16.
Magnetism
o Like poles repel
o Unlike poles attract
o Magnetic force exert either a pushing or pulling force
Bar magnets
A rectangular-shaped permanent magnet with a north pole at one end and a south pole at the other
Compass needle is like a bar magnet
How does it work?
When it’s free to rotate
o North seeking pole/north pole of the magnet point roughly in the direction of the Earth’s geographical north
pole (they are attracted because it’s earth’s magnetic south pole)
o The other end is the magnet’s south pole/south seeking pole
o The compass needle tilts downwards slightly pointing into the Earth, thus, earth is magnetized as if there was a
giant bar magnet inside it.
Non-magnetic materials
Materials that are not attracted by a magnet and cannot be magnetized. E.g. plastic and rubber
Magnetic materials
Materials that are attracted by a magnet and can be magnetized.
o Used to make magnets
o Not all magnetic materials are magnets
What’s the difference between magnets and magnetic materials?
Examples of magnetic materials?
o Ferrous materials (contains iron) such as hard steel, ferrite (ceramic material)
o Cobalt and nickel
If material contains iron, this is not a guarantee that it will be magnetic, stainless steel contains a lot of iron but it’s
not a magnetic material.
Magnetic materials are classified into two
Hard (permanent magnetic materials) Soft
A material that, once magnetized, is difficult to A material that, once magnetized, is easy to demagnetize
demagnetize
Retains magnetism well, but difficult to magnetize in the Easy to magnetize, but readily loses its magnetism
first place
Hard to bend Easy to bend
Hard steel Soft iron
Permanent magnets, compass needles, loudspeaker Cores for electromagnets transformers and radio aerials.
magnets
Two ways of making a induced magnet
Using magnetic materials near a magnet
Using electromagnets
Two types of magnets made from magnetic materials
Permanent magnets Induced magnetism
Magnetized magnetic material that When a magnetic material is only magnetized when
o produces its own magnetic field that placed in a magnetic field of a permanent magnets.
o does not get weaker with time o Have opposite polarities
Made of hard magnetic material, e.g. hard steel
Can attract or repel another permanent magnet When the permanent magnet is removed, it returns to its
Can attract other unmagnetisd magnetic materials to unmagnetised state
induce magnetism. o Or may retain small amount of magnetism
Bar magnets, compasses, fridge magnets, door of Electromagnets
refrigerators, magnetic duster on a whiteboard
Magnetic fields
A region of space around a magnet or electric current in which a magnet or magnetic material will experience a force.
Magnetic field of a bar magnet.
Rules
1. the filed lines should never
cross
2. field lines should include
arrows, pointing from the
magnetic north to magnetic
south.
Magnetic field lines
Represent the direction the magnetic force would have on the north pole of a magnet.
Ofc, the filed fills all the space around the magnet, not just those lines.
How do we interpret from the magnetic field lines pattern?
1. Direction: the direction of a magnetic field line at any point is the direction of the force on the north pole of a
magnet at that point. (filed lines come out of north poles and go in the south poles)
2. Strength: lines that are close together indicate a strong filed.
Strongest at the poles, gets weaker as the distance from the magnet increases.
Plotting filed lines
Method 1
1. Place magnet under a stiff sheet of plain paper or preferably clear plastic
2. Sprinkle iron filings over it
3. Tap it to allow fillings to move slightly so that they line up in the filed
Method 2
While plotting compass is placed in a magnetic field, its needle turns to indicate the direction of the field.
1. Place the magnet on top of a piece of paper
2. Draw a dot at one end of the magnet (near its corner)
3. Place a plotting compass next to the dot, so that one end of the needle of the compass points towards the dot
4. Use a pencil to draw a new dot at the other side of the compass needle
5. Now move the compass to the other side of the dot so that its other end points towards the new dot, and
repeat the above process
6. Keep repeating until you have a chain of dots going from one end of the magnet to the other. Then remove the
compass, and link the dots using a smooth curve – the magnetic field line
7. The direction of the field line is the same as the direction of the plotting compass
8. You can now repeat the whole process several times to create several other magnetic field lines
Uniform magnetic field
o Produced in the gaps between opposite poles
o Directed from north to south
o Same strength and direction at all point (represented by parallel magnetic field lines as equal spacing between
the lines and same direction of arrows at any point)
Don’t add the lines
between its own north
and south pole when
drawing magnetic
interaction.
What happens when two bar magnets are placed close together?
o Their magnetic fields interact
o Produce a new pattern of magnetic lines of force.
o From those new magnetic field lines pattern, it is possible to tell whether the magnets will attract or repel.
How does this explain repelling and attracting of poles?
o When like poles are placed together (look at inside the box),
o All the lines from two magnets pointing at one direction and close together indicating a stronger filed than the
other poles
o Magnets will move from stronger field to weaker field
o Like poles repel, unlike poles attract.
Electromagnets
A coil of copper wire that acts as a magnet when an electric current passes through it.
Solenoid
An electromagnet made by passing a current through a coil of wire
Why is copper wire used instead of magnetic materials like iron?
Low resistance
The coil does not have to be made from a magnetic material, t is the electric current that produces the magnetic field.
How does this work?
o A current flows through the wire
o There’s a magnetic field around the coil
How is that magnetic field?
Magnetic field is produced both inside and outside of the coil
Inside is a uniform field (field lines are parallel and same distance apart)
Outside is similar to that of a bar magnet
o One end of the coil and north pole and the other is south pole
o Current in – north, current out – south.
o Direction is from north to south.
What’s different (advantages) with the normal magnetic fields?
Can increase the strength of an electromagnet
1. Increase the current flowing through it.
2. Increase the number of turns of wire on the coil (not making the coil longer) but concentrating the field.
3. Add a soft iron core (can becomes strongly magnetized by the field – soft magnetic matrial)
Can be switched on and off by switching off the current (and magnetic field around the coil disappears)
The direction of the field can be reversed by reversing the direction of the current.
Uses of electromagnets
- Electromagnetic cranes in scrapyard
- MRI scanners uses powerful electromagnets to produce images of the organs of the body
- Electric doorbells
- Loudspeakers, microphones and earphones to sense or end soundwaves
- Electric motors
- Relays and transformers
- Steel is separated from waste using electromangets and recycled
Two ways to make induced magnets?
1. electromagnets 2. Putting soft magnetic material near magnets
Heating a magnet can demagnetize it.
