The Nature of Magnetism Flashcards
The phenomenon of magnetism was discovered by the _____ about _______.
Chinese, 2637 B.C.
The magnets used in the primitive compasses were called ________ or _______.
lodestones, leading stones
What are the crude pieces of iron ore called lodestones?
magnetite
Lodestones are classified as what type of magnets?
natural magnets
What is the only other natural magnet aside from lodestones?
the earth itself
What are all other magnets made by humans known as?
artificial magnets
These are those materials which are attracted or repelled by a magnet and which can be magnetized themselves.
magnetic materials
most common magnetic materials
iron and steel
those of hard magnetic materials that retain their magnetism when the magnetizing field is removed
permanent magnets
one that has no ability to retain a magnetized state when the magnetizing field is removed
temporary magnet
Classifications of Magnetic Materials based on applications
Ferromagnetic materials, Paramagnetic materials, Diamagnetic materials
Classifications of Magnetic Materials based on strong magnetic properties of iron
magnetic, nonmagnetic
includes iron, steel, nickel, cobalt, and commercial alloys
ferromagnetic materials
nonmagnetic materials that have ferromagnetic properties of iron
ferrites
A common application of a ferrite is a ferrite core in the coils for __________.
RF (radio-frequency) transformers
range of permeability of ferrites
50 to 3000
includes aluminum, platinum, manganese, and chromium
paramagnetic materials
relative permeability of paramagnetic materials
slightly more than 1
includes bismuth, antimony, copper, zinc, mercury, gold, and silver
diamagnetic materials
relative permeability of diamagnetic materials
less than 1
two points of a magnet opposite each other
north pole, south pole
force that exists arounds the magnet invisible to the naked eye
magnetic field
the ______ the magnet, the greater the number of lines of force and the ________ the area covered by the filed
stronger, larger
What does the direction of the lines outside the magnet shows?
path a north pole would follow in the field
What do you call an entire group of magnetic field lines which flow outward from the north pole of a magnet?
magnetic flux
What is the symbol for magnetic flux?
Greek lowercase letter phi
What is the SI unit of magnetic flux?
weber (Wb)
1 Weber = n magnetic field lines
n = ?
1 x 10^8
What is the symbol for magnetic flux density?
B
What is one weber per square meter called?
tesla
It is the magnetic flux per unit area of a section perpendicular to the direction of flux.
Magnetic Flux Density
SI unit of magnetic flux density
tesla (T)
When was the relation between magnetism and electric current discovered?
1819
Who discovered that an electric current flowing through a conductor produced a magnetic field around that conductor?
Danish scientist Oersted
A _____ current will produce many lines of force extending far from the wire, while a _____ current will produce only a few lines close to the wire.
high, low
Every section of the wire has the field of force around it in a plane _______ to the wire.
perpendicular
What is a convenient way to determine the relationship between the flow of current in a conductor and the direction of the magnetic lines of force around the conductor?
right-hand rule
two results of bending a straight conductor into the form of a single loop
magnetic fields are denser inside the loop, all lines inside the loop are aiding in the same direction
When is a coil of wire conductor formed?
more than one loop or turn
What is the symbol for magnetomotive force?
F
SI unit for magnetomotive force
ampere-turns (At)
The _____ of a magnetic field in a coil of wire depends on how much current flows in the turns of the coil.
strength
The more current, the ______ the magnetic field.
stronger
The more turns, the more ______ are the lines of force.
concentrated
the product of the current times the number of turns of the coil
magnetomotive force
formula of magnetomotive force
F = NI
formula of magnetic flux density
B = Φ/A
concentration of lines of forces; depends on how long the coil is
magnetic field intensity
If a coil with a certain number of ampere-turns is stretched out to twice its original length, the intensity of the magnetic field will be ________.
half as great
symbol of magnetic field intensity
H
formula of magnetic field intensity
H = F/l
also known as BH curve
flux-field curve
used to show how much flux density results from increasing the amount of field intensty
flux-field curve
it is the ratio of flux density to field intensity
permeability
unit of permeability
Tm/At
The average value of permeability is measured at the point where the ______ is first established
knee
permeability of air
4π x 10^-7 Tm/At
What is the opposition to the production of flux in a material called?
reluctance
symbol for reluctance
R
it is inversely proportional to permeability
reluctance
iron has high permeability and therefore ____ reluctance
low
air has ______ permeability and hence has high reluctance
low
formula of reluctance
R = l/µA
states that if a conductor “cut across” lines of magnetic force, or if lines of force cut across a conductor, an emf or voltage is induced across the ends of the conductor
Principle of Electromagnetic Induction
When was the principle of electromagnetic conduction discovered and who discovered it?
1831, Michael Faraday
The most important application of relative motion between conductor and magnetic field is made in _________.
electric generators
In a DC generator, how are fixed electromagnets arranged?
in a cylindrical housing
Since the conductors are in series in the coil, the induced voltages add together to produce the _______ of the generator.
output voltage
According to ________, the value of the induced voltage depends upon the number of turns of a coil and how fast the conductor cuts across the lines of force or flux.
Faraday’s Law
states that an induced voltage always has such a direction as to oppose the change in magnetic flux that produced it
Lenz’s Law
Three Factors determining Induced Voltage
amount of flux, number of turns, time rate of cutting
The more the lines of force that cut across the conductor, the _____ the value of induced voltage.
higher
The more turns in a coil, the ______ the induced voltage.
higher
The faster the flux cuts a conductor, or the conductor cuts the flux, the _______ the induced voltage because ______ lines of force cut the conductor within a given period of time.
higher, more
Formula for Faraday’s Law and Lenz’s Law
ε = N(ΔΦ/Δt)
