simple question power, resistance, current, etc
in [Physics]
|
From: Benj on 20 Feb 2010 17:35 On Feb 18, 11:36 pm, Sam Wormley <sworml...(a)gmail.com> wrote: > On 2/18/10 10:03 PM, none wrote: > > > Is the "power" of a battery a constant? I assumed it should be, but then > > I was thinking that if V=IR, say you have a battery with voltage 9 and > > circuits with resistance 3 or 4.5. The current should be 3 or 2, > > respectively. Then, the power would be 27, or 18, if power = voltage * > > current, right? So does the power of a batter change depending on the > > resistance of the circuit? Seems odd... > > http://hyperphysics.phy-astr.gsu.edu/Hbase/hframe.html > > Forget not that there is also a resistance in the battery itself. Which for a fresh/charged battery is usually negligible like most everything "Wormley" says. All he can do is publish stupid links like the one above that I immediately found full of errors and misstatements. But then bad physics is what he does best. You should hear him on AGW!
From: none on 20 Feb 2010 19:51 On 2010-02-18 22:03:34 -0600, none <none(a)none.com> said: > Is the "power" of a battery a constant? I assumed it should be, but > then I was thinking that if V=IR, say you have a battery with voltage 9 > and circuits with resistance 3 or 4.5. The current should be 3 or 2, > respectively. Then, the power would be 27, or 18, if power = voltage * > current, right? So does the power of a batter change depending on the > resistance of the circuit? Seems odd... OK, thanks for all of the replies! Very helpful, and now I do understand, of course the power changes, depending on the load. That's why a battery runs down at different rates when it drives a small light vs a big light, etc. Now, on to the real question behind this. We did a science project to determine the efficiency of a wind turbine, and hooked up an ammeter and voltmeter. It turned out that for many different windspeeds, if the resistance in the circuit was lower, then we got significantly higher Power being produced than if the resistance was higher. So we thought that meant that the wind turbine was more efficient at converting wind energy to electricity if the resistance was lower. But one of the judges told us that was not true - that the efficiency was actually the same. So who was right? thanks
From: Darwin123 on 20 Feb 2010 20:41 On Feb 20, 7:51 pm, none <n...(a)none.com> wrote: > On 2010-02-18 22:03:34 -0600, none <n...(a)none.com> said: > > > Is the "power" of a battery a constant? I assumed it should be, but > > then I was thinking that if V=IR, say you have a battery with voltage 9 > > and circuits with resistance 3 or 4.5. The current should be 3 or 2, > > respectively. Then, the power would be 27, or 18, if power = voltage * > > current, right? So does the power of a batter change depending on the > > resistance of the circuit? Seems odd... > > OK, thanks for all of the replies! Very helpful, and now I do > understand, of course the power changes, depending on the load. That's > why a battery runs down at different rates when it drives a small light > vs a big light, etc. > > Now, on to the real question behind this. We did a science project to > determine the efficiency of a wind turbine, and hooked up an ammeter > and voltmeter. It turned out that for many different windspeeds, if the > resistance in the circuit was lower, then we got significantly higher > Power being produced than if the resistance was higher. So we thought > that meant that the wind turbine was more efficient at converting wind > energy to electricity if the resistance was lower. But one of the > judges told us that was not true - that the efficiency was actually > the same. So who was right? > > thanks I congratulate you and your class for the completion of such an ambitious project. The answer will not detract in any way the ambition, drive and curiosity shown by such a project. I hypothesize that the judges are right based on an error you made in the first post. You really wanted to ask a question about generators. You made a statement that the power of a battery should be constant. Batteries are always DC. However, your real question was about a generator. Most practical generators are AC. If you took the design from a book or website, it is very likely that you built an AC generator. The judges may know something about AC generators that you do not. On the other hand, it is possible the opposite occurred. You built a DC generator and the judges used an AC formula to calculate efficiency. Then they may be wrong. However, the naivete with which you asked about batteries makes that an unlikely, although plausible, possibility. The discussion continues on the assumption that the judges are right. I conjecture that your problem comes from not understanding AC circuits. A battery is implicitly a DC power supply. All answers that we gave you are completely valid for a DC power supply. However, you now tell us that the question was really about a generator. Apparently, you haven't really learned the difference between a generator and a battery. Therefore, I conjecture that your class built an AC generator without realizing that it would work differently than a battery. To validate my hypothesis, I will ask a few questions 1) How did you calculate the power? I suspect you used the formula which is completely valid for DC, P=IV where P is the power, I was the current, and V was the voltage. However, this formula is unambiguous only for DC current. If the current was AC, one has to specify at what times I and V were measured. Which brings us to the next question. 2) Was the voltage output of your wind generator AC, DC or some combination? It depends on exactly how the brushes were correct. If the voltage is DC, the formula P=IV is valid and unambiguous. If the output is completely AC, however, the formula for power is: P=IV cosA where P is the time averaged power (averaged over several cycles), I is the rms current, V is the rms voltage, and A is the phase angle. The phase angle, A, could vary with resistance. If your generator produced AC power, it is possible that A increased with load resistance. Although the rms current would increase, the angle A could increase in such a way as to leave the time averaged power unchanged. The power you calculated with the DC formula would show an increase, but it is not a physically meaningful power. I will close with the following question: 3) How does an oscilloscope, in the ungrounded configuration, present the voltage of the generator. If it is a DC generator, the oscilloscope will show a high background voltage with a small AC ripple. However, if it is an AC generator, it will show a sinusoidal waveform oscillating about zero, with maybe a small DC offset.
From: Androcles on 20 Feb 2010 21:07 "none" <none(a)none.com> wrote in message news:2010022018513516807-none(a)nonecom... > Now, on to the real question behind this. We did a science project to > determine the efficiency of a wind turbine, and hooked up an ammeter and > voltmeter. It turned out that for many different windspeeds, if the > resistance in the circuit was lower, then we got significantly higher > Power being produced than if the resistance was higher. So we thought that > meant that the wind turbine was more efficient at converting wind energy > to electricity if the resistance was lower. But one of the judges told us > that was not true - that the efficiency was actually the same. So who > was right? > > thanks > "Efficiency" has more than one meaning, but in this case it means the total energy available minus (the energy lost in other ways such as heat in the turbine) divided by the total energy available * 100. If your load is an open circuit (a very high resistance load) you are not using the total wind energy available, the turbine is merely spinning. If you short-circuit the turbine (a zero resistance load) then all the wind's energy will turn the blades and be lost as heat which will damage the turbine. So somewhere in between you'll be using all the wind's energy with 50% going into the load and 50% heating the turbine (which is cooled by the wind).
From: jimp on 20 Feb 2010 21:42
none <none(a)none.com> wrote: > On 2010-02-18 22:03:34 -0600, none <none(a)none.com> said: > >> Is the "power" of a battery a constant? I assumed it should be, but >> then I was thinking that if V=IR, say you have a battery with voltage 9 >> and circuits with resistance 3 or 4.5. The current should be 3 or 2, >> respectively. Then, the power would be 27, or 18, if power = voltage * >> current, right? So does the power of a batter change depending on the >> resistance of the circuit? Seems odd... > > OK, thanks for all of the replies! Very helpful, and now I do > understand, of course the power changes, depending on the load. That's > why a battery runs down at different rates when it drives a small light > vs a big light, etc. > > Now, on to the real question behind this. We did a science project to > determine the efficiency of a wind turbine, and hooked up an ammeter > and voltmeter. It turned out that for many different windspeeds, if the > resistance in the circuit was lower, then we got significantly higher > Power being produced than if the resistance was higher. So we thought > that meant that the wind turbine was more efficient at converting wind > energy to electricity if the resistance was lower. But one of the > judges told us that was not true - that the efficiency was actually > the same. So who was right? > > thanks The judges. To measure the power capacity of the generator for a given wind speed, you load it with smaller and smaller resistance until the thing either heats to an unaccepable level from internal losses or starts to stall, which ever comes first. Then you plot the measured powers against the wind speed, keeping in mind that the available power in the wind is not a linear function of speed. -- Jim Pennino Remove .spam.sux to reply. |