Module 3.9 — Capacitance / Capacitors
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402 Terms
What is a capacitor?
A passive electrical component that stores charge and energy in an electric field.
What is another name for a capacitor?
A condenser.
What is capacitance?
The ratio of stored charge to applied voltage.
What symbol represents capacitance?
C.
What is the SI unit of capacitance?
The farad (F).
What is the main function of a capacitor?
To store electrical energy in an electric field.
Where is energy stored in a capacitor?
In the electric field associated with its charged plates.
What happens when electrons are removed from a conductor?
It becomes positively charged.
What happens when electrons are added to a conductor?
It becomes negatively charged.
Why is energy required to charge a conductor?
Work is done to establish its electric charge and electric field.
What represents an electric field in BA 3.9?
Lines of electric flux.
What is the relationship between capacitor charge and voltage?
Q is directly proportional to V when capacitance is constant.
What happens to stored charge if capacitor voltage doubles?
The charge doubles if capacitance remains constant.
What happens to stored charge if capacitor voltage halves?
The charge halves if capacitance remains constant.
What happens to capacitor voltage if stored charge doubles at constant capacitance?
The voltage doubles.
What happens to capacitor voltage if stored charge halves at constant capacitance?
The voltage halves.
What does the ratio Q/V represent?
Capacitance.
What is the formula for capacitance?
C = Q/V.
What is the formula for charge on a capacitor?
Q = CV.
What is the formula for capacitor voltage?
V = Q/C.
What does Q represent in capacitor formulas?
Charge in coulombs.
What does V represent in capacitor formulas?
Potential difference in volts.
What does C represent in capacitor formulas?
Capacitance in farads.
What does one farad mean?
One coulomb of charge stored per volt.
How is one farad expressed in fundamental units?
1 F = 1 C/V.
Why are submultiples of the farad commonly used?
One farad is very large for many practical capacitors.
What does μF stand for?
Microfarad.
What does nF stand for?
Nanofarad.
What does pF stand for?
Picofarad.
How many microfarads equal one farad?
1,000,000 μF.
How many nanofarads equal one farad?
1,000,000,000 nF.
How many picofarads equal one farad?
1,000,000,000,000 pF.
What is one microfarad in farads?
10⁻⁶ F.
What is one nanofarad in farads?
10⁻⁹ F.
What is one picofarad in farads?
10⁻¹² F.
How many nanofarads equal one microfarad?
1,000 nF.
How many picofarads equal one nanofarad?
1,000 pF.
How many picofarads equal one microfarad?
1,000,000 pF.
Convert 0.1 μF to nanofarads.
100 nF.
Convert 0.01 μF to nanofarads.
10 nF.
Convert 500 nF to microfarads.
0.5 μF.
Convert 4,700 pF to nanofarads.
4.7 nF.
Convert 2.2 μF to nanofarads.
2,200 nF.
What units must be used directly in C = Q/V?
Farads, coulombs and volts.
What is the capacitance of a capacitor storing 2 C at 4 V?
0.5 F.
What charge is stored by a 2 F capacitor at 3 V?
6 C.
What voltage is across a 4 F capacitor storing 12 C?
3 V.
What charge is stored by a 10 μF capacitor at 28 V?
280 μC.
What is 280 μC in coulombs?
280 × 10⁻⁶ C.
What charge is stored by a 2 μF capacitor at 12 V?
24 μC.
What charge is stored by a 5 μF capacitor at 6 V?
30 μC.
What is the voltage across 10 μF storing 100 μC?
10 V.
What is the capacitance when 60 μC is stored at 12 V?
5 μF.
Why does placing two conductors close together with an insulator between them increase charge-storage capability?
It creates a capacitor with a concentrated electric field.
What is the simplest form of capacitor construction?
Two conducting plates separated by an insulating dielectric.
What materials are capacitor plates commonly made from?
Metal foil or metal film.
What prevents the plates of a normal capacitor from touching?
The insulating dielectric.
What is a dielectric?
An insulating material between capacitor electrodes.
What is the purpose of a dielectric?
To separate the electrodes and support the electric field.
Name four capacitor dielectric materials mentioned by BA.
Air, paper, mica and ceramic.
Name three additional dielectric materials identified by BA.
Glass, polyester and polypropylene.
What is another dielectric material named in BA 3.9?
Mylar.
Are capacitor plates necessarily flat?
No; they may be flat, rolled or have other shapes.
What electrical quantity appears between capacitor plates when charged?
A potential difference.
What type of charges develop on the two capacitor plates?
Equal and opposite charges.
What happens to the plate connected to the battery's negative terminal?
It gains electrons and becomes negatively charged.
What happens to the plate connected to the battery's positive terminal?
It loses electrons and becomes positively charged.
Does charge normally cross an ideal capacitor dielectric during DC charging?
No; charge redistributes through the external circuit.
Why does an ideal capacitor block steady DC current?
Its dielectric does not provide a conducting path between plates.
Can current flow in the external circuit while a capacitor is charging?
Yes.
What happens to capacitor charging current as it approaches full charge?
It decreases towards zero.
When does an ideal DC capacitor stop drawing charging current?
When its voltage reaches the applied steady supply voltage.
What is the capacitor voltage after full charging from a constant DC supply?
Approximately equal to the supply voltage.
How does a capacitor behave in a steady DC circuit after full charge?
Like an open circuit.
How does a capacitor respond to an alternating voltage?
It repeatedly charges and discharges.
Why can AC current exist in a circuit containing a capacitor?
The changing electric field permits alternating current in the external circuit.
Does an ideal capacitor conduct electrons directly through its dielectric?
No.
What determines the capacitance of a basic parallel-plate capacitor?
Plate area, plate separation and dielectric permittivity.
What is permittivity?
A material property describing its electric-field response.
What symbol represents permittivity?
ε (epsilon).
What is relative permittivity?
A material's permittivity compared with free-space permittivity.
What symbol represents relative permittivity?
εr.
What is another name for relative permittivity?
Dielectric constant.
What symbol does BA use for dielectric constant?
k.
Does relative permittivity have units?
No; it is dimensionless.
What does ε0 represent?
Permittivity of free space.
What approximate value of ε0 is given in BA 3.9?
8.85 × 10⁻¹² F/m.
What is the formula for absolute permittivity?
ε = ε0 × εr.
What is the relative permittivity of a vacuum?
Approximately 1.
What relative permittivity does BA quote for air?
Approximately 1.0006.
What relative permittivity range does BA quote for paper?
2.5–3.5.
What relative permittivity range does BA quote for glass?
3–10.
What relative permittivity range does BA quote for mica?
5–7.
What relative permittivity range does BA quote for wood?
3–8.
What relative permittivity range does BA quote for metal oxide powders?
6–20.
What happens to capacitance when a dielectric with higher relative permittivity is used?
Capacitance increases if dimensions remain constant.
What happens to capacitance when relative permittivity decreases?
Capacitance decreases if dimensions remain constant.
What is the relationship between plate overlap area and capacitance?
Directly proportional.
What happens to capacitance if plate overlap area doubles?
It doubles if other factors remain constant.
What happens to capacitance if plate overlap area halves?
It halves if other factors remain constant.