Module 3.10 — Magnetism
Call Kai
Learn
Practice Test
Spaced Repetition
Match
Flashcards
Knowt Play1/332
There's no tags or description
Looks like no tags are added yet.
333 Terms
What is magnetism?
A physical phenomenon involving magnetic fields that can exert forces on magnetic materials and moving electric charges.
What natural magnetic mineral is mentioned in BA 3.10?
Lodestone.
What property of lodestone made it useful historically?
It attracts iron and can align with the Earth's magnetic field.
What direction does a freely suspended magnet tend to align with?
North–south.
Why does a freely suspended magnet align north–south?
The Earth's magnetic field exerts a turning effect on it.
What navigation instrument uses a freely suspended magnet?
A magnetic compass.
What is a ferromagnetic material?
A material that can be strongly magnetised and strongly attracted by a magnet.
Name the three common ferromagnetic metals listed by BA.
Iron, nickel and cobalt.
Are all metals ferromagnetic?
No.
Is copper normally ferromagnetic?
No.
Is aluminium normally ferromagnetic?
No.
What is a magnetic domain?
A microscopic region whose magnetic moments are aligned.
How does BA describe a magnetic domain?
Like a tiny magnet with a north and south pole.
How are magnetic domains generally arranged in an unmagnetised ferromagnetic material?
Their directions are largely random.
Why does an unmagnetised ferromagnetic object have little overall magnetic effect?
Its domains' magnetic effects largely cancel.
What happens to magnetic domains during magnetisation?
More domains align in a common direction.
What happens to the overall magnetic field as more domains align?
It becomes stronger.
What does BA Figure 1 illustrate?
Randomly oriented magnetic domains in unmagnetised material.
What does BA Figure 2 illustrate?
Magnetic domains aligned by an applied magnetic field.
What happens when magnetic domains become fully aligned?
The material approaches magnetic saturation.
Does increasing the magnetising current greatly increase flux density after saturation?
No; there is little further increase.
What are four magnetisation methods listed by BA?
Exposure to a magnetic field, magnetic stroking, a current-carrying coil, and cooling through the Curie temperature in a field.
How can an external magnetic field magnetise ferromagnetic material?
It encourages its magnetic domains to align.
What can remain after an external magnetic field is removed?
Residual magnetism.
How can an existing magnet be used to magnetise a steel object?
Stroke it repeatedly in the same direction.
How can electric current be used to magnetise a ferromagnetic material?
Pass current through a coil surrounding it.
What happens to the domains when current magnetises an iron core?
They increasingly align with the applied field.
What can happen when steel is hammered while aligned with the Earth's magnetic field?
It may acquire some magnetisation.
How does BA describe producing strong permanent magnetisation through heating and cooling?
Cool the material in a magnetic field after heating above its Curie temperature.
What is demagnetisation?
Reduction or removal of an object's magnetisation.
What is another name for demagnetisation?
Degaussing.
What are three demagnetisation methods discussed by BA?
Heating above the Curie point, a diminishing alternating magnetic field, and mechanical shock.
What happens to ferromagnetic ordering above the Curie temperature?
The material loses its ferromagnetic magnetisation.
What is the Curie temperature?
The temperature above which a ferromagnetic material loses its ferromagnetic ordering.
How can an alternating magnetic field demagnetise an object?
Apply a sufficiently strong alternating field and gradually reduce it to zero.
Why is a decreasing AC field effective for demagnetisation?
It progressively reduces the material's residual magnetisation.
How can hammering or jarring affect magnetisation?
It can disturb domain alignment and reduce magnetisation.
Can a sufficiently strong reverse magnetic field demagnetise a magnet?
Yes.
What happens if a reverse magnetic field exceeds a material's coercivity?
It can demagnetise or reverse the material's magnetisation.
How does increasing temperature generally affect a permanent magnet?
It tends to weaken its magnetisation.
Why does heating generally weaken a permanent magnet?
Thermal effects disturb the alignment of magnetic moments.
What happens to magnetisation as the Curie temperature is approached?
It decreases towards the loss of ferromagnetic ordering.
What happens to a ferromagnetic material at the Curie point?
It loses its ferromagnetic properties.
Does cooling a magnet after severe overheating necessarily restore its original strength?
No; remagnetisation may be required.
What is a permanent magnet?
A magnet that retains magnetisation without continuous external excitation.
What are the two poles of a magnet called?
North and south.
What happens when two north magnetic poles approach?
They repel.
What happens when two south magnetic poles approach?
They repel.
What happens when a north and south pole approach?
They attract.
What is the general rule for magnetic pole interactions?
Like poles repel and unlike poles attract.
Can magnetic poles exert forces without physical contact?
Yes.
What produces the force between magnetic poles?
The magnetic field.
What generally happens to magnetic-field strength as distance from a magnet increases?
It decreases.
Where is a bar magnet's field generally strongest?
Near its poles.
What does BA Figure 3 demonstrate?
A magnet attracts some materials but not wood, rubber, foam or paper.
Why can a magnet attract a steel paper clip?
Steel is ferromagnetic.
Why does an ordinary magnet not strongly attract a piece of wood?
Wood is not ferromagnetic.
What is a magnetic field?
The region in which magnetic effects or forces can occur.
Can magnetic field lines be seen directly?
No; they are a visual representation.
What can be used to reveal the pattern of a magnetic field?
Iron filings.
Why do iron filings form patterns near a magnet?
They align with the local magnetic field.
What do closely spaced magnetic field lines indicate?
A stronger field.
What do widely spaced magnetic field lines indicate?
A weaker field.
What direction do magnetic field lines follow outside a bar magnet?
From north to south.
What direction do magnetic field lines follow inside a bar magnet?
From south to north.
Do magnetic field lines form open-ended paths?
No; they form continuous closed loops.
Can magnetic field lines cross each other?
No.
What does the direction of a magnetic field line represent?
The direction of the magnetic field at that point.
What is magnetic flux?
The total magnetic field passing through a specified surface.
What symbol represents magnetic flux?
Φ (phi).
What is the SI unit of magnetic flux?
Weber (Wb).
What other name does BA use for magnetic field lines?
Lines of force.
What does BA state about magnetic flux lines and their length?
They tend to shorten themselves.
What does BA state about neighbouring magnetic flux lines?
They tend to repel sideways.
What path do magnetic flux lines preferentially follow?
A path of low magnetic reluctance.
Can ordinary electrical insulation completely block magnetic flux?
No.
What are four magnetic-field-line properties given by BA?
They form closed loops, never cross, have direction and are closer together in stronger fields.
What does BA Figure 7 illustrate?
The continuous closed-loop pattern of magnetic flux around a magnet.
What is responsible for generating most of the Earth's magnetic field?
Electrical currents associated with moving conductive material in the fluid outer core.
What conductive material is important in the Earth's outer core?
Molten iron.
What process is associated with generating the Earth's magnetic field?
The geodynamo.
What other regions contribute to the Earth's observed magnetic field?
The crust, ionosphere and magnetosphere.
How does a compass behave in the Earth's magnetic field?
Its needle tends to align with the local field.
Do the Earth's magnetic and geographic poles exactly coincide?
No.
What can affect a compass placed near another magnet?
The nearby magnet's field may dominate the Earth's field.
Why can magnetic objects interfere with compass readings?
They alter the magnetic field acting on the needle.
What is the north-seeking pole of a compass?
The end that points approximately towards geographic north.
What magnetic polarity is associated with the Earth near geographic north?
Magnetic south polarity in the simple bar-magnet model.
Why does the north-seeking end of a compass point towards geographic north?
It is attracted towards the Earth's opposite magnetic polarity.
What determines the magnetic behaviour of a material?
Its microscopic structure and electron configuration.
What four magnetic material categories are identified by BA?
Ferromagnetic, ferrimagnetic, paramagnetic and diamagnetic.
What is ferromagnetism?
Strong magnetic ordering that can produce persistent magnetisation.
What is a common example of a ferromagnetic material?
Iron.
Name two other ferromagnetic metals.
Nickel and cobalt.
What is ferrimagnetism?
Magnetic ordering with opposing unequal magnetic contributions that leave a net magnetisation.
What materials are given as ferrimagnetic examples in BA?
Ferrites, magnetite and lodestone.
What is paramagnetism?
Weak attraction to an externally applied magnetic field.
Name three paramagnetic materials listed by BA.
Platinum, aluminium and oxygen.
Are paramagnetic materials strongly or weakly attracted to ordinary magnets?
Weakly.
Do paramagnetic materials normally retain strong permanent magnetisation?
No.