Shock Value B

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Re: Shock Value B

Post by space scientist »

sean9keenan wrote:
space scientist wrote:
sean9keenan wrote:In another thread Prelude to Death asked me a question about a lab.

I'm going to ask the question in general for anyone interested in solving it, and then can provide an answer for those interested

Essentially, how do you measure the internal resistance of a battery? You only have a resistor, a multimeter and you are not supposed to use the ampmeter function of the multimeter (although that solution has value as well)

If anyone has other questions about the lab (or the other part) feel free to ask

-Sean Keenan
You measure the voltage across the resistor in series with the battery and take that value from the voltage of the battery (This gets you the voltage across the internal resistance of the battery.). Then you find the current across the resistor and use Ohm's Law to determine the internal resistance. You can check your work by finding the total resistance and using Ohm's Law to see if the current is the same across all components in the circuit. For this lab, the proctors must have a tolerance on answers due to the fact that not all resistors have the resistance that they are marked with (This is the purpose of the tolerance band on a resistor.).
Exactly, although you can also measure the resistance of the external resistor using the multimeter (even though you are told the resistance) Ultimately however if you did not so sufficient work you did not get very much credit on the problem. If you had the right ballpark answer (order of magnitude) and showed sufficient work then you would get credit. However, many teams (almost all) used the ohmmeter function of the multimeter directly on the battery, which doesn't work.

Does anyone know why using the ohmmeter function directly across the battery does not work?
Wouldn't the voltage produced by the battery cause the ohmmeter to be inaccurate since ohmmeters have a battery in the meter and measure the current across the ammeter (this design works because of Ohm's Law). Therefore, the voltage from the battery would mess up the current reading in the meter which causes the meter to give the wrong resistance, for the meter is programmed to calculate with a voltage that was much lower than the actual voltage in the circuit for the meter.
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Re: Shock Value B

Post by sean9keenan »

space scientist wrote:
sean9keenan wrote:
space scientist wrote:
You measure the voltage across the resistor in series with the battery and take that value from the voltage of the battery (This gets you the voltage across the internal resistance of the battery.). Then you find the current across the resistor and use Ohm's Law to determine the internal resistance. You can check your work by finding the total resistance and using Ohm's Law to see if the current is the same across all components in the circuit. For this lab, the proctors must have a tolerance on answers due to the fact that not all resistors have the resistance that they are marked with (This is the purpose of the tolerance band on a resistor.).
Exactly, although you can also measure the resistance of the external resistor using the multimeter (even though you are told the resistance) Ultimately however if you did not so sufficient work you did not get very much credit on the problem. If you had the right ballpark answer (order of magnitude) and showed sufficient work then you would get credit. However, many teams (almost all) used the ohmmeter function of the multimeter directly on the battery, which doesn't work.

Does anyone know why using the ohmmeter function directly across the battery does not work?
Wouldn't the voltage produced by the battery cause the ohmmeter to be inaccurate since ohmmeters have a battery in the meter and measure the current across the ammeter (this design works because of Ohm's Law). Therefore, the voltage from the battery would mess up the current reading in the meter which causes the meter to give the wrong resistance, for the meter is programmed to calculate with a voltage that was much lower than the actual voltage in the circuit for the meter.
Yup, thats the issue,I think it's very important that people realize how the instruments that they are using actually work. When you use the ohmmeter function of you multimeter you use the internal battery and measure the current through the resistor and the multimeter calculates the resistance based on the voltage it knows it's applying, and the current it's reading. This doesn't work when you have an extra voltage source on the other end!

Hope that clears it up,

Sean K
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Re: Shock Value B

Post by prelude to death »

Thanks for explaining!!!! :D

Also, great test Sean (despite the fact that I pulled an epic fail on it)! It was really challenging and now I'll know what to expect if Shock Value doesn't get replaced by Keep the Heat next year.
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Re: Shock Value B

Post by christino »

The Missouri state test was way too hard. We had more difficulty on it than last year's nationals test. I never thought that magnetic monopoles, paramagnetism, and diamagnetism would count as "basic" magnetism topics. Finding the strength of an electric field based on some numbers in newtons and grams was ridiculous.
Last edited by christino on April 17th, 2011, 3:30 pm, edited 1 time in total.
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Re: Shock Value B

Post by EpicFailOlympian »

christino wrote:The Missouri state test was way too hard. We had more difficulty on it than last year's nationals test. I never thought that magnetic monopoles, paramagnetism, and diamagnetism would count as "basic" magnetism topics. Finding the strength of an electric field based on some numbers in newtons and grams was ridiculous.
Hard tests are better. If you saw the Microbes test, it was WAY too easy, and almost everything was on the study guide. We got 10th, and we pretty much got every question right.
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Re: Shock Value B

Post by Singh1997 »

Hey guys, states is coming up on the weekend, and I'm wondering what practicals you have seen. I got first at regionals in this event, but that was with the hardest question being a lemon battery. For the lab part, we simply had to make snap circuits, and at invitational, we had to make an electromagnet. What other things have you guys done, especially at the state level? If we have to make circuits, do we have to use circuitboards, or do we just connect components with alligator clips?

Thanks in advance,
Jay
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Re: Shock Value B

Post by tuftedtitmouse12 »

huh...at il state, well i didn't do it, schedule conflict, but i saw from pictures and heard from other competitors that we had like a circuit board and you had to connect wires to it and yeah...it looked hard...people from all teams were like :shock: :shock: :shock: :shock: :shock: i don't know this... :oops: :oops:

to me, it seemed impossible :lol: but wish you lots of luck
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Re: Shock Value B

Post by space scientist »

I have seen practicals involving usage of meter, determining polarity of magnets, reading resistor coding, making a circuit, and determining whether an incandescent light bulb is brighter in a series or parallel circuit. All of those were fairly easy, and I know how to do the ones that I did incorrectly the first time. In addition, practicals should be fairly easy if you know the concepts behind them.
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Re: Shock Value B

Post by Singh1997 »

Hey, thanks for all the info. Two things

1) To determine the polarity of the magnets, would you tie the magnet to a string, and see how it aligns itself?
2) Do you know if the circuitboard they had to use was a non-solderable breadboard? Those are relatively basic, and what I imagine they would give us. I just want to know what to study

Thanks
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Re: Shock Value B

Post by prelude to death »

For #1, I'm pretty sure that would be correct. I think it says to in the Wiki (but then again, I editted it, so yeah). In the rules manual, I'm pretty sure they say that you get a compass too, but I think all the compass is for is so that you can check which pole of the magnet is north and south, but yeah. I think that's basically it. (Someone correct me if I'm wrong.)
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