21/04/23What 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

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• 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.

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• 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

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• 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

21/04/23Magnetic field lines

• The density of the field lines will indicate the relative strength of a magnetic field

Field is strongerwhere lines aremore dense

Field is weakerwhere lines areless dense

S

N

21/04/23Magnetising 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.

21/04/23Magnetising material

Unmagnetised material domains are

randomly aligned

Magnetised materialdomains are all

completely aligned

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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.

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• 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)

21/04/23Electromagnets

You can make an electromagnet easily, all you need is:wire, iron bar, battery.

1) Wind the wire around the iron bar2) Scrape some insulation off of each end of the

wire3) Attach each end to one terminal of the battery3) Use the electromagnet to pick up some iron

objects. 4) Disconnect one of the connectors and see what

happens to the iron objects

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battery

Object held by electromagnet

Electromagnets

Iron coreWire wound around theiron core

Can you make the electromagnet work without the nail?

21/04/23Electromagnets

How can we make the electromagnet stronger, allowing it to pick up larger objects

1) Increase the number of coils around core2) Increase the power from battery (increase the

current)3) Add a soft iron core to concentrate the

magnetic field through the coils

21/04/23Electromagnets

Advantages of electromagnets include:

1) Can be turned off and on

2) Strength is defined by electrical power not size of material in the magnet

21/04/23Solenoid

• 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

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21/04/23Magnetic 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?

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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)

d

I

2

B 0