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Ohm`s Law

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Subido el 6 de noviembre de 2013 por Samuel E.

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An explanation of Ohm's Law

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Okay, now we're going to talk about Ohm's Law. 00:00:00
Ohm's Law is probably the most fundamental equation in the whole world of electric circuitry. 00:00:04
Anyone who deals with circuits in any way understands and uses Ohm's Law on a regular basis. 00:00:09
And I'm going to explain this in the context of a diagram of a simple circuit here. 00:00:15
And I'll just draw a battery and a light bulb. 00:00:21
Because, once again, we're all familiar with the batteries and light bulbs. 00:00:23
We've seen things like this. 00:00:28
So this is easy to understand. Let's imagine a little light bulb here, and we run a wire from the battery over to the bulb, and then from the bulb back to the other end of the battery, and inside the bulb is the filament, and when the current goes through it, it shines. 00:00:29
whenever you have something like this a circuit element like this light bulb in the circuit there 00:00:50
are three variables to consider there's the voltage and the symbol for the for voltage is v 00:00:57
capital v and voltage is measured in volts so you would say in this case the voltage is 1.5 volts 00:01:08
or if you have the electrical outlet in your home you would say the voltage is 120 volts 00:01:19
unfortunately the symbol for volts is also V so you end up writing things like 00:01:24
this you end up writing things like here V equals 1.5 V which looks like strange 00:01:31
algebra how can V equal one and a half times V that doesn't seem to make sense 00:01:37
but that's not what this says this is not an algebraic algebraic error this 00:01:41
says the voltage the thing is 1.5 volts and this second V over here is the unit 00:01:46
they just happen to have the same letter but it's pretty easy to tell by the 00:01:52
context which is which so you have the voltage in volts you have the resistance 00:01:55
and we call that capital R resistance is exactly what the name implies its 00:02:01
resistance to current flow everything has some resistance well almost 00:02:11
everything almost everything impedes the flow of current in some way and 00:02:16
things that you put in the circuit for example the light bulb has a significant 00:02:21
amount of resistance the wires have very little resistance which is exactly why 00:02:25
we use them in fact the resistance is the of the wires is so small compared to 00:02:30
the resistance of the bulb that we consider the resistance of the wires to 00:02:36
be zero that's really a good approximation it's not quite zero but 00:02:39
it's really close so it's a good approximation to just ignore the 00:02:43
resistance of the wires the resistance to current flow in this circuit 00:02:46
primarily comes from the bulb and anything that you put into the circuit 00:02:50
you could put a light bulb there a heater a toaster anything has some 00:02:55
electrical resistance and resistance is measured in ohms ohms and you recognize 00:02:59
the name ohm there we'll talk about George ohm in just a second resistance 00:03:07
is measured in ohms and the symbol for ohms is the Greek letter Omega this is 00:03:11
the last letter of the Greek alphabet and it looks kind of like a horseshoe 00:03:17
like that so you might say something like this you might say resistance 00:03:21
equals six ohms or the resistance equals a thousand ohms that's just how that 00:03:25
symbol is used and then the third thing to consider is the the current and the 00:03:32
symbol for current is I the letter C is used for other things so that's not the 00:03:41
most convenient or intuitive symbol, but just roll with that. Just take that as a fact. Current is 00:03:47
commonly represented by the letter I, and it's measured in amps, and the symbol for amps is A, 00:03:52
which is kind of nice. That makes sense. Sometimes you hear the term, instead of amps, you hear 00:04:00
amperes, and that's the correct term. This is named after a person, Andre Ampere, a French 00:04:05
physicist who did a lot of work with electrical theory at the time this was 00:04:13
being discovered and amps is just short for amperes and you hear it said 00:04:18
both ways amps or amperes so someone might write for example the current is 00:04:21
5 amps or they might write the current is 20 amps that's just how the symbol I 00:04:27
is used and the A is used in this context I is the current and it's 00:04:34
measured in amps amps is the unit now here's Ohm's law these three things the 00:04:38
voltage the resistance and the current are all related by a nice simple 00:04:46
mathematical equation this is it V equals I times R V is the voltage I is 00:04:51
the current and R is the resistance and this equation is known as Ohm's law and 00:05:03
and it's not really a law in the sense that it that it's universally applicable 00:05:15
like other laws of physics 00:05:23
but it does work incredibly well for most conductors across a wide range of 00:05:24
temperatures and conditions 00:05:29
so this does accurately describe the way things behave in the real world 00:05:30
the voltage flowing through something is always equal to the current 00:05:35
times the resistance now here's a picture of George Ohm 00:05:39
this guy was a schoolteacher and he was conducting electrical experiments 00:05:42
and this was soon after the electric battery had been invented 00:05:47
and before the electric battery people were only able to 00:05:50
do experiments with static electric charges and they had these little devices 00:05:53
called Leiden jars 00:05:57
that they could use to store up some pretty big static charges 00:05:58
but it would discharge real quickly they couldn't do they couldn't do 00:06:02
experiments with any sustained current flow 00:06:05
once the battery was invented and that was in the year 1800 00:06:07
that allowed scientists and physicists to begin to experiment 00:06:11
with the continuous flow of current and Ohm discovered this in his experiments 00:06:15
this fundamental relationship between the voltage and the current and the 00:06:19
resistance 00:06:23
and as as I said before it's one of the most important and most fundamental 00:06:23
equations 00:06:28
in the study of electric circuits now I want to say one more thing about the 00:06:29
equation 00:06:33
Ohm's law which is commonly written like this 00:06:33
V equals IR can be rearranged 00:06:39
algebraically what if I divided both sides 00:06:43
by R and then you see over here on the right side 00:06:47
the R up top and down below will cancel and I'm left with 00:06:52
I equals V over R so let's write it like that 00:06:55
I equals V over R the equation makes a lot of sense 00:06:59
if you think about it this way remember let me write it over here 00:07:07
I equals V over R and this is just 00:07:11
the original equation V equals IR just rearranged algebraically 00:07:16
so I is the current V is the voltage 00:07:20
and R is the resistance so think of 00:07:29
think of voltage remember voltage is what motivates the current flow 00:07:39
you increase the voltage that causes more current to flow 00:07:43
and resistance is that which opposes the current flow 00:07:47
so if you put in something that has higher resistance 00:07:50
That will interfere, that will impede and cause lower current, a lower amount of current to flow. 00:07:53
This fact, these facts show up mathematically in this equation. 00:07:59
You can imagine putting two numbers in here and doing the math, just a division, V divided by R, 00:08:03
and getting out a number for I. 00:08:09
And this makes sense. 00:08:12
If you put in a big number for V right here, then when you do the calculation, 00:08:12
you'll get out a bigger number for I for the current. 00:08:17
and if you put in a big number for the resistance then when you do this calculation you'll be having 00:08:19
a large number be dividing by a large number down here in the denominator so you'll end up with a 00:08:26
smaller value for i so this makes sense increasing if you understand that voltage is what causes 00:08:31
current flow then you understand that increasing the voltage causes more current so putting in a 00:08:37
bigger number for v there gives you a bigger number for your current and if you understand 00:08:41
that resistance is interference with current flow you understand that putting in a bigger number for 00:08:46
r will give you a smaller number for current when you do this calculation so this this equation 00:08:50
makes a lot of sense and it also matches what we see in the real world and that's fundamentally 00:08:56
the test for whether or not something is true in science is does it in fact fit with the actual 00:09:02
data that we see in the real world and in this case it does ohm's law i equals b over r but more 00:09:08
commonly written like this, V equals IR. 00:09:13
And we'll come back next and do some simple example 00:09:18
problems with this equation. 00:09:21
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Idioma/s:
en
Etiquetas:
EducaMadrid
Autor/es:
Derek Owens
Subido por:
Samuel E.
Licencia:
Reconocimiento - No comercial - Compartir igual
Visualizaciones:
83
Fecha:
6 de noviembre de 2013 - 23:32
Visibilidad:
Público
Centro:
IES JOAQUIN ARAUJO
Duración:
09′ 25″
Relación de aspecto:
1.76:1
Resolución:
480x272 píxeles
Tamaño:
9.14 MBytes

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