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Energy Storage – Capacitors – Equalization of Pressure

A Low Voltage Electrolytic Capacitor

In electrical theory we have so far been dealing with unchanging or very very slowly changing values. Tonight’s post is the start of talking about changing values of current and voltage. We are still talking about Direct Current, but we are starting to change the course of the path we are on. We are going to talk about a new component called a capacitor.  I recommend you watch the video before reading the rest of this post to understand how a capacitor is constructed.

The nice thing about electrical theory is it can be used to understand many systems and that is what I attempted to do in the video with the analogy of the tank storing water and the way pressure will build in the tank. Before the development of digital computers, many physical properties were modelled and simulated on analog computers. The analog computers made much use of capacitors to store energy. We will be getting there.  I am all for the increase in effectiveness of learning several things by learning one and making analogies.

As I produced and edited the video, I realized that I have “blown past” many fundamentals in the interest of accomplishing something useful with the thermistor project.  It is time to back-up and pick up some of those fundamentals.

Matter is made of up atoms and for our purposes atoms are made up of a nucleus consisting of positive charged protons and neutral neutrons.   Floating around this nucleus is charged particles called electrons.  If all of the electrons are tightly held in a material the material will not conduct electricity and we refer to it as an insulator.   If the material only very loosely holds the electrons and they can bounce from atom to atom easily, the material is a good conductor.  No material is a perfect conductor and none is a perfect insulator.   Science has in recent years got very close to the perfect conductor with materials called superconductors.  These are very special materials that operate at very low temperatures and are of no interest to us for this discussion.

The rule for electrical charges is like charges repel and opposite charges attract.  Normally this means electrons are attracted to an area that does not have enough electrons to neutralize the positive charges of the protons.   Excess electrons is expressed as being charged negative and a deficiency of electrons is said to be positive charged.

Insulators are of interest to us, especially with respect to capacitors.  First, although the electrons are held tightly to their atoms, when it is subjected to an electric field, the electrons tend to be pulled more toward the positive charge side of the field.   This is exactly what is occurring in the capacitor because of the charge on the plates on either side of the insulator.  This distortion of the path of the electrons can make the field stronger and is different for different materials.   It is given the name the dielectric constant.

The second thing that is important to know about the insulator is there is a point where the electric field can pull even the tightly held electrons loose.  At this point the insulator breaks down and will conduct.   When this happens usually heat is created and the capacitor is destroyed.  If you notice the real capacitor in the photograph has a voltage rating.  If the voltage is exceeded the insulation in the capacitor will break down and internal arcing will destroy the capacitor.

The basic unit of measurement of charge is the coulomb.  It takes 1.602177 X10^19  electrons to equal a coulomb.  (That is the one and only time you will ever see that number from me. — It is not important to know.)  The symbol for charge is Q.

When we talked about electric current what we are really measuring is the electron flow past a point.   A current of 1 Ampere is a charge of 1 coulomb flow per second.   This can be though of to being similar to a flow of water of 1 gallon per minute (or 1 litre per minute.)

Since a capacitor can be thought of as a tank holding water, the size of the capacitor is the capacity of it.    As in the video, how fast the capacitor “fills up” depends upon the current flowing into it and the size of the capacitor.   The formula for a capacitor size is C = Q / V.  If you think of the tank in the video, if the tank is very skinny and there for smaller capacity, the pressure would increase more rapidly although the volume of water being held inside would be less.   (Q is equivalent to the volume in the tank, V is equivalent to the head pressure at the bottom of the tank.)  The unit of measurement of a capacitor is the Farad.   However, usually the largest unit used to describe real capacitors is the microfarad and often picofarads is used.

This is a lot of information being thrown at you in a short amount of time so we will leave real capacitors to a later time.

 

 

Gary

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