3.10: Magnetism Flashcards
- When the north poles of two bar magnets are brought together there will be;
(a) A force of attraction.
(b) A force of repulsion,
(c) An upward force.
(b) A force of repulsion,
- A magnetic field is made up of;
(a) Pos and Neg Charges,
(b) Magnetic grains.
(c) Flux lines
(c) Flux lines
- Magnets would usually lose their magnetism when the:
(a) magnetizing force is removed
(b) magnet is immersed in water
(c) magnets receive heavy vibration/shock
(c) magnets receive heavy vibration/shock
2 ways to demagnetise object:
- Degaussing tool
- Hammering (Shock) / vibration
- through which material will magnetic lines of force pass most readily?
(a) Copper
(b) Iron
(c) Aluminum
(b) Iron
Materials that are attracted by a magnet such as iron, steel, nickel, and cobalt, have the ability to become magnetized
Non magnetic materials:
Brass (copper and zinc), aluminium, copper, zinc, paper, wood, glass and tin
'’Iron is an extremely well-known ferromagnetic metal. It is, in fact, the strongest ferromagnetic metal
- The direction of a magnetic field is from;
(a) North to south.
(b) South to north,
(c) Back to front.
(a) North to south.
Note: The magnetic field flows from North to South outside the magnet but flows from South to North inside the magnet.
- Reluctance is;
(a) The opposition to the lines of flux.
(b) The ease, which lines of flux can flow,
(c) The sensitivity of the material.
(a) The opposition to the lines of flux.
The opposition that a material offers to the magnetic lines of force is called reluctance.
A material with a low reluctance (R), which means magnetic fields find it easy to enter the material such as soft iron or annealed silicon steel
- The unit of flux lines is the;
(a) The tesla.
(b) The Weber.
(c) Ampere-turn
(b) The Weber.
- The unit of flux density is the;
(a) The Tesla.
(b) The Weber,
(c) Ampere-turn.
(a) The Tesla.
- In a magnetic circuit the magnetic flux may be compared with what quantity in an electrical circuit?
(a) voltage
(b) current
(c) resistance
(b) current
- Introducing an air gap into a magnetic circuit has which of the following effects :
(a) decreases the reluctance
(b) has no effect
(c) increases the reluctance
(c) increases the reluctance
'’The reluctance of soft iron, for instance, is much lower than that of air’’
- Faraday’s laws of electromagnetic induction are related to :
(a) the e.m.f. of a generator
(b) the e.m.f. of a chemical cell
(c) the current flowing in a conductor
(a) the e.m.f. of a generator
Faraday’s law
Faraday’s law of induction states that the induced electromotive force in a closed loop of wire is directly proportional to the time rate change of magnetic flux through the loop’’
- When a conductor is moving parallel to magnetic lines of force, what relative value of emf is induced?
(a) Minimum
(b) Maximum
(c) Average
(a) Minimum
- Which of the following statements accurately describes the magnetic field surrounding a current-carrying conductor?
(a) It is parallel to and equal along all parts of the conductor
(b) It is parallel to and maximum at the most negative part of the conductor
(c) It is perpendicular to and equal along all parts of the conductor.
(c) It is perpendicular to and equal along all parts of the conductor.
- A material with low reluctance and high permeability such as iron or soft steel is used to make what type of magnet?
(a) Temporary
(b) Permanent
(c) Natural
(a) Temporary
'’A temporary magnet, produced from a material with a low reluctance, would have a high permeability (i.e its highly permeable)
- A paramagnetic material has a relative permeability
(a) less than unity
(b) greater than unity
(c) equal to unity
(b) greater than unity
- A paramagnetic material has a relative permeability of
(a) greater than unity
(b) less than unity
(c) zero
(a) greater than unity
- The property of a material to accept lines of flux is called
(a) permeability
(b) reluctance
(c) retentivity
(a) permeability
Note: Reluctance is how resistive the medium is
Permeability (opposite of reluctance) is a measure of how easy it is for the magnetic field to flow through a medium
- Vibration in a magnet causes
(a) flux to increase
(b) flux to decrease
(c) flux to stay the same
(b) flux to decrease
‘’The physical disruption and vibration caused by repeated shock, hammering or vibration of a magnet shakes the order of the magnetic domains within the material, and thus demagnetises it.’’
- An electromagnets magnetizing force would be increased if:
(a) its weight were increased
(b) it had an iron core
(c) it had an air core
(b) it had an iron core
Materials like soft iron have a high permeability. The magnetic field will find it easier to go through an iron core than travel through air.
Making the core of soft iron instead of air therefore significantly increases the magnetic field strength produced
- The ability of a material to reject lines of flux is called:
(a) Permeability
(b) Resistance
(c) Reluctance
(c) Reluctance
Reluctance is how resistive the medium is
Permeability (opposite of reluctance) is a measure of how easy it is for the magnetic field to flow through a medium
- What property do ferromagnetic materials posses?
(a) high reluctance
(b) high resistance
(c) low reluctance
(c) low reluctance
Remember: Diamagnetic = magnet DIE = repulsion = permeability less than 1
Paramagnetic = paraolympics = wheelchairs = going towards magnet but slowly = permeability just greater than 1
Ferromagnetic = Ferrits are fast (or are they? what even is a ferrit?) = strong force towards magnet = permeability much greater than 1
- What property do ferromagnetic materials posses?
(a) high reluctance
(b) high permeability
(c) low permeability
(b) high permeability
Remember: Diamagnetic = magnet DIE = repulsion = permeability less than 1
Paramagnetic = paraolympics = wheelchairs = going towards magnet but slowly = permeability just greater than 1
Ferromagnetic = Ferrits are fast (or are they? what even is a ferrit?) = strong force towards magnet = permeability much greater than 1
- In order to increase magnetic field strength around an electromagnet, one should increase:
(a) The number of turns and the separation between them.
(b) The number of turns and decrease the separation between them.
(c) The cross-sectional area of the conductor without changing the number of turns.
(b) The number of turns and decrease the separation between them.
‘’A coil or solenoid is simply a conductor formed into a number of loops. Can have core of air or iron, iron increases magnetic field as more permeable. The magnetic fields from each turn will merge together producing a magnetic field which is very similar to a bar magnet, called an electromagnet. To increase the strength of the magnetic field (without changing the core material) the number of turns and/or the current must be increased’‘
- Which of the following coils has the greatest magnetizing force
(a) 1 turn and 1 meter long
(b) 10 turns and 1 meter long
(c) 10 turns and 10 meters long
(b) 10 turns and 1 meter long
’A coil or solenoid is simply a conductor formed into a number of loops. Can have core of air or iron, iron increases magnetic field as more permeable. The magnetic fields from each turn will merge together producing a magnetic field which is very similar to a bar magnet, called an electromagnet. To increase the strength of the magnetic field (without changing the core material) the number of turns and/or the current must be increased’‘
- The MMF of a coil fed with 2 amps and having 10 turns is
(a) 5 ampere turns
(b) 20 ampere turns
(c) 20 amperes / turn
(b) 20 ampere turns
‘’When current is flowing in a solenoid (windings of coil forming electromagnet) it produces a Magnetic Motive Force (MMF) and its value is a product of the current and the number of turns on the coil, NI or Ampere Turns (AT).
MMF ‘’drives’’ magnetic flux through a magnetic circuit, just like voltage drives current through an electrical circuit.
MMF = N x I where N = number of turns and I = Current
- Three ways of increasing the strength of an electromagnet are:
(a) Decrease the number of turns, increase the current and use higher permeability material in the core.
(b) Increase the number of turns, increase the current and use higher permeability material in the core.
(c) Increase the number of turns, increase the voltage and increase the cross-sectional area of the coil.
(b) Increase the number of turns, increase the current and use higher permeability material in the core.
Materials like soft iron have a high permeability. The magnetic field will find it easier to go through an iron core than travel through air. To increase the strength of the magnetic field (without changing the core material) the number of turns and/or the current must be increased.
- The hysteresis loop for a magnetic material is on a graph with
(a) total flux against flux density
(b) current against flux density
(c) flux density against magnetising force
(c) flux density against magnetising force
‘’A hysteresis loop is a B-H curve under the influence of an AC magnetizing force. Values of flux density B are shown on the vertical axis and are in Tesla. Magnetizing force H is plotted on the horizontal axis. It shows how much flux is obtained for a given magnetising force.’’
Remember Hysteresis means something behaves one way when being increased and another way when being decreased.
- To determine the direction of the magnetic field around a conductor you would use
(a) the corkscrew rule
(b) Fleming’s left hand rule
(c) Fleming’s right hand rule
(a) the corkscrew rule
To determine the direction of the lines of magnetic flux around a conductor, Maxwells Corkscrew rule and the right hand grasp rule are used.
Note: Flemings let hand rule is used for motors
- A material with a narrow hysteresis loop
(a) will have high retentivity
(b) will have low retentivity
(c) cannot be magnetised
(b) will have low retentivity
‘’The ability of a material to retain an amount of residual magnetism is called the retentivity of the material’’
- If a bar magnet is cut in half
(a) the magnet is destroyed
(b) two bar magnets are formed
(c) one bar magnet and one non-magnet is formed
(b) two bar magnets are formed
- Copper is a
(a) paramagnetic materials
(b) ferromagnetic material
(c) diamagnetic material
(a) diamagnetic material
- Storage of magnets should be
(a) in pairs end to end
(b) in a non magnetic pox
(c) in pairs with keeper plates
(c) in pairs with keeper plates
‘’Modern magnet materials do lose a very small fraction of their magnetism over time. A magnet should always be stored with a keeper, a soft iron bar used to join the magnetic poles. By using the keeper while the magnet is being stored, the magnetic flux will continuously circulate through the magnet and not leak off into space’’
- Of the following which pair of materials would most readily become magnetized?
(a) copper and steel
(b) nickel and bronze
(c) iron and steel
(c) iron and steel
‘’Ferromagnetic materials are those which are relatively easy to magnetize and demagnetise, such as iron, steel, cobalt and the alloys such as Alnico and Permalloy.
Diamagnetic (non-magnetic) - Water, Copper and Gold
Paramagnetic - (slightly magnetic) - Sodium and Aluminium
Ferromagnetic - (very magnetic) - Steel/iron
- When magnetizing a piece of material, magnetic strength will rise
(a) linearly with coercive force
(b) linearly with magnetic force
(c) non-linearly with magnetic force
(c) non-linearly with magnetic force
‘’In magnetic circuits, hysteresis is the property whereby when increasing the magnetising force on a sample of material it behaves in one way but when decreasing the magnetising force it behaves in a different manner due to retained magnetism’’
- The core material used for an electromagnet is soft iron because:
(a) it is lighter than hard steel
(b) it de-magnetizes easily
(c) its magnetism is not easily destroyed.
(b) it de-magnetizes easily
‘’Ferromagnetic materials are those which are relatively easy to magnetize and demagnetise, such as iron, steel, cobalt, and the alloys such as Alnico and Permalloy’’
- Which of the following would be classified as a ferromagnetic material? (a) Copper
(b) Iron
(c) Bakelite
(b) Iron
‘’Ferromagnetic materials are those which are relatively easy to magnetize and demagnetise, such as iron, steel, cobalt, and the alloys such as Alnico and Permalloy’’
Note: Copper is considered Diamagnetic (doesn’t magnetise)
- The ability of a material to reject lines of magnetic flux is called:
(a) Permittivity
(b) Permeability
(c) Reluctance
(c) Reluctance
Option A: below a certain temperature
Option C: diamagnetic material
Option A: 20
Note: When current is flowing in a solenoid it produces a Magnetic Motive Force (MMF) and its value is a product of the current and the number of turns on the coil, NI or Ampere Turns (AT)
MMF ‘’drives’’ magnetic flux through a magnetic circuit, just like voltage drives current through an electrical circuit. MMF = N * I
Option B: Soft iron
Option B: Right Hand Clasp Rule
Option C: Magnetic poles
‘’Flux density is greatest at the magnetic poles, where all the flux lines converge’’
Option C: repulsion between them will be increased
Option C: Equator
‘’Magnetic dip is another name for inclination. It is least at the equator, and maximum (90 degrees) at the magnetic poles’’
Option C: can be used to store binary code
‘’Magnetic tape uses tiny ferrite particles coated onto a plastic tape’’
Option A: multiplied by current
Ampere turns means AMPS * TURNS
Option C: High remanence
Option C: repel each other
‘’Assuming this means parallel AND in the same direction, they will repel each other. Like the magnetic fields around two wires, carrying current in opposite directions’’
Option A: equator
Option B: attract
Option B: weakest outside the coil
Option C: u(micro)
Option C: B(capital)
Elon, owner of Tesla, dense brain - flux density also owns Boring company (B for Boring, flux density, Tesla)
Option C: greater than unity
‘’A paramagnetic material has a greater permeability than free space (vacuum). so its relative permeability is greater than 1’’
Option B: Iron
‘’Iron (especially soft iron) has the greatest permeability’’
Option A: high reluctance, high coercive force
Coercivity is the resistance of a magnetic material to changes in magnetization, equivalent to the field intensity necessary to demagnetize the fully magnetized material
Option C: flux density
‘’Flux density is the number of flux lines per unit cross sectional area’’
Option B: it has HIGH permeability and LOW coercivity
Note: Coercivity is the resistance of a magnetic material to changes in magnetization
Option B: has no permeability
Option C: The flux density of the original magnetic field produced by the coil is increased
Option C: increase linearly with magnetic flux
‘’Flux density = magnetic flux lines per unit cross sectional area’’
Option B: flux density against magnetising foce
Option C: diamagnetic material
Option C: 20 ampere turns
MMF (symbol H) is amps * turns. Unit is Ampere Turns
Option B: iron
‘’Iron is a paramagnetic (ferromagnetic actually), the other 2 are diamagnetic’’
Option B: flux to decrease
‘’A good way to destroy a magnet is to drop it or subject it to high frequency vibrations’’
Option A: flux density / MMF
Option C: be harder to magnetise
‘’The ‘curie’ temperature is the temperature above which the material cannot be magnetised’’
Option A: ferromagnetic
Option A: greater than unity
‘’Cobalt is a hard ferromagnetic silver-white element’’
Option A: exist in all space around the magnet
Option B: permeability
Option A: the magnetised medium will accept no further lines of flux
Option A: Copper braiding
Option B: ferromagnetic materials
B. Soft iron
B. It concentrates the magnetic field
C. Iron
C. Tesla
B
B. It is easy to magnetise and demagnetise
A. The magnetic north pole
B. High permeability
A. Strongly influenced by magnetic fields
B. Exposing it to large shock or vibration
A. Soft Iron should be placed at its core
A. Motors
To remember:
Lionel Messi - Left hand rule for Motors
Ryan Giggs - Right hand rule for Generators
C. A 100 turn coil 10 cm iron core and current of 0.75
The magnetic field strength (symbol H) of a solenoid is defined as the Magneto Motive Force per unit length (1 Metre) and is therefore measured in Ampere-Turns per metre
B. hysteresis
B. the number of lines of force in webers
The total number of magnetic lines of force leaving or entering the pole of a magnet is called magnetic flux Φ (measured in Weber’s Wb). The number of flux lines per unit area is known as flux density B (measured in Tesla T)
C. Magnetising force H and Flux density B
