3.10 Magnetism Flashcards
Magnetism is a phenomenon caused by?
The motion of electric charges
The region in which magnetic effects are observed is called?
A magnetic field
What is the earths magnetic field also known as?
Geo-magnetic field
The earth’s magnetic field extends into space creating a region called?
Magnetosphere
What generates the earths magnetic field?
Electrical currents flowing in the slowly moving molten iron core
Why doesn’t a o pass point exactly toward geographic north?
Due to pockets of magnets (lodestones) in the crust causing deviation
Which direction does the magnetic field lines of the earth travel?
Away from the south pole and towards the north pole (opposite to a normal magnet)
What is the light weight magnet inside a magnetic compass that points towards north called?
A needle
What happens if a compass is placed next to a magnet?
It will begin to point towards the magnet instead of north or south
Which magnetic pole is the point at which the earths magnetic field lines point vertically down?
The North Pole
What is a magnetic material made from?
Materials that contain one or more of the magnetic elements
Where is the force of attraction strongest?
At each end (pole) of the magnet
What is a plotting compass used for?
To plot the magnetic field associated with a magnet
Then lines plotted on a magnetic field are called?
Lines of magnetic flux
Why do we use different shape magnets?
Because the have different magnetic field patterns
What does it mean when the magnetic fox lines are closer together?
The magnetic field is stronger
What is meant by flux density?
The amount of magnetic flux flowing through a area
What does it mean by lines of magnetic flux are closed loops?
They leave at one pole and must enter the opposite pole
What is Weiss domains?
When a material contains very small magnets (atom sized)
When a Weiss domains is subjected to a strong magnetic field what happens to the magnets inside?
They reinforce one another and line up
What methods are used to magnetise a material?
-stroking with another magnet
-hammering a piece of magnetic material inline with a magnetic field
-heating apiece of magnetic material and cooling in a magnetic field
-passing through an electric coil
When a magnet is broken down into smaller pieces, what happens to the pieces?
Each piece acts a smaller permanent magnet with a north and South Pole
What methods are used to demagnetise a magnet?
- turning the current off an electro-magnet
-slowly moving the item away from a magnetic field
-mechanical force
-heating beyond the curie temperature
What is a curie temperature?
The temperature at which the material looses its magnetism
What’s the main down side to demagnetising?
You normally have to repeat it many times for permanent loss of magnetism
What is magnetic shielding?
A shield is created to protect a component from magnetic interference
Can you stop magnetic flux flowing from north to south?
No but a good conductor of magnetic flux can redirect the lines
What are the three types of magnetic material?
-paramagnets
-diamagnets
-Ferromagnets
What type of natural material do ferromagnets contain?
-Iron
-cobalt
-nickel
What happens to a soft ferromagnetic material when removed from a magnetic field?
It becomes demagnetised spontaneously
What happens to a hard ferromagnetic material once removed from a magnetic field?
It retains its magnetism
What material has the greatest magnetism amongst all the natural minerals on the earth?
Magnetite
What do all ferromagnets have that will cause demagnetism?
A curie temperature
What is it called when a material has the tendency to remember their magnetic history?
Hysteresis
What is meant by the romance of the material?
The fraction of the saturation magnetism which is retained when the driving field is removed
What is magnetostriction?
The property of magnetic materials that causes them to change shape or dimensions during the process of magnetism
How do ferromagnetic materials respond to an impressed magnetic field?
They mechanically change (change length slightly in the direction of the applied field)
How do paramagnets react to magnets?
They’re weakly attracted to magnets
What happens to the electrons inside of paramagnets?
They spin in the same direction
How can you measure the magnetic force of paramagnets?
With sensitive instruments (due to it being so small)
What happens to a paramagnets magnetic properties when a external magnetic field is removed?
It looses its properties
What happens to a paramagnets magnetic properties when temperature is increased?
They decrease
What happens when a diamagnetic material enters a magnetic field?
It is repelled
It is said that all materials have diamagnetic properties but how is this true?
The diamagnetic properties are just weaker than the ferromagnetic and paramagnetic properties so it’s easily overcome
What causes diamagnetic materials to repulse magnetic fields?
There electrons are paired
What is an electromagnet?
A type of magnet in which the magnetic field is produced by an electric current
In an electromagnet what is each loop of a coil called?
A turn
In a electromagnet what type of material is inside the coil?
A ferromagnetic material
What is the main advantage of an electromagnet?
It can change its magnetic strength depending on how much electric current is used
What is the main downside of an electromagnet compared to a permanent magnet?
It has to be constantly supplied by electric current
In an electromagnetic what happens when the current direction changes?
The magnetic field also changes direction
What does it mean when the lines of magnetic flux are closer together?
The magnetic field is stronger in this area
What are the three things needed for an electromagnet?
-a conductor coil
-an electrical supply
-a core
Does an electric current produce a magnetic field?
Yes it does
What is the standard way to denote the direction of current flow in a conductor?
Think of the current arrow
-the direction of travel will be a dot (arrow head)
-the opposite direction will be a plus (arrow flights)
What is maxwells right hand rule?
By using your right hand you can understand the direction of current or magnetic fields
What is a magnetic circuit?
A complete closed path with any group of magnetic flux lines
What is magnetomotive force?
The amount of flux produced
How do you calculate magnetomotive force?
Current x number of turns
What is magnetic field strength measured in?
Ampere turns/ metre
What is the SI unit of flux?
Weber (Wb)
What is the equation for magnetic flux?
Flux density x cross-sectional area
What is permeability of free space?
The ratio of flux density to magnetic field strength at any point in free space
What is meant by a material being magnetically hard?
Material is difficult to magnetise or demagnetise
What is meant be a material being magnetically soft?
Material that is easy to magnetise or demagnetise
What is retentivity?
A materials ability to retain a certain amount of residual magnetic field when the magnetising force is removed after achieving saturation
What is retentivity a necessary quality for?
A permanent magnet
What is coercive force?
The amount of reverse magnetic field needed to return a materials magnetic flux to zero
What is the saturation point of a magnet?
The point at which increasing the magnetic field applied to a material stops increasing its magnetism
What are eddy currents?
Loops of electrical current induced within conductors by a changing magnetic field, due to faradays law of induction
What method is used to prevent bad effects from eddy currents?
Lamination (layerings of metal)
What are strong permanent magnets sensitive to?
Physical shock
What should permanent magnets be kept away from?
External magnetic fields to prevent from demagnetisation
What temperature can cause a reduction in magnetic fields?
250˚C
If magnets are stored long term what is important to remember?
The magnetic circuit must not be changed by removing the magnet from its housing
What are ferromagnetic components called ‘keepers’?
They’re used in storing magnets to connect between poles so that the lines of flux flow through them
Where is the force of attraction of a magnet the strongest?
At the poles of the magnet
