3.10 Magnetism Flashcards
What has been the historical significance of magnetism?
Utilised since the invention of the magnetic compass during the Chinese Han dynasty, around 2000 years ago.
List some applications of permanent magnets.
- Motors
- Generators
- Microphones
- Loudspeakers
- Hard drives
- Sensors
- Switches
Who began the discovery of the connection between electricity and magnetism?
Michael Faraday in 1831.
What is the major discipline that emerged from the study of electricity and magnetism?
Electromagnetism.
What is magnetism caused by?
The motion of electric charges.
What is the region in which magnetic effects are observed called?
Magnetic field.
Name three elements that are described as being magnetic.
- Iron
- Nickel
- Cobalt
What happens to items containing magnetic elements when near permanent magnets?
They are attracted and can be magnetised.
Why do the magnetic fields of most elements cancel out?
Due to the many electrons present and their arrangements.
True or False: Each atom of a magnetic element acts like a small, permanent magnet.
True.
Fill in the blank: The understanding of electromagnetism is fundamental to the design and maintenance of _______.
[electrical components and systems].
What is the effect of the particular arrangements of electrons in magnetic elements?
They create a small, residual magnetic effect.
What is the Earth’s magnetic field also known as?
Geo-magnetic Field
How is the Earth’s magnetic field generated?
By a self-exciting dynamo process in the planet’s interior
What generates the magnetic field in the Earth’s outer core?
Electrical currents flowing in slowly moving molten iron
What is the process in the Earth’s outer core likened to?
A naturally occurring electrical generator
What causes the deviation of a compass from true north?
Pockets of magnets (Lodestones) in the crust or rocks
What is the angle called that represents the deviation of the compass from true north?
Declination or magnetic declination
What is a significant problem for aircraft relying on magnetic compasses?
The variation of magnetic declination with geographic location and time
If the Earth is pictured as a permanent magnet, which pole attracts the north pole of a compass?
The south pole of the Earth
What is the north magnetic pole?
The point at which the Earth’s magnetic field lines point vertically downwards
What happens when a permanent magnet is freely suspended?
It comes to rest with one end pointing towards the north pole of the Earth
What is the north-seeking pole of a permanent magnet?
The end that points towards the north pole of the Earth
What is the primary use of a magnetic compass?
To find the direction of magnetic north and therefore all other directions
What causes the compass needle to deviate from magnetic north?
Pockets of magnets and shifting magnetic fields
What does a magnetic compass contain that allows it to function?
A small, light magnet called a needle
What happens to the compass needle in the presence of a stronger magnetic field?
It aligns with the field around the stronger magnet
What are the three typical representations of the Earth’s magnetic fields?
A: Theoretical magnetic field with smooth lines, B: Kinks causing deviation, C: Snapshot of shifting magnetic fields
Fill in the blank: The deviation of the compass from true north is an angle called _______.
declination
What materials does a permanent magnet attract?
Magnetic materials containing one or more magnetic elements.
What is the force of attraction of a magnet strongest?
At each end (pole) of the magnet.
What happens when like poles of magnets interact?
They repel each other.
What happens when unlike poles of magnets interact?
They attract each other.
What is a plotting compass used for?
To map the direction of magnetic force around a magnet.
What do the lines produced by a plotting compass represent?
Lines of magnetic flux.
What direction does magnetic flux flow according to the model?
From north to south around the magnet.
What happens to the strength of the magnetic field where lines of magnetic flux are closer together?
The magnetic field is stronger.
What is the shape of the lines of magnetic flux?
Closed loops.
Where do lines of magnetic flux exit and enter a magnet?
Exit at the north pole and enter at the south pole.
What direction do lines of magnetic flux point externally?
Away from the north pole and towards the south pole.
Fill in the blank: Magnetic lines of flux always form _______.
closed loops.
True or False: Magnetic lines of flux can cross each other.
False.
What is the effect of magnetic lines of flux on each other?
They repel each other sideways.
What is the behavior of magnetic lines of flux in terms of their length?
They try to shorten themselves.
What path do magnetic lines of flux always take?
The path of least reluctance.
Is there a known insulator for magnetic lines of flux?
No known insulator exists.
What are Weiss domains?
Small magnets within materials where the magnetic fields of individual atoms are aligned in the same direction
Weiss domains are crucial in understanding how materials can become magnetised.
What happens to an iron bar when subjected to a strong magnetic field?
The magnetic domains align in the same direction, reinforcing each other’s magnetic fields
This results in the iron bar becoming magnetised and exhibiting a magnetic field.
What is a permanent magnet?
An iron bar that has been magnetised and exhibits a magnetic field
The alignment of Weiss domains contributes to its permanent magnetism.
List methods for magnetising materials.
- Stroking with another magnet
- Hammering in the direction of a magnetic field
- Heating in a magnetic field
- Passing an electric current through a coil (electro-magnet)
These methods utilize various physical principles to align the magnetic domains.
What occurs when a permanent magnet is broken into pieces?
Each piece acts as a smaller permanent magnet with its own north and south pole
This phenomenon illustrates that magnetism is a property of the material at a fundamental level.
What is demagnetisation?
The process of removing magnetism from a workpiece or tool
Demagnetisation may be necessary for various applications, especially in electronics.
How can demagnetisation be achieved?
- Turning off the current of an electro-magnet
- Slowly moving away from a strong alternating magnetic field (degaussing)
- Using mechanical force (striking or dropping)
- Heating beyond the Curie temperature
Each method affects the alignment of magnetic domains in different ways.
What is degaussing?
The process of slowly moving an item away from a strong alternating magnetic field to demagnetise it
This is often necessary for components after exposure to strong electromagnetic events like lightning strikes.
What is the Curie temperature for iron?
769 °C
Heating beyond this temperature may demagnetise iron, but magnetism can recur if the temperature falls below it.
What happens if you heat a magnetic material beyond its Curie temperature?
The magnetism may be lost, but it can recur if the temperature drops below the Curie temperature
Regular exposure to high temperatures can lead to permanent loss of magnetism.
Fill in the blank: The Curie temperature for cobalt is _______.
1075 °C
This temperature is significant for understanding the thermal stability of cobalt’s magnetic properties.
True or False: Nickel has a Curie temperature of 356 °C.
True
This temperature is essential for applications involving nickel in magnetic fields.
What range of Curie temperatures do some ferrites have?
50 – 600 °C
Ferrites exhibit varying thermal properties which can affect their magnetic behavior.
What is magnetic shielding?
Magnetic shielding is used to create a shield for a device or component that requires protection from magnetic interference.
What does a steel ring do in magnetic shielding?
The steel ring attracts the lines of magnetic flux to itself because it has high permeability.
What happens to components placed inside a steel ring in the context of magnetic shielding?
Components placed inside the ring should not be subjected to a magnetic field applied in the direction shown.
True or False: Nothing can stop magnetic flux from flowing from north to south.
True.
What is the principle behind magnetic shielding?
It redirects lines of magnetic flux using good conductors.
What are some examples of devices that may require magnetic shielding?
Electro-mechanical instruments such as ammeter and voltmeters.
What can affect electro-mechanical instruments in legacy aircraft?
Stray magnetic fields, such as the earth’s and fields created in cables when a current is passing through them.
How can stray magnetic fields be mitigated in sensitive instruments?
By encasing them in a metal case.
What two materials are compared in terms of reluctance in magnetic screening?
Air and soft iron.
Fill in the blank: Air has a high _______ while soft iron has a low _______.
reluctance.
What happens to magnetic flux when equipment is surrounded by soft iron?
Most of the flux will pass through the soft iron, rather than the air inside it.
What is reluctance analogous to?
Resistance.
What property does soft iron exhibit when under the influence of a magnetizing force?
Soft iron readily magnetizes and demagnetizes.
What are the three types of magnetic materials?
Ferromagnets, Paramagnets, Diamagnets
What is a characteristic of ferromagnetic materials?
Readily magnetises and easily orient their electron spins to an external magnetic field
What distinguishes soft ferromagnetic materials from hard ferromagnetic materials?
Soft materials become demagnetised spontaneously; hard materials retain their magnetism
List some applications of ferromagnetic materials.
- Electric motors
- Generators
- Transformers
- Telephones
- Loudspeakers
- Magnetic stripes on credit cards
Name common examples of ferromagnetic materials.
- Iron
- Cobalt
- Nickel
- Metallic alloys
- Rare earth magnets
What is magnetite?
A ferromagnetic material formed by the oxidation of iron into an oxide
What is the Curie temperature?
The maximum temperature where the ferromagnetic property disappears due to thermal agitation
What is the Curie temperature of iron?
About 1043 K
What is hysteresis in ferromagnetic materials?
The tendency to remember their magnetic history after an external magnetic field is removed
What is remanence?
The fraction of saturation magnetisation retained when the driving field is removed
What is magnetostriction?
The mechanical response of ferromagnetic materials to an impressed magnetic field
What are paramagnetic materials?
Materials with a small and positive susceptibility to magnetic fields
List some examples of paramagnetic materials.
- Aluminium
- Titanium
- Oxygen
- Iron oxide
What occurs to paramagnetic materials when the external magnetic field is removed?
They do not retain their magnetic properties
What is Curie’s law in relation to paramagnetic materials?
Magnetic properties decrease as temperature increases at a constant external magnetic field
What are diamagnetic materials?
Materials that are repelled by a magnetic field
Give examples of diamagnetic materials.
- Water
- Plastics
- Gold
- Copper
- Mercury
How can you determine if an object is diamagnetic or paramagnetic?
By observing how it aligns itself in a magnetic field
What causes the repulsion in diamagnetic materials?
Induced magnetic field occurs in the opposite direction of the external magnetic field
What distinguishes diamagnetic materials from paramagnetic and ferromagnetic materials?
Diamagnetic materials have all paired electrons, while others have unpaired electrons
What is an electromagnet?
A type of magnet where the magnetic field is produced by an electric current.
What happens to the magnetic field of an electromagnet when the current is turned off?
The magnetic field is not present.
What are the main components of an electromagnet?
- Insulated wire wound into a coil
- A magnetic core made from ferromagnetic or ferrimagnetic material
What is a ‘turn’ in the context of an electromagnet?
Each loop of the coil.
What is the advantage of an electromagnet over a permanent magnet?
The magnetic field can be quickly changed by controlling the amount of electric current.
Does an electromagnet require a power supply?
Yes, it requires a continuous supply of electric current.
List some uses of electromagnets in aircraft systems.
- Motors
- Generators
- Relays
- Loudspeakers
- Hard-disc drives
Why is it difficult to obtain very large strength magnetic fields with permanent magnets?
The material becomes magnetically saturated.
What is the principle of electromagnetism?
A magnetic field is created by passing an electric current through a wire.
Describe the shape of the magnetic field produced by a straight conductor.
Circular and concentric to the conductor.
What happens to iron filings sprinkled around a conductor when current passes through it?
The filings take up a shape indicating the magnetic field.
What indicates the strength of a magnetic field?
The concentration of the lines of flux; tighter lines indicate a stronger field.
What three components are required to create an electromagnet?
- A conductor (usually a helical coil)
- An electrical supply and associated circuitry
- A core to increase strength and efficiency
What occurs when the switch of a solenoid is closed?
The current flows through the coil, producing a magnetic field around the conductor.
Fill in the blank: An electromagnet is sometimes known as a _______.
[solenoid]
True or False: Electromagnets do not require any external power supply.
False.
What is generated around a conductor when an electric current flows through it?
A magnetic field
Every electric current produces a magnetic field.
What happens to a compass needle placed near a wire carrying a current?
It is deflected due to the magnetic field
If the current is sufficiently large, the compass needle comes to rest in a direction perpendicular to the wire.
What is the Corkscrew Rule also known as?
- Maxwell’s Right Hand Thumb Rule
- Right Hand Grasp Rule
- Right Hand Grip Rule
- Corkscrew Rule
It predicts the direction of the magnetic field around a current-carrying conductor.
How does the Corkscrew Rule predict the direction of the magnetic field?
By visualizing the corkscrew being turned in a clockwise direction
This clockwise direction indicates the magnetic field direction when viewed from above.
In the Corkscrew Rule, what does the direction of the corkscrew’s movement represent?
The direction of current flow in the conductor
The corkscrew moves away from the observer as the current flows.
How is the direction of current flow in a conductor typically represented?
As an arrow
A cross represents current flowing into the page, while a dot represents current flowing out of the page.
What is the Hand Clasp Rule used for?
To determine the direction of the magnetic field lines and current around a straight current-carrying conductor, solenoid, or coil inductor
It utilizes the right hand to show the relationship between current and magnetic field direction.
In the Hand Clasp Rule, what does the thumb represent?
The direction of current
The curled fingers show the direction of the magnetic field lines of force.
What does the Right Hand Rule for Determining Polarity of a Solenoid indicate?
The direction of current flow determines the polarity of the coil
Clockwise current flow results in a South Pole at one end and an North Pole at the other end.
What happens to the polarity of a coil if the current flows anti-clockwise?
The end of the coil becomes the North Pole and the other end becomes the South Pole
This is determined when facing the coil’s end.
What is magnetomotive force (mmf)?
A measure that causes a magnetic flux in a circuit, similar to electric current
mmf is produced by current flowing through a coil and is often expressed in ampere-turns (At)
How is magnetomotive force calculated?
mmf = Current x Number of Turns (At)
F = I x N, where I is the current in amps and N is the number of turns
What is the SI unit of magnetomotive force?
Ampere (A)
Although often expressed as ampere-turns (At), this does not conform to SI standards
What is magnetic field strength (H)?
A measure of the intensity of magnetic effects at any point in the magnetic field
Defined as mmf per unit length, measured in ampere-turns per metre (At/m)
How is magnetic field strength calculated?
H = IN/l
Where I is the current, N is the number of turns, and l is the length of the magnetic circuit in metres
What is the relationship between current flow and magnetic field strength?
Magnetic field strength is proportional to current flow
If current increases, magnetic field strength increases and vice versa
What is magnetic flux (Φ)?
The total magnetic flux in a magnetic circuit, comparable to current flow in an electric circuit
Once established, maintaining flux does not require energy dissipation
How is magnetic flux calculated?
Φ = B x A
Where B is the flux density and A is the cross-sectional area of the magnetic circuit
What is the SI unit of magnetic flux?
Weber (Wb)
What is magnetic flux density (B)?
The amount of flux per unit cross-sectional area in a magnetic field
It is analogous to current density in an electric circuit
How is magnetic flux density calculated?
B = Φ / A (T)
Where Φ is magnetic flux and A is the area in m²
What is permeability (μ)?
The ratio of flux density (B) to magnetic field strength (H)
In free space, represented by μo and valued at 4π x 10^-7 (1.257 x 10^-6) in SI units
What effect does inserting an iron core into a coil have on flux density?
Increases the flux density
This is due to the higher permeability of the core material
What is relative permeability (μr)?
The ratio of flux density of the core to flux density of air
The relative permeability of air is considered to be one
How is absolute permeability (μ) defined?
μ = μoμr
Where μo is the permeability of free space and μr is the relative permeability of the medium
True or False: Magnetic flux density weakens with increasing distance from a straight current-carrying wire.
True
What is the relationship between magnetic flux density and current in the vicinity of a current-carrying wire?
Magnetic flux density is directly proportional to the current in amperes
What does the hysteresis loop show?
The relationship between the applied magnetising force H and the resulting flux density B over one complete cycle of alternating current.
What happens to flux density B when the core is saturated?
It cannot conduct any more flux, resulting in a plateau in the graph.
What is residual magnetism?
The magnetism that remains in a material after the applied field drops to zero.
Define magnetic remanence.
The residual magnetism represented by the value Br, which indicates the internal flux density after the magnetising force is removed.
What is coercive field strength Hc?
The amount of applied field needed to reduce the flux density B to zero after achieving magnetic remanence.
What is the significance of the saturation point?
It is the point at which increasing the magnetic field no longer increases magnetisation.
What is permeability?
The ratio of Flux Density (B) to the magnetising force (H).
What is the standard value of permeability of free space?
4π x 10^-7 H/m.
What does retentivity measure?
The ability of a material to retain a certain amount of residual magnetic field after the magnetising force is removed.
What is coercive force?
The amount of reverse magnetic field needed to return a material’s magnetic flux to zero.
Define reluctance.
The opposition that a ferromagnetic material shows to the establishment of a magnetic field.
What is the unit of reluctance?
At/Weber.
What are eddy currents?
Loops of electrical current induced within conductors by a changing magnetic field.
Fill in the blank: Eddy currents flow in closed loops, along planes ______ to the magnetic field.
perpendicular
What effect do eddy currents have according to Lenz’s law?
They create a magnetic field that opposes the magnetic field that created them.
True or False: Eddy currents can only be induced in moving conductors.
False
What is a key application of eddy currents?
Induction heating furnaces.
How can eddy currents be minimized in machinery?
By using laminated magnetic cores.
What does the area inside the hysteresis curve represent?
The energy lost from the circuit per unit volume of the core, mainly as heat.
What characterizes magnetically hard materials?
They are difficult to magnetise and demagnetise.
What characterizes magnetically soft materials?
They are easy to magnetise and demagnetise.
What type of materials are strong permanent magnets made from?
Magnetically hard materials
What happens to the magnetic field strength of permanent magnets if they are struck or dropped?
It is reduced
What should be avoided to prevent damage to permanent magnets?
Intense vibration
Why should permanent magnets be kept away from external magnetic fields?
To prevent demagnetisation
What is the maximum temperature that can be tolerated by magnets before reduction in magnetic flow occurs?
250°C
What happens to magnets at temperatures higher than 250°C?
Permanent demagnetisation occurs
What can be done to revert the magnetic properties of a magnet after reducing its temperature?
Decreasing the temperature will revert it to its previous state
What must not be changed when storing magnets long-term?
The magnetic circuit
What are ‘keepers’ in relation to magnet storage?
Ferromagnetic components connected between the poles of the magnet
What is the purpose of using keepers when storing magnets?
To allow lines of flux to flow through them
