Is the above law true, where I work we have Been testing that theory On a kettle, but the Amps drop when taking mains voltage of 240 to 220 ?
Volts lower amps increase
- Thread starter markymark2406
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Ohm's Law states that that the current through a conductor between two points is directly proportional to the potential difference across the two points. (provided physical factors remain constant) i.e. Current = Potential Difference / Resistance.
If the resistance is constant, then as the PD increases, the current will also increase.
Example. Resistance =200 ohms. PD = 50 volts. I=50/200 = 0.25A
R = 200 ohms PD=200V I=200/200 = 1A
R=200 PD=500V I=500/200 = 2.5A
QED
If the resistance is constant, then as the PD increases, the current will also increase.
Example. Resistance =200 ohms. PD = 50 volts. I=50/200 = 0.25A
R = 200 ohms PD=200V I=200/200 = 1A
R=200 PD=500V I=500/200 = 2.5A
QED
It all depends on the characteristics of the load. Some loads have a (roughly) constant resistance, some loads have a (roughly) constant power draw. Some have a (roughly) constant current draw, some may have a voltage/current response that doesn't fit into any of those categories.
If resistance is roughly constant then as voltage goes down current (and power) will go down. This is what you've observed with the kettle. Of course the lower power will make the kettle take longer to boil so you aren't really saving anything.
If power is roughly constant then as voltage goes down current will go up. This is what you would observe if you tried the test on a PC power supply.
If resistance is roughly constant then as voltage goes down current (and power) will go down. This is what you've observed with the kettle. Of course the lower power will make the kettle take longer to boil so you aren't really saving anything.
If power is roughly constant then as voltage goes down current will go up. This is what you would observe if you tried the test on a PC power supply.
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I have to say this displays a truly scary lack of basic electrical knowledge.
You've only quoted part of the "theory(?)".
It should be:
"To maintain the same power output, if the voltage drops then the current must increase". Useful for (say) cable size calculations; look at the large wires to your car headlamps; with 12V they need about 5 amps each, whereas a 240V lamp of the same power would only need about one quarter of one amp and a smaller wire. The voltage is pressure and your kettle or headlamp bulb is a fixed resistance, so, as you lower the pressure less electric current (amps) will get pushed through the kettle or bulb and the power output is reduced. You can easily see this with the headlamp bulb when the car battery needs charging as the bulbs will be dim.
It should be:
"To maintain the same power output, if the voltage drops then the current must increase". Useful for (say) cable size calculations; look at the large wires to your car headlamps; with 12V they need about 5 amps each, whereas a 240V lamp of the same power would only need about one quarter of one amp and a smaller wire. The voltage is pressure and your kettle or headlamp bulb is a fixed resistance, so, as you lower the pressure less electric current (amps) will get pushed through the kettle or bulb and the power output is reduced. You can easily see this with the headlamp bulb when the car battery needs charging as the bulbs will be dim.
You are absolutely correct...
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A physical law or scientific law is "a theoretical principle deduced from particular facts, applicable to a defined group or class of phenomena, and expressible by the statement that a particular phenomenon always occurs if certain conditions be present."
The law you have tried to quote from has three variables and is for DC current. Forgetting about the fact your using AC so there will be some errors what you state is true if (and this is the point) the third variable remains static. That third variable is power.
Watts = Volts x Amps (In AC circuits also x power factor correction) so as long as watts remain static the law you quote is true. So an electric motor pumping water (Assuming motor revs remain consent) will need more amps as the voltage drops until the point where it fails. Because it will likely slow down a little it will not be exactly following the law.
As already stated a switched mode power supply as used in a computer will auto compensate for voltage variation and deliver the same power so drop the volts and amps will go up.
However the Watt = Joule/Second so to keep the power steady it must do the work in a set time. The kettle with a lower voltage will take longer so you are altering both volts and watts so amps will drop not rise.
There is a device claimed to reduce the power used but one can only reduce the power used if one is producing some form of power that is not required. So a light that also produces heat by stopping it producing heat one can use less power. This of course only works if you don't want the heat. So if we consider an electric only building using CFU in the winter is unlikely to save energy. In fact because with radiated heat given off from tungsten light bulbs the comfort zone for air temperature is reduced so the tungsten light bulbs will waste less energy in winter than the CFU but in summer they will waste more energy as we don't want the heat generated.
With the kettle example the faster it boils the less energy escapes so the faster it boils the better. If however we put a heat retaining jacket on our kettle so heat was not lost we could boil it slower.
The most wasteful way to boil water is likely with a gas hob as more heat escapes into the atmosphere both as pure heat and as steam as the hob does not condense the water out of the exhaust gases. Plus to remove the steam requires an extractor which removes even more heat from the room so central heating has to work harder to replace the heat removed to outside.
However solid fuel is likely similar again with loads of heat going up the chimney but harder to work out how much is wasted.
The law you have tried to quote from has three variables and is for DC current. Forgetting about the fact your using AC so there will be some errors what you state is true if (and this is the point) the third variable remains static. That third variable is power.
Watts = Volts x Amps (In AC circuits also x power factor correction) so as long as watts remain static the law you quote is true. So an electric motor pumping water (Assuming motor revs remain consent) will need more amps as the voltage drops until the point where it fails. Because it will likely slow down a little it will not be exactly following the law.
As already stated a switched mode power supply as used in a computer will auto compensate for voltage variation and deliver the same power so drop the volts and amps will go up.
However the Watt = Joule/Second so to keep the power steady it must do the work in a set time. The kettle with a lower voltage will take longer so you are altering both volts and watts so amps will drop not rise.
There is a device claimed to reduce the power used but one can only reduce the power used if one is producing some form of power that is not required. So a light that also produces heat by stopping it producing heat one can use less power. This of course only works if you don't want the heat. So if we consider an electric only building using CFU in the winter is unlikely to save energy. In fact because with radiated heat given off from tungsten light bulbs the comfort zone for air temperature is reduced so the tungsten light bulbs will waste less energy in winter than the CFU but in summer they will waste more energy as we don't want the heat generated.
With the kettle example the faster it boils the less energy escapes so the faster it boils the better. If however we put a heat retaining jacket on our kettle so heat was not lost we could boil it slower.
The most wasteful way to boil water is likely with a gas hob as more heat escapes into the atmosphere both as pure heat and as steam as the hob does not condense the water out of the exhaust gases. Plus to remove the steam requires an extractor which removes even more heat from the room so central heating has to work harder to replace the heat removed to outside.
However solid fuel is likely similar again with loads of heat going up the chimney but harder to work out how much is wasted.
In devices involving a state change, rising current with falling voltage is part of their normal workings.
Probaly the most common examples of this are devices that use a plasma, such as CCFLs or TIG welders. A high (or higher than usual running) voltage breaks down the gas and initiates an arc. Once arced over the 'resistance' of the gas (well, now a plasma) reduces dramatically and if the current wasn't limited the driving circuit and/or device would most likely be damaged. The ballast in the CCFL circuit is often considered in voltage mode before the lamp is struck and in current mode after it has been struck. The ballast automatically manages the two modes and the transistion between the two.
One other notable widespread 'device' that also can exhibit negative resistance is the human body. Wet salty bags of Spoo that we are, if we're unlucky enough to establish a current path between that inquisitive finger and our club foot then the resistance of that path can fall as the current flows due to chemical changes.
Google 'necrotic tissue damage electrocution' for some interesting reading. And for that heightened sense of horror, check out some its delayed effects.
Probaly the most common examples of this are devices that use a plasma, such as CCFLs or TIG welders. A high (or higher than usual running) voltage breaks down the gas and initiates an arc. Once arced over the 'resistance' of the gas (well, now a plasma) reduces dramatically and if the current wasn't limited the driving circuit and/or device would most likely be damaged. The ballast in the CCFL circuit is often considered in voltage mode before the lamp is struck and in current mode after it has been struck. The ballast automatically manages the two modes and the transistion between the two.
One other notable widespread 'device' that also can exhibit negative resistance is the human body. Wet salty bags of Spoo that we are, if we're unlucky enough to establish a current path between that inquisitive finger and our club foot then the resistance of that path can fall as the current flows due to chemical changes.
Google 'necrotic tissue damage electrocution' for some interesting reading. And for that heightened sense of horror, check out some its delayed effects.
I`m not sure I would really call that `negative resistance` in the usual sense, but it`s certainly true - as you say, due to )ften irreversible) `state changes`. Ultimately, of course, sufficient electrical current though any biological tissue will turn it into carbon, a pretty conductive material! However, that`s a very irreversible process (turning lumps of carbon back into a human being would get you a Nobel Prize!), so not really `negative resistance`!
Kind Regards, John
I can't believe this is even being discussed again 
I can't believe this is even being discussed again
Sheeesh who's done that!
When it comes to resurrection of a well loved thread I am but a mere amateur compared to these guys.......
//www.diynot.com/forums/viewtopic.php?t=34549&start=165
Started in 2005, went strong in 2006 then Woody's post from mid 2008 said it all
Nooooooooooooooooooooooooooooooooo
What have you done
And still going strong now. I've put a note in next years diary that if it was looking a bit quiet over there that I'd look under the stairs for some damp and join in. It'll probably only stop when the post counter hits Maxint
When it's just a conceptual discussion (rather than some practical issue which one hopes was solved months or years ago), I see no harm in someone 'resurrecting' it if they feel so inclinedI can't believe this is even being discussed again
In any event, I doubt that there are many of us who have not, on occasions, failed to notice that what they were responding to was a resurrected old thread! I will freely admit that, on this occasion, I did not notice.Kind Rgards.
Very much so John. If it's a new problem then it's thinking cap on and best neuron forward to help out. But my resurrecting post was aimed at provoking thought with the OP or those who may pass by in the future. The OP sounded like somebody who may be studying or learning. And since constant power had been done I added my 2pennith.
So there you have it; in these days of green living and austerity measures, another thread that's been further used, topped up with fresh characters, when it could of so easily been consigned to the bin.
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