Capacitance And Capacitive Reactance
what is the unit of measurement for reactive power
volt-amperes reactive
what happens when the potential differnce is equal in an ac circuit?
the flow of electrons is zero and the capacitor is fully charged
what does this mean: 200wvdc
the highest dc voltage that can be applied is 200 volts
what the equation for electric charge
current x time Q= I x t
if wvdc is on the capacitor what must you do?
make sure that if you apply it to an ac circuit that the peak voltage of an ac sine wave does not exceed the wvdc rating: for example: 200wvdc must never have more then 141 v rms applied to it 200x.707=141
what is the unit of measurement for capacitance?
microfarad or farad
symbol for capacitive reactance
Xc
what is the symbol for capacitive reactance?
Xc
what is the formula used to determine the capacitive reactance of a capacitor
Xc= 1/((2)(3.14)(f)(C)) C is in farads
capacitance is directly/indirectly proportional to the area of the plates
directly
the time required to charge a capacitor varies directly/indirectly with the resistance and capacitance
directly
the time constant of a capacitor circuit is directly or indirectly proportional
directly by varying the capacitance and/or resistance the total charge or discharge time can be changed
dielectric constant of air
1
what can C stand for
1) represent capacitance 2) as the SI unit for coulombs
dielectric constant of paper
2-2.5
which capacitors are non polarized?
Air,oil-filled,paper,mica and ceramic
symbol for capacitance
C
symbol for electric charge
Q
the symbol for reactive power is ____
Q
the equation for reactive power is
Q= (Vx)(Ix) Reactive power= reactive voltage x reactive current
different equation for electric charge using farad capacitor
Q= C x V
V2= ____ + (Vmax - V1)(63.2%)
V1
what circuits are to be used with non polarized electrolytic capacitors?
ac
what are six common capacitor types?
air oil-filled paper(tubular) mica ceramic(disc) electrolytic
what does it mean to have a non polarized capacitor?
can be connected to ac or dc sources, without having terminals identified for polarities
what is a farad
capable of storing one coulomb of charge for every volt across it
the ability of an electrical device to oppose any change in voltage across that device
capacitance
what happens if the distance between plates decreases?
capacitance increases
a basic capacitor consists of two _______ plates seperates by an insulating material called the _______
conducting, dielectric
what is the unit of measurement of electric charge
coulomb
what circuits must be used with polarized capacitors?
dc, can be hazardous if connected incorrectly
define capacitive reactance
determines the current due to changing voltage across a capacitor in an ac circuit
what happens to energy in one portion of a cycle?
energy will be temporarily stored in the electrostatic field of the capacitor then returned to the corcuit when te electrostatic field discharges
Oil filled capacitors have a very (high/low) current and voltage ratings
high
The capacitance is directly/inversely proportional to the distance between the plates
inversely
capacitance reactance is (inversely/ directly) proportional to frequency
inversely
capacitive reactance is (inversely/ directly) proportional to capacitance
inversely
10-6⃣F
is one micro farad
define reactive power
it is the rate at which energy is stored and returned to the circuit also can be defined as the product of the reactive voltage and the reactive current
what happens if a capacitor become connected to excessively high voltage
it will cause dielectric breakdown- the capacitor will short out
list two factors that affect the capacitive reactance of a capacitor
its capacitance the frequency of te voltage across it
In a purely capacitive circuit current (lags/leads) rhe voltage across the capacitor by __%
leads, 90%
how are mica capacitors made up?
made by inserting a thin sheet of mica between two metal plates
If the capacitance of a capacitor is high what happens
more electrons flow to and from platea, more electron flow means more current, which indicatea the ohmic value of capacitive reactance is low ( low Xc)
if the resistance is high and initial current is low is there more or less time required
more time
can all capacitors be used for both dc and ac?
no due to their construction some capacitors are designed for use in dc only
do all dielectrics withstand the same voltage
no, some d/e can withstand greater voltages then others of the same physcak thickness
unit of measurement of capacitive reactance
ohm
what is the unit of measuremet for capacitive reactance?
ohm
10-1⃣2⃣F
picofarads
equation for calculating one time constant in an rc circuit
r= RxC r- is te time constant in seconds R- resistace in. ohms C - capacitance in farads
state the effect upon time constant if the value of C is decreased?
r= RxC time decreases
state the effect upon time constant if the R value increased
r= RxC time is increased
The energy stored and returned into the circuit is called
reactive energy
the time require for the capacitor curret to decrease to zero is the same te for the capacitor voltage to reach maximim is the ______
same
State four different capacitor applications-
timers rectifier smoothing and filtering power factor correction motor starting
every dielectric has the same capacitance true or false
false, it varies between material,
SI unit of measurement for capacitance is
farad
most capacitors have a _____ microfarad value but sime are _______ or _______
fixed, variable, adjustable
define dielectric strength
is the strength of the dielectric material measured in kilovolts per millimetre (kV/mm)
what happens when the distance between plates increases
the capacitance decreases
if the capacitance is low what happens to the capacitive reactance
the electron flow is reduced, therefore the ohmic value of capacitive reactance is high ( high Xc)
non polarized generally have smaller microfarad rating and higher voltage ratings then electrolytic types true or false?
true
what ia volt-amperea reactive abbreviation
var
define dielectric constant
is the rating of the dielectric effectiveness of insulating material compared to air
how many families are there in the electrolytic capacitors
2 polarized non polarized
what are three factors that affect the capacitance of a capacitor
-the area of the plates -the distance between the plates -type of dielectric material that seperates the platea
during each time constant the capacitor voltage will increase 63.2% of the difference between the maximum value an present value
...
how do you calculatr power in watts for a resistor? how do you calculate reactive power in vars for a capacitor?
1) P®= I2⃣R 2)Qxc=I2⃣Xc
dielectric strength of paper
20 kV/mm
dielectric strength of mica
200 kV/mm
dielectric strength of air
3 kv/mm
dielectric constant of transformer oil
4
dielectric constant of mica
5
how many periods are there for the capacitor voltage to reach its maximum value
5 equal periods the length of each period is defined as an one time constant
dielectric constant of porcelain
6
V1= Vmax x ____%
63.2
dielectric constant of some types of ceramic
7500
dielectric strength of porcelain
8 kV/ mm
state the relationship between Vxc and Ixc
Ixc always leads Vxc by 90degrees
symbol for the unit of measurement
F
Since the stored energy of the capacitor is returned into the circuit when it dischargrs it (does/does not) contribute to then work done or energy used by the circuit.
does not
if the plate area is doubled the capacitance is -
doubled
constant =
effectiveness
in an inductive circuit, the current lags the voltage. A capacitor in a circuit has the (same/opposite) effect, it causes the current to (lag/lead)
opposite, lead
define capacitance
the ability of an electrical device to store energy in an electrostatic field or to release energy from an electrostatic field to an electric circuit
⬇frequency causes a (⬆/⬇) in Xc
⬆
⬆ frequency creates a (⬆/⬇) in Xc
⬇
unit of measurement symbol of electric charge
C- coulomb
how are paper capacitors made up
like sandwich of wax paper between two sheets of aluminum or tin foil, the layers are rolled up and enclosed in tubular container, gives the tin foil plates a very large area in a compact package
what happens if a switch is closed in a dc circuit?
the instant the switch is closed there is a rush of electrons onto the plate of the capacitor from the negative terminal, and a rush of electrons away frim the plate connected tonyhe positive terminal, the flow of current eill continue until the potential difference across the plates is equal to the source voltage Vcapacitor=E supply
the unit of measurement for time constant is
the second
reactive power is sometimes referred to as
wattless or quadrature power
define the rc time constant
when a dc voltage is applied to a capacitor, the initial current flow is very high and limited only ny the resistance of the circuit. as the charge niolds on the playes the potential difference between the plates increases, opposing the applied voltage. as the opposing voltage increases, the current decreases until the voltage across the plates equals the applied dc voltage and the current becomes zero