A3 - UP Flashcards
Area 3 from UP Reviewers (Farm elect, thermody, heat transfer,
Certain types of crystals produce a voltage when subjected to pressure.
Pressure (Piezoelectricity)
Voltage is produced when the junction of two unlike metals is heated. Thermocouples use the principle of thermoelectricity.
Heat (Thermoelectricity).
solar or photo cells convert radiant energy to electrical energy.
Light (Photoelectricity
Symbol and unit for conductance
G, siemens
if an object contains more number of total electrons than the total protons
Negatively charged
if an atom contains fewer number of total electrons than the total protons
positively charged
- The result of non-random movement of electrons
- Defined as the rate at which electrical charge flows
- Measured in amperes (A)
- One ampere is equal to one coulomb per second
current
- The ability of a material to resist the flow of electrical charge when subjected to a given potential difference
- Electrical resistance is measured in ohm (Ω)
Electrical Resistance
The lower the resistivity of the material, the _____ conductor it is
better
Conductors are materials which has ____ resistance to electron flow
low resistance
Insulators are materials which has ____ resistance to electron flow
high resistance
_____ are materials having an intermediate resistance to electron flow; usually used in construction of solid state electronic devices such as diodes and transistors
Semiconductors
Conductance is the reciprocal of _____, measured in mhos
resistance
phenomenon when conductors lose their resistance in extreme cold
superconcuctivity
States that the current flowing in a circuit varies directly with the electrical pressure and inversely with the opposition
ohm’s law
Because charge cannot be created but must be conserved, the sum of the current in a node must be equal to zero
Kirchhoff’s current law
the ability of material to conduct electricity; reciprocal of resistivity
conductivity
The net voltage around a close circuit is zero
Kirchhoff’s voltage law
- The power dissipated by an electrical circuit is given by:
- The common unit of electric power is watt.
- An instrument called a wattmeter measures power. The wattmeter gives a reading of the product of the current and voltage in a circuit.
Power Calculations (Joule’s Law)
The value of the _____depends on the kind of opposition offered. It is never greater than 1.
power factor
Power factor is equal to ____ for heaters, electric ranges and incandescent lamps.
The opposition is pure resistance.
one (1)
Power factor is between ____ for electric motors.
0.6 to 0.8
From the point of the view of the load, any network composed of ideal voltage and current sources, and of linear resistors, may be represented by an equivalent circuit
consisting of an ideal voltage source in series with an equivalent resistance.
Norton’s theorem
Characterized by non-variant current flow in one direction at all times
Common source are batteries, thermocouples, solar cells, and rotating _____ generators
direct current (DC)
Characterized by an alternating flow of in two directions
Most common variation is of a sine wave
Alternating Current (AC)
When voltage and current are in phase, all the power is ____
positive
_____, in VAR units, is the power below the axis. It is “watt-less” and does no useful work. VAR means volt-ampere-reactive.
Reactive power
Coils of wire such as those found in transformers, relays, and motors have ______
inductive reactance
the property to oppose current change.
inductance
Current in AC circuit with pure inductance will lag the voltage by ____
90 deg
The amount of electric charge that a capacitor receives for each volt of applied potential is called its ___
measured in farads but practical devices are rated in terms of
microfarads.
capacitance
Power factor improvement can be accomplished either by ____ of series or parallel-connected capacitors.
addition
are lines of force surrounding a permanent magnet or a moving
charged particle.
magnetic fields
the quantity of magnetic field force or “push”; also known as magnetic potential difference; analogous to electric voltage (emf).
Magnetomotive force (mmf)
the quantity of total field effect or “substance” of the field; analogous
to electric current.
magnetic flux
the amount of field force (mmf) distributed over the length of the electromagnet.
magnetic field strength (intensity)
the amount of magnetic field flux concentrated in a given area.
magnetic flux density
Completely describes the laws of electromagnetism and its effect based on four equations or theories developed by Gauss, Ampere and Faraday.
Maxwell’s Equations
describes that the electric field due to electric charges can be derived by Coulomb’s Law.
Gauss’s law for electricity
states that the net magnetic flux through any (real or imaginary) closed surface is zero. Magnetic field lines do not start or stop at any point in space, but form a close loop thus any magnetic field line entering a closed surface must also leave that surface, so the net flux is zero.
Gauss’ law for magnetism
describes the production of magnetic fields not only by electric currents but by changing electric field as well.
Ampere’s law
describes the production of electric fields as a result of changing magnetic fields.
Faraday’s law of induction
voltage induced by a changing magnetic flux
induced emf
current produced during electromagnetic induction
induced current
There are three standard types of DC generators:
(a) shunt generator;
(b) series generator; and
(c) compound generator
There are three general classes of synchronous generators:
a) slow-speed engine-driven type
b) moderate-speed waterwheel-driven type
c) high-speed turbine-driven type
an engine that has 75-90 rpm, though runs as high as 150 rpm
slow-speed engine-driven type
an engine that has 80-90 rpm and higher
moderate-speed waterwheel-driven type
The total power output for a 3-phase system is a _____
constant value
The main reason for the use of 3-phase systems is that for the same voltage and current, a 3-wire, 3-phase system will deliver ______ as much power as a 2-wire, single-phase system.
1.73 times
an engine that has 720-3600 rpm
high-speed turbine-driven type
________ are the most common in the home, on the farm and in light industry. It requires some type of starting mechanism.
Single-phase motors
__________ are inherently self-starting. They are common in ratings from ½ hp to 400 hp.
Three-phase motors
If the equipment can be successfully operated by hand, a ______ motor will do.
¼ hp motor
common motor enclosure types
a) drip proof
b) splash-proof
c) totally enclosed
Transformers only work with ____ voltages, not _____ voltage. Thus, they may be classified as an AC device and not a DC device.
changing voltages, not steady voltages
The primary winding has more turns than the secondary.
Step-up transformer
The primary winding has fewer turns than the secondary.
Step-down transformer
States that the induced voltage in a coil is always in a direction so as to oppose the effect which is producing it.
The Lenz’s law
refers to the portion of electric conductors that extends from the service entrance to an outlet or a certain group of outlets. It connects the loads to the final overcurrent protection device.
Branch circuit
No point along the floor line in any wall space should be more than _____ from
a receptacle outlet.
Any wall space greater than 0.6 m (2 ft) shall have an outlet.
1.8 m (6 ft)
Any wall space greater than _____shall have an outlet.
0.6 m (2 ft)
Provide one 20 A circuit for each ______ or one 15 A circuit for each 35 m2 (375 ft2) floor area.
46 m2 (500 ft2)
At least _____ wall switch controlled lighting outlet shall be installed in every habitable
room.
one
Allow _____ outlets for each 15 A circuit and ______ for each 20 A circuit.
8 to 10 outlets
10 to 12 outlets
A factor of ________ is used for all floor area excluding open porches, garages, basements or unused or unfinished spaces not adaptable for future use.
32 watts/m2 (3 watts/ft2)
the use of colors of conductor covers in the wiring connections
polarizing
white wire is always the _____ wire
neutral wire
_____ are those which are larger than the wires.
cables
Wires and cables are either stranded or solid.
is grounding of equipment not intended to be at a voltage potential different from the earth.
It is necessary to prevent electric shock to persons coming into contact with metallic objects which, either intentionally or accidentally, form part of the electric system.
equipment grounding
The resistance of the human body varies from about _____ Ω when dry to about
____Ω when wet (including the effects of perspiration).
500,000 Ω
300 Ω
Therefore, voltages as low as 30V can cause sufficient current to be fatal
is grounding of current-carrying portions of a system.
The principal reason is to limit the voltage between any conductor and ground from rising above a safe level due to some fault outside a building.
system grounding
green wire is the ____wire
grounding wire
reed or black wires are ____ conductors
hot conductors
white wires are to be connected to _____ colored terminals, and red or black wires to ____colored terminals
silver-colored terminals
brass-colored terminals
all switches are in the ____ conductors
hot conductors
Two basic classes of devices are ________
fuses and circuit breakers
______ is never to be interrupted by a fuse, circuit breaker, switch or any other device.
Grounded neutral
types of distribution centers
a) indoors
b) outdoor
c) pole
The pole type should be located at the ______
electrical load center of the
farmstead
The best location for indoor type distribution center is the position closest to the equipment having the largest wattage rating
