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ch.12 - Magnetism


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What is symbol and units for Magnetic Flux?
Φ , Webers (Wb)
What is the Symbol and unit for flux density?
B , Teslas (T)
A magnetic field is a _____ field.
With Faraday's flux concept, the density of lines represents the _____ of the field and their direction represents the _____ of the field.
strength, direction
Three ferromagnetic materials are what?
Iron, nickel, and cobalt
The direction of a magnetic field is from ____ to ____ outside a magnet.
North, South
True or False. Plastic has a an effect on a magnetic field, but much smaller than iron.
False. Plastic has no effect.
Flux density B is defined as the ratio Φ/A, where A is the area _______ to Φ.
To figure out the direction of the magnetic field about a current-carrying conductor, use the ____ hand rule, where your _____ points in the direction of the current.
Right, Thumb
Define fringing
A decrease in flux density in an air gap between two ferromagnetic materials.
What is the "stacking factor"?
The ratio of ACTUAL area of ferrous material to the PHYSICAL area of the core.
What does the symbol "H" stand for and what are it's units with respect to magnetism?
Magnetic Field Intensity, Ampere-turns/length (At/m)
What symbol represents Magnetomotive Force or MMF?
£ <- supposed to be cursive F
How are H and £ related?
H=£/l (magnetomotive force / length)
What happens to H (Magnetic Field Intensity) when the core's length increases?
It decreases
Permeability is a measure of how ____ it is to establish flux in a material.
Define Reluctance.
The opposition to flux in a magnetic circuit.
How do £ (magnetomotive force), Number of turns in a coil, and I (current) relate?
How do B (flux density) and H (magnetic field intensity) relate?
B=μH where μ is permeability
How is RMS voltage calculated for any waveform?
squareroot of area under v squared curve divided by the base
Why are most cheaper multimeters not accurate when calculating non-sinusoidal RMS waveforms?
(average responding meters) They rectify AC to DC. Then take the average value and use a multiplier to calculate RMS- only works for sine waves.
How is the effective value calculated given an AC signal added to a DC signal?
Square root of AC squared plus DC squared. (Distance formula)
What does j*j= ?
What are the units for reactive power?
VAR (volt-amps reactive)
How do you total the power in a parallel circuit?
By summing- same for series
What is the symbol for Apparent Power and units?
a. S
b. VA (volt-amps)
How is the Power Factor calculated?
Fp = P / S
Define Eddy Currents.
Unwanted current that is induced in the core of an inductor or transformer by changing core flux.
Define Hysteresis.
Power loss in a ferromagnetic material caused by the reversal of magnetic domains in a time varying magnetic field.

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