| Topic: |
Science > Physics |
| User: |
"John" |
| Date: |
13 May 2007 10:18:54 AM |
| Object: |
Elementary electrics |
Hi
I don't even understand the basic elements of electrics. It seems
contradictory to me.
Let's say I have two DC power adapters. One 12 V, 1 A and one 12 V 2
A. I connect each adapter to an R ohm resistance. According to Ohms
law, the current flowing through the resistor is I=12/R in both
circuits. So what difference does it make that one power adapter
supports greater currents?
Suppose the resistance is really large. Then maybe the 1 A power
adapter will not be enough? But according to Ohms law, the current
will be smaller if the resistance is larger so maybe even a 12 V 0.1 A
power adapter will be enough. But this seems strange...
My intuition is: "The greater the resistance, the more powerful power
adapter you'll need". The theory seems to say the opposite.
Where do I think incorrectly?
.
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| User: "Androcles" |
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| Title: Re: Elementary electrics |
13 May 2007 01:22:11 PM |
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"John" <janzon@gmail.com> wrote in message
news:1179069534.864009.103460@l77g2000hsb.googlegroups.com...
: Hi
:
: I don't even understand the basic elements of electrics. It seems
: contradictory to me.
:
: Let's say I have two DC power adapters. One 12 V, 1 A and one 12 V 2
: A. I connect each adapter to an R ohm resistance. According to Ohms
: law, the current flowing through the resistor is I=12/R in both
: circuits. So what difference does it make that one power adapter
: supports greater currents?
:
: Suppose the resistance is really large. Then maybe the 1 A power
: adapter will not be enough? But according to Ohms law, the current
: will be smaller if the resistance is larger so maybe even a 12 V 0.1 A
: power adapter will be enough. But this seems strange...
:
: My intuition is: "The greater the resistance, the more powerful power
: adapter you'll need". The theory seems to say the opposite.
:
: Where do I think incorrectly?
:
"Resist" means try to stop.
A resistor tries to stop the current.
A big resistor is more successful than a small one at STOPPING.
In your kitchen you have a tap. It stops the water. It resists
the water if you let it drip, and it has a low resistance when
fully opened.
Switches have low resistance or infinitely high resistance.
They let the current flow or they resist it so well it stops a
altogether.
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| User: "Genius" |
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| Title: Re: Elementary electrics |
13 May 2007 01:51:19 PM |
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John <janzon@gmail.com> whispered:
Hi
I don't even understand the basic elements of electrics. It seems
contradictory to me.
Let's say I have two DC power adapters. One 12 V, 1 A and one 12 V 2
A.
The 1A and 2A indicated the MAXIMUM current the adapter can deliver without producing smoke
and setting your place to fire for example.
Think Watts, 12 x 1 = 12 W, and 12 x 2 = 24 W.
So if you connect a 6 Ohm resistor to the 1A adapter it will eh... die.
Genius
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| User: "H. Wabnig .... .-- .- -... -. .. --. @ .- --- -. DOT .- -" |
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| Title: Re: Elementary electrics |
13 May 2007 11:48:27 AM |
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On 13 May 2007 08:18:54 -0700, John <janzon@gmail.com> wrote:
Hi
I don't even understand the basic elements of electrics. It seems
contradictory to me.
Let's say I have two DC power adapters. One 12 V, 1 A and one 12 V 2
A. I connect each adapter to an R ohm resistance. According to Ohms
law, the current flowing through the resistor is I=12/R in both
circuits. So what difference does it make that one power adapter
supports greater currents?
Suppose the resistance is really large. Then maybe the 1 A power
adapter will not be enough? But according to Ohms law, the current
will be smaller if the resistance is larger so maybe even a 12 V 0.1 A
power adapter will be enough. But this seems strange...
My intuition is: "The greater the resistance, the more powerful power
adapter you'll need". The theory seems to say the opposite.
Where do I think incorrectly?
Practice says the opposite.
Ohms Law Triangle, draw it on paper,
with your thumb cover one and read the other two:
U
R I
U = R * I
or
I = U / R
or
R = U / I
the electric power is P = U * I
P = (U * U) / R
or
P =( I * I) *R
w.
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| User: "PD" |
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| Title: Re: Elementary electrics |
14 May 2007 07:17:11 AM |
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On May 13, 10:18 am, John <jan...@gmail.com> wrote:
Hi
I don't even understand the basic elements of electrics. It seems
contradictory to me.
Let's say I have two DC power adapters. One 12 V, 1 A and one 12 V 2
A. I connect each adapter to an R ohm resistance. According to Ohms
law, the current flowing through the resistor is I=12/R in both
circuits. So what difference does it make that one power adapter
supports greater currents?
Suppose the resistance is really large. Then maybe the 1 A power
adapter will not be enough? But according to Ohms law, the current
will be smaller if the resistance is larger so maybe even a 12 V 0.1 A
power adapter will be enough. But this seems strange...
My intuition is: "The greater the resistance, the more powerful power
adapter you'll need". The theory seems to say the opposite.
Where do I think incorrectly?
Hooked up to the same load (resistance), two 12V adapters will drive
the *same* current, up to the current rating of the adapter. That is,
if the resistance is 100 ohms, the current out of either adapter would
be 0.12A.
The larger adapter doesn't FORCE more current through the load, it's
just capable of driving more current if the resistance is *low* enough
to want to pull that much current.
Remember Ohm's law: I = V/R, so the lower the resistance in a load,
the more current will flow through it, given a particular voltage.
To be absolutely explicit, suppose the load were 9 ohms. Then 12 V
applied across this load would demand 1.3A. The 12V/2A adapter would
be able to supply that, but the 12V/1A adapter would not be able to
supply that. What would actually happen in the latter case depends on
the details of the adapter: the wires might melt, a fuse might blow,
or the voltage might sag to about 9V.
PD
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