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© Boardworks Ltd 2003
Electromagnetism : Magnetic?
Which of the metals below are magnetic metals?
Aluminium(Al)
Silver (Ag)
Iron
(Fe)
Gold (Au)
Nickel (Ni)
Cobalt (Co)
Copper (Cu)
Zinc(Zn)
Magnesium (Mg)
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Al ZnAg
Cu
Au
Mg
Electromagnetism : Magnetic?
N
S
Fe
NiCo
Only Iron [Fe], Nickel [Ni] and Cobalt [Co] are :
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What is a magnetic field ?
NS
The iron filings locate the magnetic field and show the direction of the forces in this region.
The region around a magnet where it has a magnetic effect is called its magnetic field.
If you put a magnetic material in that field it will experience a force.
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Electromagnetism :Magnetic Fields
What does the magnetic field around a bar magnet look like ? [shape and direction].
N S
Strongest at poles
Weaker further away
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Magnetic Fields - Experiments
Put the north poles of two bar magnets together. What happens?
If you put the two magnets under a piece of paper with the N poles together and then carefully scatter iron filings on the paper.
This is what you see:
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Magnetic field pattern with N poles together
NNS S
Notice the pattern of lines offorce in this region.
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Magnetic Fields - Experiments
Put the north and south poles of two bar magnets together. What happens?
If put the two magnets under a piece of paper with N and S poles together and then carefully scatter iron filings on the paper.
This is what you see:
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Magnetic field pattern with N and S poles together
NNS S
Notice the pattern of lines of force in this region. How are they different from before?
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Magnetic Fields - Summary
1. When two like poles (e.g. two North poles or 2 South poles) are put together, they repel each other.
2. When two unlike poles (e.g. a North and a South pole) are put together, they attract each other.
3. Scattering iron filings around the bar magnets allows us to see the lines of force of the magnetic field.
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Making a magnet
It is possible to make a magnet by magnetising a material which can be attracted to a magnet, e.g. a paperclip. This can be done by:
• Stroking a magnet along the paperclip from one end to the other and then starting from the same place, repeat the movement. The more times this is done, the more magnetic the clip becomes.
• Holding a nail in a magnetic field and hitting it with a hammer.
• Putting a magnetic material in strong magnetic field.
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Electromagnetism : Magnetic Fields
What does the magnetic field around a current carrying wire look like?
Current (I)
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Electromagnetism : Magnetic Fields
Using an iron nail as a solenoid the properties of the magnetic field can be investigated.
Keeping the current the same, how many paper clips will the solenoid support as the number of coils is varied?
Keeping the number of coils of wire the same, how many paper clips will the solenoid support as the current is varied.
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Electromagnets Animation
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Electromagnetism : Electromagnets
How the number of coils affects the strength of an electromagnet
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0 10 20 30 40 50 60 70 80 90
Number of coils
Nu
mb
er o
f cl
ips
attr
acte
d
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Electromagnetism : Electromagnets
How the current affects the strength of an electromagnet
-10
0
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0 1 2 3 4 5
Current [A]
Nu
mb
er
of cl
ips
attra
cte
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Electromagnetism : Electromagnets
We use large electromagnets in scrap yards to pick up scrap metal:
What metals would the electromagnet attract?
What advantages does an electromagnet have over a permanent magnet?
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Electromagnetism : Electromagnets
Door bells use electromagnets:
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Electromagnetism : Electromagnets
Lifts, cars and other large electrical machines use high currents. A relay can be used to allow a small current in one circuit to control a large current in another circuit.
