Transcript Ohm`s law.pps

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Georg Simon Ohm
Georg Simon Ohm, 1789-1854, German physicist.
Born: 16 March 1789 in Erlangen, Bavaria
(now Germany)
Died: 6 July 1854 in Munich, Bavaria, Germany
Ohm's most important discovery (of 1826) now
bears his name:
.Ohm's importance was not recognized through most
of his lifetime, but in 1852 he became professor of
physics at the University of Munich.
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The unit of resistance is called Ohm in honor of a
German scientice by the name of Georg Simon
Ohm, who discovered that when a conductor has
a resistance of 1 ohm than an emf of 1 volt will
cause a current of 1 amp to flow through a
conductor.
This is a very simple bit of physics.
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.
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This equation is what Ohm's Law says.
•V=IxR
• I = V /R
•R=V/I
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An easy way to remember the formulas
is by using this diagram.
To determine a missing value, cover it
with your finger. The horizontal line in
the middle means to divide the two
remaining values. The "X" in the bottom
section of the circle means to multiply
the remaining values.
• If you are calculating voltage, cover it
and you have I X R left (V= I times R).
• If you are calculating amperage, cover
it, and you have V divided by R left
(I=V/R).
• If you are calculating resistance, cover
it, and you have V divide by I left (R=V/I).
Note: The letter E is sometimes used
instead of V for voltage.
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In this simple circuit there is
a current of 12 amps (12A)
and a resistive load of 1 Ohm
(1W). Using the first formula
from above we determine the
Voltage:
V = 12 x 1 : V = 12 Volts
(12V)
If we knew the battery was
suppling 12 volt of pressure
(voltage), and there was a
resistive load of 1 Ohm
placed in series, the current
would be:
I = 12 / 1 : I = 12 Amps (12A)
If we knew the battery was suppling
12V and the current being
generated was 12A, then the
Resistance would be:
R = 12/12 : R = 1W
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Voltage
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
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Voltage
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.
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Voltage
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.
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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.
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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.
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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.
Resistance is measured in ohms.
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AH/AS Type MH/MV Type GF/MF Type QH Type
CF/QF Type
CR/QR Type
CD Type
CH Type
CE Type
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http://www.purchon.co.uk/radio/ohmslaw.html
http://www.electronics-tutorials.com
http://webhome.idirect.com/~jadams/electronics/ohm.htm
http://www.energy.ca.gov/education/scientists/ohm.html
http://www.english.upenn.edu~jlynch/Frank/People/ohm.html
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