Magnetism D3 Review Flashcards
What is reluctance?
The opposition to the setting up of lines of flux.
When selecting a core material for an AC electro-magnet, what two characteristics of the material should be considered when selecting the best material to be used?
A high electrical resistance ( the opposition to current flow in an AC or DC cct) and a low retentivity (the material’s ability to retain magnetism after the magnetism force has been removed).
What is the reason for using laminated cores?
To reduce the eddy current loses
What is a B-H graph used for?
To identify an electro-magnet core permeability (the measurement of a material’s ability to conduct magnetic lines of flux. The standard is air with a value of 1)
In magnetism, what is the ‘Ohms Law’ equivalent for magnetic circuits?
Flux = magnetomotive force / reluctance
Amps = electromotive force / resistance
What determines an electromagnets North and South polarity?
The polarity of the source
Given the choice between hard steel, soft iron, copper or aluminium, which would be the best choice for a magnetic screen?
Soft iron because of its low reluctance (resistance to magnetism)
When the air gap is increased in a magnetic circuit, what happens to the flux density (the number of magnetic lines contained in a certain area)?
Decreases
Define coercive force
The magnetizing force to overcome residual magnetism (the amount of magnetism left in an object AFTER the magnetizing force has been removed).
What if affected, if anything, if a hard iron core is replaced with an aluminium core in an electromagnet?
The flux (magnetic lines of force) weakens
When two parallel conductors, in closed proximity, are carrying currents in the same direction, they would X each other.
Attract
When related to an electric circuit, what term would permeability (measurement of a material’s ability to conduct magnetic lines of flux) be closest compared to?
Conductivity ( the measure of the ability of the material to conduct an electrical current).
What is the term that relates to the fact that the flux density lags behind the magnetizing force?
Hysterisis
Hysteresis loss: Power loss in a conductive material caused by molecular friction. Hysteresis loss is proportional to frequency.
Materials that exhibit the same features as iron in a magnetic field are referred to as X materials
Ferromagnetic materials. A material that will conduct magnetic lines of force easily such as iron. Ferromagnetic materials have a permeability much greater than air (1).
Define reluctance
The opposition to the setting up lines of flux.
When selecting a core material for an AC electro-magnet, what two characteristics of the material should be considered when selecting the best material to be used?
1- High electrical reluctance; the higher resistance, the lower the current will flow
2- Low retentivity; so that after removing the electric current to the substance doesn’t get magnetized and return to its original state.
What is the reason for using laminated cores?
To reduce the eddy current loses; by making the core of a stack of plates electrically insulted from each others, rather than a solid block.
All transformers operating at low frequency use laminated or similar cores.
What is used the B-H for?
To calculate an electromagnet core permeability by establishing the relationship between B ( flux density) and H( Magnetizing force) to determine the permeability ( ‘mu’).
What determines an electromagnets north and South polarity?
- The polarity of the source
- The way the magnet wire is wound around the core
Given the choice between hard steel, soft iron, copper or aluminium, which would be the best choice for a magnetic screen?
Soft iron; because of its low reluctance (easily magnetized materials)
When the air gap is increased in a magnetic circuit, what happens to the flux density?
Flux density: : the electric flux passing through a unit area perpendicular to the direction of the flux.
Decreases
What is affected, if anything, if a hard iron core is replaced with an aluminum core in an electromagnet
The flux weakens
When two parallel conductors, in close proximity, are carrying in the same direction, they would X each other.
Attract each other
When related to an electric circuit, what term would permeability be closest compared to?
Conductivity
Ie How much voltage is required to get an amount of electric current flow.
What is the term that relates to the fact that the flux density lags behind the magnetizing force?
Hysteresis
It results in a power loss as heat in many electrical machines.
List five characteristics of magnetic lines of force.
- Flux lines are directional; North to south ( via the air) and South to North ( inside the conductor)
- Flux lines do not cross each other
- Flux lines tend to be as short as possible
- Flux lines form a complete loop
- Flux lines in the same direction attract each other.
Materials that exhibit the same features as iron in a magnetic field are referred to as X materials.
Ferromagnetic materials
Ie: those that exhibit strong magnetic effects.
Examples: iron, nickel, cobalt and permanent magnetic called neodymium magnets.
