# Capacitor Tutorial Summary

A capacitor consists of two metal plates separated by a dielectric.

• A capacitor consists of two metal plates separated by a dielectric.
• The dielectric can be made of many insulating materials such as air, glass, paper, plastic etc.
• A capacitor is capable of storing electrical charge and energy.
• The higher the value of capacitance, the more charge the capacitor can store.
• The larger the area of the plates or the smaller their separation the more charge the capacitor can store.
• A capacitor is said to be “Fully Charged” when the voltage across its plates equals the supply voltage.
• The symbol for electrical charge is Q and its unit is the Coulomb.
• Electrolytic capacitors are polarized. They have a +ve and a -ve terminal.
• Capacitance is measured in Farads, which is a very large unit so micro-FaraduF ), nano-FaradnF ) and pico-FaradpF ) are generally used.
• Capacitors that are daisy chained together in a line are said to be connected in Series.
• Capacitors that have both of their respective terminals connected to each terminal of another capacitor are said to be connected in Parallel.
• Parallel connected capacitors have a common supply voltage across them.
• Series connected capacitors have a common current flowing through them.
• Capacitive reactance is the opposition to current flow in AC circuits.
• In AC capacitive circuits the voltage “lags” the current by 90o.

The basic construction and symbol for a parallel plate capacitor is given as:

• W
Wilson E. Stevens

Good job. I liked the math as I had forgotten most of that. However I still don’t know how or why it works in a crystal radio. I made a capacitor with two 6-inch foil squares separated by a piece of 20 lb paper. The you slide them back and fourth uncovering part to adjust the radio. Just how does this work? Going to be teaching a group of boy scouts how to build one, and describe and show how it works. But I need to know more myself. I think I understood it 60 years ago when I built my first radio, but now I just don’t remember. Thanks.

• Wayne Storr

The Crystal Radio is the most basic AM receiver circuit which has no battery supply as it self powers itself, thus it has no amplification. A crystal radio circuit is nothing more than an LC tank circuit with a germanium diode for rectification. A fixed loop antenna provides the inductive coil and a variable capacitance is used for tuning. The variable capacitor can be home made as you describe. Moving one foil square with regards to the other changes its capacitance value, while the paper acts as the dielectric. For it to work successfully you will also need a sensitive high impedance earphone and a very good ground terminal for the coil antenna.

• B
Bill Pilgrim

As an older tradie in the Electrical Industry, who originally studied by correspondence in a Five year Apprenticeship in Queensland Australia, from 1952, I appreciated the clear and simple manner in which this series is presented. I worked mainly in industry and various types of mining, along the East Coast of Australia & have studied most of my working life. I have had Heart surgery several times and also a slight stroke, but I managed to study and gain a fair sort of result to become a Technical Teacher in TAFE, until our government of that period made many teachers redundant. I have maintained my Qld Electrical Fitter & Mechanic’s Licence for the last 60 years. I enjoy keeping my brain working and frequently browse these courses simply to keep the cogs in my head from becoming rusty with disuse.

Good work, well presented, but clear and concise. I shall look for more of the same.
Regards Bill P.

• M
Minh Thanh

This is an excellent tutorials.

• V
V.M.RANJITH

VERY NICELY EXPLAINED

• N
Norven Baylan

Very useful information in Basic Electronics.

• C
Colin Campbell

Wayne, Do you have a tutorial on a DC cct describing the formulae for the charge &discharge current and voltage on an RC circuit?

• R
Robert Lyon

Hi Wayne,
Thanks for this excellent resource.
Quick question.
I’m planning to replace a couple non-polarized electrolytics in an old pair of Advents.
researching the web, many people seem to think replacing these with metal film or poly non-electrolytics is a better way to go.
Being a newbie, I don’t understand a lot of these finer points, but it’s my understanding that the electrolytics offer higher current capabilities as their main advantage. Soo, isn’t replacing them with non-electrolytics compromising the design?
Thanks!

• Wayne Storr

Hello Robert, Electrolytic capacitors have very high capacitance ratings for their physical size making them ideal for DC power supply reservoir and smoothing applications. However, standard electrolytics can create distortion when they are used for amplifier coupling and DC blocking circuits and also have much wider tolerances (+/-20%) than other capacitors. Replacing them with metallized film types is better if you can find them for your voltage and capacitance value, they are also self-healing so can handle surge currents better.

• d
domesh kumar

When the negativ charge applied the first plate then shall we get the result

• P
Paul C

Wayne, first of all, thanks for the wonderful series. I am a 30yrs old beginner in electronics, and this site is godsend to me; and your capacitor series is comprehensive but also easily grasped.

Second, what I think Stevens was referring to:
1: In a DC circuit, a plate will charge positive, and one negative. For electrolytic caps, they have to be properly placed so this takes place. For the rest, it doesn’t matter how you place them. When you discharge them, the charge will go (in conventional electronics) from the positive pole of the capacitor (corresponding with the positive pole of the battery) to the negative one, connected to ground.

2: I have no idea about this description either. Resistance is resistance, and current is never constant, but depends on voltage and resistance – which for capacitors depends on the charge.

3. A question of my own: What if, when touching a charged capacitor lead with a grounded metal object, you do not touch the other lead? You only ground ONE lead?

Thanks again for the great articles!

• Wayne Storr

Regarding question 3. Capacitors store energy as a result of their ability to store an electrical charge. When connected to a supply, the resulting electron flow results in one plate of the capacitor holding a +ve charge while the other plate holds an equal and opposite -ve charge. As a result, the supply voltage uses energy to create and maintain the charge on the two plates with the amount of charge stored depending on the supply voltage applied to it. This charge creates an electro-static field across the dielectric that stores the energy between the plates of the capacitor. This energy can be regarded as potential energy.

Assuming an ideal capacitor, when the supply voltage is removed, the capacitor maintains this charge indefinitely as it has nowhere else to go. However, if one leg or lead of the capacitor is grounded, there becomes an imbalance across the electric field causing the capacitor to discharge as the electro-static field looses energy.

• E
ElectricalScience

thank for your explanataion wayne, you are great for answer… my own anwser is anything electrical component that stores energy if one negative pole grounded it can cause discharge to low potential (ground) ,like only one negative pole in capacitor we touch with our hand then this will discharge, caused capasitor store electron and then send them to the earth( in case lack of electron) , plus to minus, balance the electron needs between earth and capasitors make it close circuit..

otherwise if we touch only positive pole with our hand the capasitor will not discharge its capacity.. because its not contain electron, only in negative pole eletron fall in line.

• M
M. Stevens

Couple ‘burning’ questions:
1.) I understand the ‘purpose’ of electrolytic caps (what they do). But when non polarized caps become fully charged; then what? Where does it (Q) go when released?

2.) As simple as Ohm’s Law is, I can’t understand why I keep reading about dropping current but not voltage drops in ‘capacative reactance’. Early on in electronics it’s repeatedly stressed that current is constant – does not change – so why are capacitors described as current resistors and voltage enhancers? What EXACTLY do they do? Purpose!?

• Wayne Storr

Regarding 1.) A capacitor stores energy on its two plates in the form of an electric field and it is the charge on the plates that forms the electric field. When current flows into a capacitor, charging it, the electric field becomes stronger, it stores more energy, and when current flows out of the capacitor, the voltage across its plates decreases and the strength of the electric field also decreases. The strength of the charging current is maximum when the two plates are uncharged. Then capacitors store energy as a result of their ability to store charge and the property of a capacitor to store this energy is called its capacitance. There is a direct relationship between capacitance, charge and voltage. A pure capacitor will hold its charge indefinitely, but in reality the DC or AC circuit in which a capacitor is connected will constantly charge and discharge it as the flow of electrical energy moves back and forth.

Regarding 2.) I have never described capacitors as: “current resistors and voltage enhancers”. You need to ask this question to the person who refers to them as such.

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