What is magnetism Magnetism is a force which cannot be seen and does not require contact to be felt. It exists between two pieces of magnetic material, termed ferromagnetic material, which means Iron like. Magnetism will attract any ferromagnetic material without any specific magnetic orientation, e.g. iron is attracted to a magnet

However if the ferromagnetic material is magnetised then the interaction will be stronger and two out comes are possible. The pieces of magnetic material will either attract or repel each other depending upon whether we bring opposite poles together or the same poles together. All magnets have a North and a South pole. Like poles will repel each other and unlike poles will attract each other.

A magnetic field is defined as being from the North pole of the magnet to the South pole of the magnet. This is shown as directional arrow on lines drawn on diagrams to represent field lines. E.g. N S

When we consider a magnetic field orientated into or out of the plane of the page, then we need to define the direction of the field. We use cross for a field going into the page (north out, south in) and a point for a field coming out of the page These two images represent the tail feathers and the sharp point of a dart entering or leaving the plane of the page. X ● Into Out of

Magnetic field lines The density of the field lines will indicate the relative strength of a magnetic field Field is weaker where lines are less dense S Field is stronger where lines are more dense N

Magnetising material Material like iron has many thousands of little magnets inside them called domains. These are all randomly aligned. When the iron is magnetised all these domains line up. This can be achieved by drawing a magnet along the length of the metal in the same direction repeatedly.

Magnetising material Unmagnetised material domains are randomly aligned Magnetised material domains are all completely aligned

Magnetic field around a wire We know that a current carrying wire generates a magnetic field around it The direction of this magnetic field is defined by the right hand grip rule This states that if we grip the wire with our thumb pointing in the direction of the current thro’ the wire then our fingers will curl around the wire in the direction of the magnetic field.

Image of Right hand grip rule Thumb points in direction of current Fingers curls around in direction of magnetic field (in this case, North at back , South in front)

Electromagnets You can make an electromagnet easily, all you need is: wire, iron bar, battery. 1) Wind the wire around the iron bar 2) Scrape some insulation off of each end of the wire 3) Attach each end to one terminal of the battery 3) Use the electromagnet to pick up some iron objects. 4) Disconnect one of the connectors and see what happens to the iron objects

Object held by electromagnet Electromagnets Wire wound around the iron core Iron core Object held by electromagnet battery Can you make the electromagnet work without the nail?

Electromagnets How can we make the electromagnet stronger, allowing it to pick up larger objects Increase the number of coils around core Increase the power from battery (increase the current) Add a soft iron core to concentrate the magnetic field through the coils

Electromagnets Advantages of electromagnets include: Can be turned off and on Strength is defined by electrical power not size of material in the magnet

Solenoid The coil of wire around the iron core is known as a solenoid. If we now consider the application of the right hand grip rule around a solenoid, we can see how its magnetic field develops We can then determine direction of the magnetic field for the whole solenoid http://en.wikipedia.org/wiki/Solenoid

Magnetic field strength The unit used to measure magnetic field strength is the Tesla (T) Magnetic field is given the symbol B We understand that a current carrying wire generates a magnetic field, but how is the strength of the magnetic field affected by the size of the current through the wire and distance from the wire?

Calculating magnetic field strength We can determine the formula : Where B = magnetic field strength in Tesla (T) I = current in Amps (A) d = distance from current carrying wire (m)  = 3.14 μ0 = permeability of free space (air) = 1.26 x 10-6 (TmA-1)