Circuit Lab B/C

ignorantcircuitboi
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Re: Circuit Lab B/C

Postby ignorantcircuitboi » February 19th, 2019, 4:18 pm

Although I know this is incorrect, during the competition we ended up putting the voltmeter in series with the various resistors. When put into parallel we got a reading of 0V but got positive values when in series... does anyone know why this would be the case?

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Re: Circuit Lab B/C

Postby Schrodingerscat » February 19th, 2019, 4:50 pm

ignorantcircuitboi wrote:Although I know this is incorrect, during the competition we ended up putting the voltmeter in series with the various resistors. When put into parallel we got a reading of 0V but got positive values when in series... does anyone know why this would be the case?


Assuming you connected everything as you say, that would only make sense to me if the resistors were either somehow shorting or had too low of resistance to generate a measurable voltage in your circuit.

Edit: did all of the resistors read zero volts when connected in series and measured in parallel?

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Re: Circuit Lab B/C

Postby ignorantcircuitboi » February 19th, 2019, 5:15 pm

I am not sure about all but the 3 out of the 5 I tested did so. I am fairly sure that all connections were solid as clean alligator clips were used.

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Re: Circuit Lab B/C

Postby UTF-8 U+6211 U+662F » February 19th, 2019, 5:23 pm

ignorantcircuitboi wrote:I have a question. How do I solve the following question (like step by step process)?

I am given the following materials:
a voltmeter
1 known resistor with resistance of 1,000 ohms
5 unknown resistors
a breadboard
alligator clips
a 3 V battery


How do I find the resistance value of each of the unknown resistors?

This was my approach, which turned out to be blatantly wrong:
First, I connected all the resistors in series. My thought process was that since I knew the resistance of one resistor, I could just use the voltmeter to find the find the voltage across the known resistor, and then I could find the current across that resistor. That current should be the same as the total current and the current across all the resistors because the circuit was in series. Using this, I could then measure the voltage across all the unknown resistors. Since I knew the current value, if I knew the voltage across the resistors, I could find all the resistance values.

Then, I put the leads of the voltmeter on the ends of the resistors (so I connected it in parallel). But the reading for the voltage was 0, so I could not find the value of the resistors. I think my thought process is wrong, does anyone know what I should do? Thanks!

Did you attach the battery

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Re: Circuit Lab B/C

Postby ElPotato » February 19th, 2019, 5:58 pm

ignorantcircuitboi wrote:I am not sure about all but the 3 out of the 5 I tested did so. I am fairly sure that all connections were solid as clean alligator clips were used.


Maybe the voltage across the resistors were so low that it did not register on the voltmeter's scale? Was this at Cornell b/c the Cornell Circuits lab had a similar setup and I remember one of the resistors having a really high resistance. Perhaps that threw off the measurements?
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Re: Circuit Lab B/C

Postby satvik03 » February 19th, 2019, 6:06 pm

Yeah could be. I had a similar issue. I'm trash at lab and average about 4% on the lab section lol. I tried to put the resistors in series, and measure the voltage across the resistors by putting the leads on the ends of circuit. I'm pretty sure I gotta learn how to measure voltage drop correctly.
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Re: Circuit Lab B/C

Postby satvik03 » February 22nd, 2019, 8:20 am

Uh what happens if you place the black lead closer to the voltage source and the red lead on the other side of the resistor (when measuring voltage across a resistor)? This should just give the negative value for voltage, but the magnitude should be the same right?
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Re: Circuit Lab B/C

Postby mdv2o5 » February 22nd, 2019, 12:14 pm

satvik03 wrote:Uh what happens if you place the black lead closer to the voltage source and the red lead on the other side of the resistor (when measuring voltage across a resistor)? This should just give the negative value for voltage, but the magnitude should be the same right?

Yes. Assuming your black lead is plugged into the COM or ground port on your meter and the red lead is plugged into the port for DC volts, the meter measures the potential of the red lead relative to the black lead (Vred - Vblack).

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Re: Circuit Lab B/C

Postby biz11 » February 23rd, 2019, 9:51 am

What types of questions de they ask on transformers? Almost everything would require knowledge of inductance which they say you don't need to know.

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Re: Circuit Lab B/C

Postby Pastaman202 » February 24th, 2019, 7:12 am

At one of my invites, there was a switch that connected with 2 terminals, and you are given a capacitor, battery, lamp and resistor. The question was to set up a schematic diagram of how when the switch is connected to one terminal, the light bulb starts dim and get brighter and brighter, while the other starts bright and gets dimmer and dimmer. Does anyone know how to set this up? P.S. this was a div B test. Thanks!
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Re: Circuit Lab B/C

Postby UTF-8 U+6211 U+662F » February 24th, 2019, 9:03 am

Pastaman202 wrote:At one of my invites, there was a switch that connected with 2 terminals, and you are given a capacitor, battery, lamp and resistor. The question was to set up a schematic diagram of how when the switch is connected to one terminal, the light bulb starts dim and get brighter and brighter, while the other starts bright and gets dimmer and dimmer. Does anyone know how to set this up? P.S. this was a div B test. Thanks!

You'd have to set it up so that the capacitor discharges for the one switch position and recharges for the other switch position. So you'd have two RC circuits with a switch going in between them.

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Re: Circuit Lab B/C

Postby JavaScriptCoder » March 6th, 2019, 11:01 am

I've been looking at past practice tests after doing some research on circuit lab, and I saw a y-shaped load, with a resistance on each resistor. I can't really figure out what a y-delta transform is and how that's supposed to work, and I also don't really understand how the current is supposed to flow in the y. Can someone tell me:

a) this is an AC concept, will it be tested
b) if it is tested, how can I best understand it?

Thanks~!
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Re: Circuit Lab B/C

Postby UTF-8 U+6211 U+662F » March 6th, 2019, 11:33 am

JavaScriptCoder wrote:I've been looking at past practice tests after doing some research on circuit lab, and I saw a y-shaped load, with a resistance on each resistor. I can't really figure out what a y-delta transform is and how that's supposed to work, and I also don't really understand how the current is supposed to flow in the y. Can someone tell me:

a) this is an AC concept, will it be tested
b) if it is tested, how can I best understand it?

Thanks~!

Y-Delta transform is a way to simplify circuits. The two circuit pieces are equivalent and can exist in either DC or AC circuits. Current won't flow in the Y by itself: the purpose is the transform is to simplify a smaller piece in a larger and more complicated circuit. Check out this: https://www.khanacademy.org/science/ele ... r-networks
https://www.allaboutcircuits.com/textbo ... nversions/

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Re: Circuit Lab B/C

Postby JavaScriptCoder » March 6th, 2019, 1:29 pm

Thank you!
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Re: Circuit Lab B/C

Postby jaggie34 » April 4th, 2019, 9:53 pm

Anyone think it would be useful looking into magnetic moment?
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