Capacitance and Capacitive Reactance Flashcards
What is a capacitor?
Two metallic plates, separated by a dielectric material.
What is the insulating material between a capacitor’s plates called?
the dielectric.
There are six types of non-polarized capacitors, what are they? What is their primary useful function.
Air (capacitance can be easily varied),
Oil- filled (usable in power circuits with high I and V),
Paper (large capacitance in a small package),
Plastic film (like paper, but utilize a different dielectric),
Mica (high capacitance, with a strong dielectric),
Ceramic (high capacitance for their small size).
When a symbol for a core is drawn with two parallel lines next to it, whet is signified by this addition?
The core is a magnetic material like iron.
What factors will affect capacitance?
The area of the plates,
distance between the plates,
and the type of dielectric.
The symbol for capacitance is ______, it is measured in ______.
C,
Farads (F), typically microfarads.
What is capacitance?
The property of a circuit to oppose a change in voltage.
It may also be defined as the ability to store electric charge.
What are the two basic ratings of a dielectric?
Dielectric strength: voltage insulating strength,
Dielectric constant: material’s ability to polarize and store charge, compared to air.
When connecting a capacitor rated in WVDC into an AC circuit, what must be observed.
The peak AC voltage must not exceed the WVDC rating.
Which capacitors may be only used in DC, how can they be modified to be used in an AC circuit?
Polarized electrolytic.
By connecting two of the same polarity plates in a back to back series arrangement, it is certain that in an AC circuit one side always has the correct polarity.
What are some common capacitor applications?
Timing,
Rectifier smoothing and filtering,
Power factor correction,
and Single phase motor starting.
How could one calculate the charge held by a capacitor?
Multiply the measured capacitor voltage by it’s capacitance.
When a capacitor is placed in a DC circuit, how will the voltage across the capacitor rise to it’s maximum value as the circuit is closed?
The voltage across the capacitor will rise quickly at first and slow down gradually. It may never reach 100% of it’s steady value.
How can we define the time it takes for capacitor voltage to reach it’s steady value?
We can define it as five time constants, which are defined as the product of capacitance and resistance.
How much will the capacitor voltage change in each time constant?
Voltage will increase by 63.2% of the remaining value to reach it's maximum, after every time constant. Tc1 = 63.2% Tc2 = 86.4% Tc3 = 95% Tc4 = 98.1% Tc5 = 99.4%, Which is essentially 100.
Why must a capacitor have a discharge resistor?
So that a dangerous stored charge is not unexpectedly released on an innocent electrician.
What is the effect of a capacitor in an AC circuit?
In AC the voltage continually changes. Because a capacitor will always try to mirror the voltage across it, there is a constant charging and discharging, which opposes the source voltage. We can reason that there will be a continual opposition to current flow because of this. We define this effect as capacitive reactance.
How is capacitive reactance represented? What is it measured in?
Capacitive reactance is X_C, and is measured in Ohms.
What factors will affect capacitive reactance?
The frequency of the supply, and the capacitance present in the circuit.
How is an capacitive circuit classed, regarding time relationships?
We call this type of circuit a LEADING circuit. Because the current leads the voltage.
When Reactive power in a capacitive circuit is positive, how is energy being used?
Current is taken into the capacitor to establish the electrostatic field.
When Reactive power in a capacitive circuit is negative, how is energy being used?
current is delivered back into the circuit by the capacitor’s collapsing electrostatic field.
