5.4a Magnetic fields

Arctic light - Frank Olsen / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)

THE IB SAYS

Essential idea: The effect scientists call magnetism arises when one charge moves in the vicinity of another moving charge.

Understandings: Magnetic fields; Magnetic force

Applications and skills: Sketching and interpreting magnetic field patterns

Guidance: Magnetic field patterns will be restricted to long straight conductors, solenoids, and bar magnets

Data booklet equations: None

My Vodcast

Magnetic fields

You're probably familiar with magnets. Metal objects, often in the shape of a horseshoe or a bar, that exert magnetic force that can affect other magnets or certain other materials, such as those made from iron or steel.

Certain rocks or ores can be easily shaped into weak magnets, hence magnetism was an effect observed by early civilisations. These civilisations discovered that when suspended, needle-shapes of these magnetic materials could align themselves, with one end (or pole) always pointing towards the geographic north. This is why the ends are called poles, as this is short for the magnet's North-seeking (or South-seeking) pole.

What the ancients didn't realise is because this is the Earth itself has a magnetic field, and acts as a magnet.

File:VFPt Earths Magnetic Field Confusion.svg: Geek3 / derivative work: MikeRun / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)

This image shows the magnetic field diagram of the Earth. All magnetic field diagrams show how a small suspended magnet would behave in the field. They would line up along the lines with the arrow pointing towards the south pole of the larger magnet - in this case the Earth - because opposite poles, like charges, attract. Note that the Earth's magnetic south pole is near the Earth's geographic north pole, a confusing situation!

Note that magnetic field lines (like the field lines for gravity and charge):

Always have a direction (from north to south - showing the direction a compass would point in)
Show the strength of the field by the density of the lines of flux
Never cross - they can't indicate two directions at once
Don't meander - they take the shortest possible path given the conditions

To experiment with how small bar magnets (known as a compass) behave in magnetic fields, I strongly recommend this simulator.

Magnetic fields are caused by moving charges.

In permanent magnets the magnetism comes from the magnetic effects of electrons in the shell of a ferromagnetic material. These electrons give atoms a magnetic moment (makes them act as magnets) and billions of these atoms line up in tiny domains. If these domains are randomly orientated then the object will have no overall magnetism (though it will be affected by magnets). If the domains can be lined up then you have an overall magnet.

ikeRun / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)

Increasingly aligned domains lead to the formation of a magnet.

Magnetic fields of electrical currents

The flow of charges in a straight wire will create a "cylindrical" magnetic field as shown below.

To predict the direction and orientation of this field, use your right hand with your fingers curled - if you put your thumb in the direction of current flow, your fingers will show the direction of the field.

If we extend the straight wire and loop it into a circle then a torus or "doughnut" shaped field will result.