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Ohm's law is about three things:
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(V) |
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(I) |
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(R) |
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Just remember: |
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V = I x R |
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This equation is what Ohm's Law says.
It is a very simple bit of physics. |
This is sometimes called potential difference or PD. It is
measured in Volts. If you have not learnt about voltage before, you will certainly have
seen something like 1.5v on the side of the batteries which you put in your walkman. The
"V" in the formula given above stands for voltage. Here are
some typical values:
| 70mV |
the voltage across the inside and outside of a human
nerve |
| 1.5v |
the voltage of a walkman battery |
| 6v |
the voltage of a moped battery |
| 12v |
the voltage of a car or motorcycle battery |
| 24v |
the voltage of a 50 seater coach battery |
| 110v |
mains voltage in the USA & some continental
countries |
| 240v |
nominal mains voltage in the UK |
| Thousands of volts |
voltages in amateurs' antennas whilst transmitting |
| Tens of thousands of volts |
voltages in overhead power cables |
As you can see, there is a lot of difference between the voltages
in our nerves and muscles and the voltages in the mains power supply. It does not make a
lot of sense to put your fingers in the mains power sockets! So be careful not to hang
onto my long wire antenna when I am transmitting. You can charge yourself up with a van de
Graaf generator to a few thousand volts, but the currents involved are quite small, it is
not the same with the mains which could give you a lethal shock. I have tried it once and
it was not a pleasant experience.
1.5 volts might be enough to light up a small tent with a torch,
but not enough to light up your living room.
1mv (one millivolt) is 1 thousandth of a volt.
1Mv (one Megavolt) is 1 million volts.
Batteries, the mains, dynamos and electrical generators provide
the energy to force electrons around electric circuits. The bigger the voltage is, the
greater the "force" making electrons go round a circuit. You can think of it as
being like a hill: if you fall down the hill you could roll to the bottom. The steeper the
hill is the quicker you will roll down it.
Current is measured in amperes or amps for short. We use the
symbol "I" in the formula to represent current. (The reason for
using "I" rather than "C", is that
"C" is already used for something else.) The kinds of current
flowing in our nerves and muscles is only a few microamps: the currents flowing in the
mains might be as much as 13 amps. The currents flowing in my antenna could be very much
higher! Please don't touch my antenna whilst I am transmitting.
Resistance is to do with how easy it is for the electric current
to flow through a material, e.g. a piece of copper wire. Although your physics teacher
will tell you that copper is a very good conductor of electricity, it does have a
measurable resistance. Some materials have virtually no resistance when they are cooled
down to absolute zero, they are called super conductors. Mercury will do this.
Materials like plastic, wood, polythene, ceramics and rubber have
very high resistances so that it is almost impossible for electric currents to flow
through them. These materials are called insulators. They may not be perfect.
Materials like copper, silver, and gold have very low
resistances. In fact all metals will conduct electricity. They are called conductors. Even
so, they do have some resistance to the flow of electrons through them. A perfect
conductor is called a superconductor, it has zero resistance; very cold
mercury acts as a superconductor.
Resistance is measured in Ohms.
Ohm's Law says that there is a relationship between these three
factors. So if you know two of the values you can easily work out the third one.
| V = I x R |
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V |
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| I = V / R |
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| R = V / I |
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I |
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R |
The "V over I and R
triangle" should help you to remember these
three equations. If you know the current and resistance and want to calculate the voltage,
you use the first equation. If you know the voltage and resistance and want to calculate
the current, you use the second equation. Lastly, if you know the voltage and current and
want to calculate the resistance, you use the third equation.
Note: these pages were first written for students of the Radio
Amateurs Exam
I have written a page about power
which is measured in watts. It is quite easy to calculate the power dissipated in a
circuit using the equation W = V x I (some people use P = V x I). It is
important for us to know about power because our licences limit how much power we are
allowed to use. In the UK the maximum power allowed is 400w, though we are limited to
smaller power levels on some bands. If you cannot calculate and measure power levels you
will end up by breaching the terms and conditions of your licence and losing it.
You will find lots of multiple choice questions in "HOW
TO PASS THE RAE" which you can obtain from the Radio Society of Great
Britain. ISBN 1 872309 17 8.
Nigel D Purchon G0WKM.
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