Basic Electricity and Electronics
OHM'S LAW
Well let's start at step one, the atom. We know that atoms are composed of many particles, but for our purpose
we only need two, proton (nucleus) and electron. Electrons orbit the nucleus, in some elements the electrons are
firmly fixed in their orbits, we call these materials insulators. In other materials there are many loosely attached
electrons, these are easily dislodged and are constantly moving from atom to atom in a random manner, these are
conductors. There is also a group of materials that fall between these two extremes which we call
semiconductors.
When an Electro Motive Force (EMF) is applied across a conductor the positively charged pole attracts the
negatively charged electrons. As these electrons move toward the positive pole, the area they move from
becomes more positive and attract electrons from the adjacent area, as this continues a flow of electrons from
the negatively charged pole to the positively charged pole is created. At the same time electrons expelled from
the negatively charged pole replace those that move into the positively charged pole, leaving the conductor with
a net neutral electrical charge when the EMF is removed. We measure the EMF in units called "volts" often
designated "E" in formulas and the electron flow as "current" designated as "I". There is one other essential
element involved in this and that is "resistance" designated simply as "R" and is the characteristic of resisting
the flow of electrons. The more free electrons the less resistance and vice versa.
These are the three major factors involved in electrical and electronic equipment, "Voltage" the applied force,
"Current" the quantity of electrons flowing in a conductor and "Resistance" the opposition to electron flow.
There is a definite mathematical relationship between these values. This relationship is expressed in it's simplest
form by "OHM'S LAW" , which says:
E = IR
I = E/R
R = E/I
Where "E"= EMF in volts, "I" = current in Amperes and "R" = resistance in ohms
Let's look at a few examples:
100 watt light bulb,
To simplify this we will say that your house is supplied with and operates from 100 volt circuits.
(in fact the voltage in most homes vary from about 107 to 128 volts)
Ok, now we have two known quantities 1. 100volts and 2. 100watt bulb
We only have one of the essentials for OHM'S LAW "VOLTAGE" or "E", however we can derive
another by simple math.
Power (WATTS) = VOLTAGE (E) X CURRENT (I)
or
W = E * I
By simply inverting this simple equation we get:
E = W/I and I= W/E
Using I = W/E I = 100/100 I = 1 amp.
We now have both "E" (100) and "I" (1)
From Ohm's Law:
R = E/I
R = 100/1
R = 100 ohms
The value of resistance which will allow 1 ampere of current to flow with 100 volts applied is 100 ohms.
Therefore a 100 watt bulb supplied with 100 volts has 100 ohms resistance and draws 1 amp. of current.
How Much "I" would it draw at 50 volts? How many watts would that be?
(HINT: it is not 50 watts)
" The Analogy"
A common analogy that is often used is water flowing through a pipe.
The pressure which forces the water to flow is equivalent to the EMF (voltage "E")
The water itself represents the electrons.(current "I")
The pipe represents resistance ("R")
When the pressure is increased (voltage "E") the amount of water ( current "I") forced to flow in the pipe
increases. If the pressure ("E") decreases less water("I") flows.
or
As voltage increases, current flow also increases.
As voltage decreases, current flow also decreases.
If the pressure ("E") remains unchanged and the size of the pipe is decreased, the resistance to flow("R")
increases and the amount of water ("I") flow is decreased. If the pipe is made larger the resistance
to flow("R") decreases and more water ("I") flows .
or
As resistance increases, current flow decreases.
As resistance decreases, current flow increases.
As you can see current "I" and therefore the amount of power (W = EI) in a circuit
can be manipulated by changing either the voltage "E" or resistance "R" or both.
These are the three primary factors in electrical circuits ("E" "I" "R") and
OHM"S LAW defines their relationship.