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