Q: Magnetic repulsion occurs between...

A: North pole and iron

B: Opposite poles

C: South pole and iron

D: Like poles

What you need to know

Magnetic forces are strongest at the poles, they exert forces on other magnets - like poles repel and opposite poles attract. This is a non-contact force. Permanent magnets are always producing a magnetic field based on the domains within. Electromagnets have their fields induced by the current flowing around them.

The four common magnetic metals are iron, steel, cobalt and nickel. The region around these magnets is called the magnetic field. The further away you get from a magnet, the weaker the force. A compass has a tiny bar magnet that is free to rotate which aligns itself with the Earth's magnetic field.

You need to be able to explain how to draw a magnetic field using a bar magnet and a small plotting compass.

When current flows through a wire that is wrapped around an iron core, it produces an electromagnet. The force, like with a permanent magnet is stronger the closer you are to it. You can also make the magnetic force higher by adding more turns of wire around the core or increasing the current. The wire on its own is called a solenoid. You need to draw the field lines again and show the directions North to South.

Extra topics needed for the Higher Tier papers:

Fleming's left-hand rule allows us to show how conductors in a magnetic field move based on the direction of current flow. This movement of a current carrier in a field is called the motor-effect.

First Finger Field (points from North to South)

• SeCond Finger Current (current flows from the hand towards the finger tip)

• ThuMb Movement (Wire moves from the hand towards the tip of the thumb).

When this conductor is conducting at right angles to the field, we can not only work out the direction it will move but also the force applied to it.

Force (N) = Magnetic Flux Density (T) x Current (A) x Length (m)

F = B x I x l

Electric motors rely on this principle, the current on either sides of the coil is travelling in opposite directions so the force is "up" on one side and "down" on the other which causes it to rotate. You need to be able to explain how one works.

This page was updated on: 31st March 2024