Download Ohms Law for Beginners.

Electric circuits
Tech 101.
Ohms Law for Beginners.
Supplement
Prepared by Mike Crompton. (Rev. 6 February 2009)
Ohms Law for Beginners.
When we look at the world around us it is obvious that there are thousands of different
materials ranging from ‘man made’ to what ‘man is made from’. However in reality there
are actually only just over 100 materials called ‘elements’, all other materials are made
from combinations of these 100+ elements some of which are very rare and some very
common. Each one of the elements is made up of atoms, and each atom has very similar
components differing mainly in the number of each component. The nucleus is the center
of each atom and it’s main contents are particles called ‘Neutrons’ and ‘Protons’.
Orbiting around the nucleus are the third particles called ‘Electrons’. All 100+ elements
break down in their natural state into different types of groups e.g. gasses, solids, good
conductors of heat or electricity and other various technical categories.
The simplest element is the gas ‘Hydrogen’ which
Hydrogen Atom.
has one neutron and one proton in it’s nucleus with
Eone single electron orbiting around it, as shown at
Electron
-ve
right. Next is the gas ‘Helium’ with two protons in
Nucleus
the nucleus and two electrons in orbit. To all
Orbit
intents and purposes from an electrical/electronic
P
point of view, we can ignore the neutrons as they
+
Proton
Neutron
play very little part in understanding electricity.
N
+ve
no charge
Each atom has it’s own ‘Atomic Number’ which
actually denotes how many protons or electrons it
contains. Therefore hydrogen is #1 and helium is
#2. Some other examples are: Aluminum #13, Iron #26, Copper #29, Tin #50 and Lead
#82. To be stable, each atom will have the same number of electrons as protons. When an
atom has a large number of electrons they arrange themselves in ‘layers’ with each layer
having a specific number of electrons but increasing in number as they get further away
from the nucleus. The very outside layer is called the ‘Valence’ layer and it plays a very
important part not only in electronics, but whether an atom will readily combine with
other atoms, or start a chemical reaction. For example we know that hydrogen and
oxygen will combine to make water and oxygen and iron combine to make rust.
Atoms of course are very, very small with millions able to fit on the point of a pin. They
do however contain a large amount of energy compared to their size. We know for
example that splitting the nucleus of a uranium atom can cause the ‘broken bits’ to hit
and split more and more atoms causing a ‘chain reaction’ that is known as a nuclear
explosion (an atomic bomb).
Some of the energy stored in an atom comes from two of the three particles (electrons
and protons) containing an electrical charge (a small amount of electrical energy).
Protons have a positive (+ve) charge and electrons a negative (-ve), neutrons have no
charge. In electricity, charges that are the same (like charges) exert a force that tends to
repel each other, while charges that are different (unlike charges) exert a force that
attracts them. These two forces combine to give each atom a balance of energy, the
electrons (–ve) are kept in orbit and prevented from flying off by the attraction of the
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nucleus’s protons (+ve), but are stopped from crashing into the nucleus by the centrifugal
force of their circular orbit.
Remember those electrons in the outer (valence) layer? Well in some elements
(particularly the metallic group) these electrons are designated as ‘free electrons’. Their
number and distance from the nucleus determines the ability of the element to conduct
electricity. i.e. be a good or poor conductor. With 1, 2 or 3 free electrons (a valence of 1,
2 or 3) an element is a good conductor. Silver is the best electrical conductor followed by
copper, gold and aluminum. Elements with a valence of 5, 6, 7,or 8 are poor conductors.
This leaves elements with a valence of 4 as ‘Semi Conductors’, a whole different subject.
Under normal conditions there is random movement of free electrons in all directions
within any good conductor. If we take copper for example, the random movement within
a piece of wire has no noticeable effect, BUT if all the free electrons move in one
direction only we have electrical current flow! It takes a lot of electrons to make up a
measurable amount of current flow. A man called ‘Coulomb’ decided that 6.28 x 1018
(that is 6,280,000,000,000,000,000) electrons was a measurable amount. That is one heck
of a lot of electrons. A man called ‘Ampere’ decided that if that number of electrons
flowed past a point in one second it would be called a current flow of one AMP.
So now we know about electron movement and current flow but we need to know what
will make the electrons all move in one direction. Well of course it has to be some kind
of force or pressure and there are several sources. Magnetic force is one, heat is another
but it tends to cause random movement, chemicals can also provide movement but the
force that we are interested in was discovered by a man called ‘Voltaire’. That force is
called ‘voltage’. Now voltage is a little more difficult to visualize than current flow. It is
easy to see (in your mind’s eye) those electrons scooting along that piece of wire, but
voltage????? Well think of voltage this way; empty a bucket full of atoms on to the table,
and take one electron from each atom and put them in a separate pile. There would be a
real force of attraction between the atom pile and the electron pile. Each atom is short of
an electron and therefore is in a state of unbalance, with more positive protons than
negative electrons (the atoms would be positive ions). The electrons would therefore be
attracted (unlike charges attract) to the atoms. That force is called voltage. A shortage of
electrons at one point and an excess of electrons at another. If a path is provided from one
pile to the other, the negative electrons would rush to the positive ions and behold we
would have current flow. If those electrons had to flow through any kind of device (a
lamp for example) that device would be activated as long as the voltage was pushing the
electrons.
Voltage is a static force. It doesn’t flow through devices like current. It is a potential
energy. It appears across two points like pressure. The difference between two voltages,
say zero volts and five volts, is actually called a ‘potential difference’ or an ‘electro
motive force’ (EMF). Literally an ‘electron moving force’. The greater the voltage the
more force it can exert, or the more electrons it can move, or the more current it can
generate. You can’t have current without voltage but you can have voltage without
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current (There is voltage in the wall socket even though there is nothing connected to it.
Don’t stick your fingers in to prove it!!!). But there is a fly in the ointment.
What if some thing is acting against the voltage, reducing the amount of current flow?
What if something is ‘resisting’ the electrons? Well something is, and naturally it is
called ‘resistance’. Let us look at pushing a concrete block along the floor. A certain
amount of push (voltage) will result in the block moving a certain distance (like current
flow). If the block is resting on carpet, friction will only allow that push to move the
block a little. If the block is on ice that same push moves the block a lot further. To move
the same distance on carpet we would need more push. The ice offers much less
‘resistance’ than the carpet. In electronics, each conducting metal will oppose electron
movement by varying amounts due to its physical characteristics. However, if we had a
fixed amount of push (V) we could control the amount of movement (current) by
deliberately adding resistance. To accomplish this devices called ‘Resistors’ were
manufactured, each one having a different but controlled amount of resistance. So it
obvious that there is a relationship between ‘push’, ‘resistance’ and ‘movement’. This is
also true in electronics. There is a definite relationship between ‘voltage’ ‘resistance’ and
‘current flow’.
Mr. Ohm, bless his heart, defined that relationship, made up a set of laws and naturally
called the whole thing OHM’S LAW. First, each entity was given a designated letter and
or symbol to identify it. Voltage was given the letter ‘V’, Current flow ‘I’ measured in
Amps ‘A’, Resistance ‘R’ measured in Ohms and using the symbol Ω. So one could now
say “ a voltage ‘V’, would produce a current flow ‘I’ of so many amps ‘A’, against a
resistance ‘R’ of so many ‘Ohms’ (Ω). He also defined the inter relationship by stating
that “the amount of current flow can be determined by the amount of voltage pushing
against the amount of resistance” In mathematical terms that is: Current is equal to the
Voltage divided by the Resistance
I = V/R
And by transposing that formula for V and R we also get:
V=IxR
and
R = V/I
It was also established that when voltage pushed current through resistance there was
work done. The work done was named after a person called Watt. To calculate the
amount of work done in ‘watts’ (W) you simply multiply the voltage by the current:
W+VxI
If you didn’t have the value of V or I then the Ohm’s law equivalent could be substituted
for either one, this gives us:
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W = V x (V/R) or W = V /R
and
W = (IxR) x I or W = I2R
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In the first case the Ohm’s law equivalent of I (in brackets) was substituted and in the
second case the equivalent of V (in brackets).
The values of voltage, current and resistance vary drastically from circumstance to
circumstance. The value of each may be preceded by a symbol or letter to change it from
it’s base amount to a larger or smaller amount. For example; 1 Amp is very little in
electrical terms. A 100 Watt light bulb has approximately 1 Amp flowing through it
whereas a burner on a stove may have 10 to 15 Amps flowing through it. However, in
electronics 1Amp is a huge amount. It is more likely for current flow to be in the
thousandths or millionths of an Amp. To indicate these much smaller amounts we use
‘mili-Amps’ (mA) for thousandths of an Amp and ‘micro-Amps’ (μA) for millionths of
an Amp. For example I = 5mA (5x10-3Amps) or I = 200μA (200x10-6 Amps).
The opposite is true for resistance. Values of 1 to 100Ω can be relatively high in electric
terms but quite low in electronics. Values in the thousands or millions of Ohms are very
common so resistance is often designated with a ‘k’ (kilo) for thousands of Ohms and
‘M’ (Mega) for millions. For example 33kΩ for 33,000Ω or 1.2MΩ for 1,200,000Ω.
If 2 or more resistors (or any
R1
R2
R3
devices) are connected end to end
and then to a voltage source, say a
3 Volts
6 Volts
5 Volts
0 Volts
2k
1k
3k
battery, the result is an electronic
circuit called a ‘Series circuit’. A
Components connected end to end forming a series circuit.
circuit is an electronic ‘blueprint’
showing how various parts can be
6V Battery
Electron (current) flow
connected together to form a device
+ _
that will perform certain functions.
See the diagram at right. In a series
circuit (cct.) the current has only one path, and has to flow through all the components.
There will be a voltage loss or drop across each component. The amount dropped is
directly proportional to the component’s resistance, ending at zero volts at the end of the
cct. There are a set of laws/rules that apply to this type of cct. Of course Ohm’s law is the
main one but others are:
1. The current is the same through all parts and components in the cct.
2. The sum of the individual voltage drops is equal to the supply voltage.
3. The total resistance (RTOTAL) is equal to the sum of the individual resistances.
These rules apply to all series circuits regardless of what the components are, be they
resistors, stoves, lamps, etc.
Voltage the same across
all components.
6V
6V
6V
Components all
If 2 or more components are connected
connected to the
same points or a
across a voltage source, one end of each
'Parallel' circuit.
+
R1
R2
R3
component connected to the same voltage
30
20
10
source terminal and the other end of each
Total
- current
Vsupply
flow
component connected to the other source
(electron.)
6V
terminal, the circuit is said to be a
‘Parallel’ circuit. See the diagram at right.
Divided current flow (electron).
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This means that each component of a parallel cct has the same voltage across it since they
are all connected to the same points. However the cct current will divide and some will
flow through each branch/component. The ratio of current flow will be inversely
proportional to the component’s resistance. The largest resistance having the least amount
of current and the smallest resistor having the most.
As with the series circuit there are different laws/rules that govern a parallel cct.
1. The voltage is the same across all components.
2. The sum of the individual branch currents is equal to the total current.
3. Total resistance:
a) If all resistances are the same, the value of one resistance divided by
the number of resistors. e.g. three 90Ω resistors = 90/3 = 30Ω.
b) With 2 (only) unequal resistors, the product divided by the sum. e.g. a
100Ω and a 10Ω = 1000/110 = 9.9Ω.
c) With 3 resistors of different values, 1/RTOTAL = 1/R1 + 1/R2 + 1/R3
(etc). e.g. a 10Ω a 20Ω and a 30Ω = 1/10 + 1/20 + 1/30 = 0.18333.
1/0.18333 = 5.45Ω This formula will work for any number of resistors
including a) and b) above.
d) RTOTAL will always be less than the smallest resistor.
The wiring in your house connects all the lights and other devices in parallel. That way
each has the full 110 Volts supplied to it, and varying amounts of current flowing through
it. It also means that if one device ‘blows up’ all the other devices will continue to
function. Were they all in series, the 110 Volts would have to be divided up amongst all
the devices and none would receive sufficient voltage to function. Also, should one
device ‘blow up’ it would ‘open’ the circuit and all other devices could not operate.
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