Electrical Properties

This section is a reference section used by other topics.  It discuses the properties of capacitance and inductance which exist in our layouts anyplace where conductors exist.   Capacitance and Inductance is found as a parasitic property of wires and coils (motor) or an intentional property in the form of an electrical component such as a Capacitor or Inductor.

There electrical properties have always existed in our layouts, but under DC power control, these properties never were observed and often ignored or left intentionally forgotten.   With DCC, we are now dealing will all forms of electricity that are in various forms of AC electricity.  AC makes these properties come alive and make themselves observable if one know where to look.  Stated another way, these properties go straight to a understanding of problems that may appear strange or magical.
I will be using the KISS (Keep It Simple Stupid) principle here when I talk about these properties. My goal here is less about technical accuracy and more about getting a feel about these properties.

When current flows down a wire and/or voltage is present on a wire, there are 5 physical properties or effects that can come into play.

PROPERTY UNITS OF MEASURE
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1) Wire Resistance Ohms
2) Wire Magnetic Fields Gauss or Tesla's
3) Wire Electric Fields Volts/Meter
4) Wire Inductance Henries
5) Wire Capacitance Farads

Lets look at each one for I bet you quickly realize you already have seen these effects in one form or another:

#1) RESISTANCE
Resistance is easiest to understand for that is what most people are familiar if you work with any electricity.  Resistance is always present in all conductors.   We buy wire based on resistance per foot for example in the form of different wire gauge.  Ohms law allows you to calculate the voltage drop for a given wire resistance and a DC current.  We say DC current for there are other additional forms of resistance when AC is present as will be discussed later.  We use Ohm meters to measure DC resistance.  We call resistance "resistance" because this property opposes the flow of electricity.   The fact that the voltage at the end of the wire is less than what you started with means you have a power loss factor.






#2) MAGNETIC FIELD
Magnetic Field are not so strange if you every work with electro magnets as a kid in science class.  IF you ever wrapped some wire around a nail to make a coil and connected it to a battery, you made a electro magnet.  You can see the effect of the magnet when you pick up other metal objects off the ground with the magnet but as soon as you disconnect the battery, all the metal object fall back to the ground.  Current flow is what creates the Magnetic Field.   The higher the current goes, the stronger the magnetic field gets.  What is also true is the more turns of the wire you give the coil, the stronger the magnetic field gets too.

What is importance to remember is that this is a "field" which means it flows through the air effecting other things including other wires.  A magnetic field is one of two ways wires can couple electricity back and forth between each other.  The closer the two wires get to each other, the more effect they can have on each other.  Stated another way, the electrical coupling between them gets stronger.

#3) ELECTRIC FIELD
An ELectric Field is also not new to you for in fact you often encounter it in the form of Static Electricity.  Ever gotten a static shock from anyone when you tough them or touch some object?  Anyway the picture on the left graphically shows you the electric field that is created with static electricity.  Voltage is what creates the electric Field.  The higher the voltage goes, the stronger the electric field gets.  

What is importance to remember is that this is a "field" which means it flows through the air effecting other things including other wires.  Stated another way, an electric field is one of two ways wires can couple electricity back and forth between each other.  The closer the two wires get to each other, the more effect they can have on each other.  Stated another way, the electrical coupling between them gets stronger.





#4) INDUCTANCE 
If you do not know what an inductor or inductance is, go here: Inductance

Inductance is an effect that can be calculated and measure but cannot be seen.  However you see high levels of inductance every time you look at a coil of wire as in the example of the #2 or magnetic field.  A coil is simply another name of an Inductor but is more visually descriptive of what most people see.  A picture of a wire wound inductor is on the left.  Inductors have inductance!  Inductance can best be understood if you think of it as being a another form or cousin of Resistance in that it to can oppose the flow of current in the wire.  The fact that the voltage at the end of the wire is less than what you started with means you have a power loss factor.

Inductance is one of two properties that fall into the class of "AC resistance" more accurately knows as Impedance with units of Ohms.  Inductance makes itself known when the current flowing through the inductor is changing in amplitude or is NOT constant (Not DC).  In other words, Inductance only effects AC current flow.  Like resistance, inductance is in series with the wire.  Also like resistance it opposes the flow of current by creating a voltage drop across itself.  It uses the Magnetic Field to make this happen.  How much is current opposition it can offer is proportional to the value of the inductance.

With pure DC, inductance disappears which is why we never talk about this wire property much when dealing with DC powered layout.  To learn more about inductance as it relates to wire and track, go here:Track & Wire Inductance

#5) CAPACITANCE
 If you do not know what a capacitor or capacitance is, go here:Capacitance

Capacitance is an effect that can be calculated and measure but cannot be seen.  However you see high levels of capacitance every time you look at a Capacitor.  A picture of a large capacitors are on the left.  Capacitors have Capacitance!  Capacitance only effects AC current.  Capacitance can best be understood if you think of it as being a another form or cousin of Resistance.  However instead of opposing current flow in a wire, it simply redirects it elsewhere by providing another path for it to go.  The fact that the current at the end of the wire is less than what you started with means you have a power loss factor.

Capacitance is one of two properties that fall into the class of AC resistance more accurately knows as Impedance with units of Ohms.  Capacitance makes itself known when the votlage on a wire is changing in amplitude or is NOT constant (Not DC).  In other words, Capacitance only effects AC voltage.  Unlike inductance which is in series with the wire, capacitance works between two wires.   Typically we talking about the pair of wires (think bus) that are carrying the current from the start (Source) out to the destination (load) and back to the source.  A good example is a track bus that carries current from the booster out to the track and back.
Capacitance opposes changes in voltage levels by creating a virtual "short circuit" in the air between the two wires.  It use the Electric Field to make this happen.  By redirecting ONLY the changing current flow that is attempting to force the change in voltage, it allows the voltage to remain constant on the wire.  Stated another way, the voltage at the end (load) of the wire bus will be more constant unlike the voltage at the beginning of the wire bus (Booster).  Another way to look at this is the capacitance if filtering (picking) out the AC current and redirecting right back to the booster.  How much current redirection it can offer is proportional to the value of the capacitance.

With pure DC, capacitance disappears which is why we never talk about this wire property much when dealing with DC powered layout.  To learn more about capacitance as it relates to wire and track, go here: Track and Wire Capacitance


Reference Impedance: http://en.wikipedia.org/wiki/Electrical_impedance    
Subpages (2): Capacitance Inductance