caa ac and dc Flashcards
dont die :)
if an electron has less than four electrons in it’s outer shell, its a ?
if it has more than four then its a?
conductor
Insulator
what does a gold leaf electroscope do?
it deteects the charge of an item place on top. the charge will either repel or attract the needle, indicating the charge.
obtaining charge without contact is called?
induction
what type of ions are more efficient at carrying electrical current?
high energy, small ions
why dont gases usually conduct very well?
low pressure gasses are very spread out, so it takes more energy for electrons to attract to the next ion.
what are the two ways of controlling curent?
potential difference, and resistance.
what 4 factors effect resiwstance?
size of wire, length of wire, type of material and its ro value, temperature of wire.
potential difference from crushing?
piezo electric crystal
potential difference from friction?
triboelectric effect
potetnial difference from heat?
seabeck effect
potential difference from light?
photo-voltaic
what factor effect emf?
internal resistance, size of discharge current, the charge state of the cell.
components of a lead acid battery?
positive plate, negative plate, cell connector, vent cap, cell cover, terminal posts.
chemical in lead acid batteries?
lead peroxide, lead, sulfuric acid.
lead sulphate forms on positive plate.
s.g for charged and discharged lead acid batteries?
1.280 sg charged, 1.150 sg discharged
internal resistance formula
r=E-v / I
faults of lead acid batteries
sulphation, buckling, sedimentation.
nickel cadium battery’s checmicals?
nickel hydroxides on pos, cadmium on the neg, electrolyte is potassium hydroxide.
pros of nickel cadmium batteries
long life, constant terminal voltage, charged and discharged at higher currents, discharged to very low voltages without damage.
cons of nickel cadmium batteries
expensive, low cell voltage, thermal runaway is easier
how does the wheatstone bridge determine an unknown resistor value?
a volt meter is connected in parallel between the know and unknown resistors, resistance value is figured out based on the voltage reading and the value of the variable resistor.
points of a hysteresis curve
flux density, magnetising force.
opposite directional flux d and mag force
eddy currents are increased by?
stronger magnetic fields, faster caging fields, thicker materials, low resistivity.
eddy currents are decreased by?
weaker magnetic fields, slower changing fields, thinner materials, slotted/ laminated material, higher resistance materials, very fast changing fields.
unit of mmf
ampere turns
unit of reluctance
ampere turn/weber
what is back emf
back emf, self inductance, is an opposite emf caused by collapsing magnetic fields that oppose current flow.
what is armature reaction?
armature reaction is the distortion of the magnetic field due to the armature field being 90 to the main field.
back emf=
-L * rate of change of current
impedance in parallel
1/ (1/r^2+ (1/Xl - 1/Xc)^2)
transformer loading vectors and autotransformers
generator loading
what is a balanced load?
when the phase currents are equal in magnitude and are displaced by 120 degrees.
what is a symmetrical load?
when the phase voltages are equal in magnitude and displaced by 120 degrees
