Self tests 1-3 Flashcards
What is another name for magnetic lines of force?
magnetic flux
why don’t magnetic lines of force collapse into the centre of a magnet?
the magnetic lines of force do not collapse due to mutual repulsion
T/F? Magnetic lines of force never cross
True
T/F? Magnetic lines of force start at the north pole and end at the south pole
False
T/F? It is not possible to have a north pole without having an accompanying south pole
True
T/F? The north pole of the earth is actually a south magnetic pole
True
T/F? The arrows on the magnetic lines of force always point from north to south outside of the magnet, and south to north on the inside of the magnet
True
T/F? Unlike poles of magnets repel each other
False
T/F? Magnetic screens are made of materials with a low reluctance
True
T/F? Magnetic screens block magnetic flux lines
False
List three ways of creating magnets?
- using electrical current
- stroking with a magnet
- by linking with another magnetic field and tapping
List three ways of demagnetizing a material?
- Using heat
- AC current
- tapping or vibrating excessively
Which method of demagnetizing is commonly used by watchmakers?
AC current method
What is meant by the term “curie” temperature?
temperature at which the molecular structure changes
When a piece of ferromagnetic material is brought near a magnet, the ferromagnetic material becomes magnetized. This process is referred to as ?
Magnetic induction
Is it true that any time a charge moves, there must be a magnetic field set up around that charge?
Yes
why are there no magnetic poles in the field surroundings current-carrying conductor?
there is no magnetic material
As the direction of current in a conductor is reversed, what happens to compasses surrounding the conductors?
the compass also reverses direction
As the current in a conductor is increased, what happens to the magnetic lines of force surrounding the conductor?
More flux lines are made and forced outwards
When current remains steady, does the field disappear?
No it remains steady
Is the field around the conductor more or less dense closer to the conductor?
more dense
As the current in a conductor is decreased, what happens to the magnetic lines of force surrounding the conductor?
They collapse inward
I’m moving four times as far away from a current-carrying conductor, the density of the flux lines will have ____________ by ____________ times?
decreased by four
Parallel conductor carrying currents in the same direction _________ each other and parallel conductors carrying currents in the opposite directions ___________ each other?
attract , repel
Using the left hand rule for coils, the thumb points in the direction of?
North pole of coil
four things that affect the strength of an electromagnetic coil?
- number of turns
- core material
- spacing between
- vary current through the coil
Materials that act in a similar way to iron in a magnetic field are referred to as?
ferromagnetic
Two examples of non-magnetic material?
air, wood
Most practical way of varying the strength of an electromagnet?
vary the current through the coil
What device connected in series with the electromagnet is normally used to do this?
rheostat
why must electrical bus bars be securely braced?
Because strong magnetic forces occur between bus bars if there is a short
In the dot-and-cross convention used to indicate the direction of current flow a ___________ represents a current coming out of the page at you?
dot
The magnetic flux in a circuit is directly proportional to the ___________ and inversely proportional to the ___________ of the magnetic circuit?
Magnemotive force, reluctance
Opposition to the formation of flux lines in magnetic circuit is called?
Reluctance
The point at which all of the domains in the magnetic material have aligned themselves with the magnetic field is referred to as?
theoretical saturation
Residual magnetism is a measure of the amount of __________ remaining in the magnetic material after the magnetizing force has been removed?
flux density
The amount of reverse magnetizing force required to remove the residual magnetism is called the?
coercive force
The type of core material used for lifting electromagnets would have a ___________ retentivity.
low
The spreading out of the flux lines in an air gap is referred to as _____
fringing
Magnetic cores that are constructed of alternating layers of magnetic material and electrical insulator are said to be ____________
laminated
As the reluctance of a magnetic circuit is increased, the flux density of the circuit is ___________?
decreased
if the magnemotive force applied to a magnetic circuit is increased, the flux density of the circuit is __________?
increased
The ease with which flux is set up in a unit length and cross-section of a material is referred to as the _________?
permeability
The term for the lag of flux density behind magnetizing intensity is __________?
hysteresis
Magnetic cores are usually constructed from materials with a high ___________ and a low ___________ ?
permeability, reluctance
The amount of energy lost due to the constant reversals of the magnetic field in AC circuits is referred to as ____________ loss
hysteresis
What form does this energy loss normally take?
Heat
