There are many questions you can expect to answer after studying Magnetism! Why is the North pole actually a South? Why do we define magnetic field in terms of rotation not acceleration? What is the connection between magnetic fields and electric current? And why don't magnetic waves exist without electric waves?
Key Concepts
Iron is the most common magnetic material. This means that it contains magnetic domains that can be aligned to make the material into a magnet. Iron is an essential component in the alloy steel. Nichol and cobalt are also magnetic.
There are several ways of categorising magnets:
- Permanent magnets (produced by aligning the magnetic domains in a material, made from hard magnetic materials like steel) - usually purchased as bar magnets
- Temporary magnets (also produced by aligning magnetic domains, made from soft magnetic materials like iron)
- Electromagnets (produced by moving charges, made from any electrical conductor) - usually the wire is wrapped into a coil, known as a solenoid
NB: How could you test if an object is a magnet?
Magnetic flux density is equivalent to magnetic field strength. Magnetic flux density (B) is measured in Tesla (T).
The term flux comes from a time when magnetic field was thought to be caused by the flow of a substance through space.
Definition: A flux density of 1 T will cause a force of 1 N to be experienced by a wire of 1 m in length carrying a current of 1 A perpendicular to the field.
Permanent magnets
Bar magnets and solenoids have two poles: north and south. A theoretical isolated north pole is attracted to a south pole and repelled by a north pole. Similar to charges: unlike poles attract, like poles repel
The north-seeking pole of a magnet points towards the geographic north pole of the Earth. This is because the earth actually contains a south pole in its northern hemisphere due to the direction of rotation of the liquid core.
Magnetic field lines show the direction of in which a test north pole would be accelerated. The closer the field lines, the stronger the magnetic force.
In the following diagram, A is a north pole.
NB: Notice the following details when drawing your own diagrams:
- Field lines never touch or cross, even at the poles
- Field lines are closest at the poles
- The shape of the field lines is symmetrical, but the arrows are not
Electromagnets
A straight electrical conductor produces a circular magnetic field.
An easy way to determine the direction of the field is the right hand grip rule.
- Make a thumbs up sign with your right hand.
- Put your thumb in the direction of the current.
- Your fingers will curl in the direction of magnetic field.
When a wire is made into a loop, the magnetic field becomes straight as it is the sum of all the fields from each section of the turn added together. A solenoid is wound so that each coil lies next to the one before to form a cylinder. Inside this coil of multiple turns, the fields add inside to give the pattern below.
The direction of the field inside can be determined using another right hand grip rule.
- Make a thumbs up sign with your right hand.
- Curl your fingers in the direction of the conventional current within the turns of the coil.
- Your thumb will point to show the position of the north pole.
Force on a current-carrying conductor
A current-carrying conductor (i.e. an electromagnet) within an existing magnetic field will experience a force due to the interactions of the magnetic fields. This is often known as the motor effect.
If the field is perpendicular to the current the the force is perpendicular to both.
Fleming's left hand rule can be used to work out the direction of the force acting on the current-carrying conductor.
- Splay the thumb and first two fingers of your left hand to create three right angles. Practise rotating this arrangement at the wrist, elbow and shoulder without disrupting the right angles.
- Your first finger represents the external magnetic field. Line this orientation up first towards south.
- Keeping the field direction, rotate your left hand until your second finger lines up with the conventional current.
- Your thumb will now be pointing in a direction that is perpendicular to both of the previous directions. Your thumb is pointing in the direction that the current-carrying conductor will move.
Not all fields and currents will be in the same plane as the paper:
Calculating forces - conductors
The size of the force (in N) on a current-carrying conductor depends on:
- flux density (B) in T
- current (I) in A
- length of conductor in the field (L) in m
\(F=BIL\)
This force is used to define the ampere: 1 A is the current that would cause a force of 2 x 10-7 Nm-1 between two long parallel conductors placed in a vacuum.
Your data booklet includes \(\sin \theta\) in this equation. This is to ensure that you only use the perpendicular component of the field.
Calculating forces - individual charges
Consider a section of wire in a perpendicular B field. A force acts on each individual charge within the field, depending on:
- flux density (B) in T
- charge (q) in C
- velocity at which the charge is moving (v) in ms-1
\(F=Bqv\)
Your data booklet includes \(\sin \theta\) in this equation. This is to ensure that you only use the perpendicular component of the field.
How much of Magnetic fields have you understood?
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