Capacitance

Question 1

Explain a parallel plate capacitor and what factors affect its ability to store charge

Answer 1

A parallel plate capacitor is two metal plates close but not touching. with a voltage across them the plates will store charge on a surface. The amount of charge that can be stored increases with the surface area of the plates and decreases with the distance apart. The permittivity (ε) of material between the two plates (dielectric) also affects the ability to hold charge.

C  = εA/d
Capacitance = permitivity(ε)×area/distance

Question 2

What is the relationship between permittivity, capacitance and area.

Answer 2

Permittivity is usually expressed in capacitance divided by area.
ε = farads/meters

Note. A farad is an impractically large unit and most real equations are expressed in 10⁻³ 10⁻⁶ or 10⁻⁹ farads.

Question 3

What is the relationship between capacitance, charge and voltage

Answer 3

A capacitor has a capacitance of one farad if it stores a charge of one coulomb when one volt is applied.

Question 4

Explain the water tank analogy of a capacitor

Answer 4

In this analogy the footprint of the tank is the capacitance, the water pressure is analogous voltage and the quantity of water in the tank is analogous to charge held. A tank with a larger base can hold more water for an equivalent water pressure (Voltage). In two tanks of the same size the water will reach a higher level if one is supplied by a higher pressure.

Question 5

Explain permittivity

Answer 5

Some substances will for a given electric field give a higher number of flux lines than others. Glass with give five times the flux density for the same electric field than air. This is because glass ‘permits’ more electric flux than air. The variable is epsilon (ε).
Permittivity is either expressed in units of absolute permittivity (coulombs per volt per meter).
A vacuum will permit flux although badly. The permittivity of a vacuum is the base unit for which the permittivity of all other substances is compared. The more common unit for the permittivity of a substance is relative permittivity (ε r) a multiple of the permittivity of a vacuum.

Question 6

What is the effect of having capacitors in parallel.

Answer 6

The total charge adds up like having resistors in series.

Question 7

What is the effect of having capacitors in series

Answer 7

Because the current is the same at every point in a series circuits each capacitor will hold the same charge.

The total charge is calculated using the resistors in parallel formula.

C total = (C₁⁻¹ + C₂⁻¹)⁻¹

Question 8

Explain energy loss in a capacitor

Answer 8

Like all electrical components capacitors are not perfect.

Energy will be lost by leakage current passing from one plate to another through the dielectric.

Energy will also be lost to heat by the repeated creation and destruction of the electric field. This is more relevant in AC applications.
This is called dielectric losses.

Question 9

Explain dielectric strength

Answer 9

This is the maximum voltage that can be applied to a given size and thickness of a dielectric material without reducing the functioning or damaging the component. This is measured in volts per meter. The higher the value the greater the voltage that can be safely applied to a given area and thickness of dielectric material.

Question 10

Explain the working voltage of a capacitor

Answer 10

This is the voltage that can be safely and continually applied across a capacitor without causing electrical breakdown.

Question 11

Explain a variable air capacitor

Answer 11

A series of plates separated by air. A knob can increase or decrease the distance between them, thus adjusting the capacitance. Variables are designed to adjusted by the user while presets are not. Presets are usually used in pcb applications and have very low values.

Question 12

Describe a paper capacitor

Answer 12

Layers of aluminium foil and paper (impregnated with anti moisture agents) rolled up into a protective can. This are used in high voltage applications. They are more susceptible to temperature change and have a shorter life than many other types of capacitors.

Question 13

Explain mica capacitors

Answer 13

Mica has high dielectric strength and high permittivity. Mica capacitors usually have values of less than 20 nF and are of a parallel multi plate construction. They are expensive compared to other types. Because mica is ridged they cannot be rolled up into a can like paper capacitors and thus give less capacitance for a given size.
The capacitance values are stable and do not vary with temperature. They have a long life. They are used in high frequency applications.

Question 14

Explain plastic capacitors

Answer 14

Similar to paper capacitors but with a plastic dielectric instead of paper. A roll of layered paper and plastic in a protective can. High stability and long service life. High resistance to temperature changes. Used for general purpose capacitative circuit work.

Question 15

Explain capacitance

Answer 15

Capacitance is the ability to store electric charge. A device designed to store electric charge is called a capacitor.
Symbol: C
Unit: Farads (F)

Question 16

Explain current through a capacitor

Answer 16

Current does not really flow THROUGH a capacitor. What appears to be a current is in fact the flow of electrons to the plate but they do not pass through the Dielectric, they build up on the plate. This creates an electric field that then pushes away the electrons on the opposite plate. Hence an illusion of current is created, but unlike a normal current it only continues to flow until the surface of plates are saturated with electrons. In theory a current can continue to flow if the voltage is increasing continually but in practice every capacitor has its safe voltage limit.