Kaplan FL #3 PS Q#34 - Circuits

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tshank

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Assume that the values for V and R are known. What other information is needed to calculate the voltage drop across the speaker?





A - The resistance of the speaker's internal circuitry.





B - The duration of the discharge between the wire and the cylinder.





C - The current flowing through the resistor.





D - No other information is required.

upload_2014-8-28_22-32-45.png


Kaplan's Explanation:
The circuit diagram indicates that the resistor and speaker are wired in parallel. Therefore, the voltage across the resistor is the same as the voltage across the speaker. We can calculate the voltage across the resistor using V = IR. The only additional information we need is the value of I, the current flowing through the resistor.

(A) Distortion.We need to know the current through the speaker, as well as its internal resistance, to determine the voltage across the speaker. The resistance alone is insufficient.

(B) Distortion.The duration of the discharge would not provide us with any useful information. If we knew the charge transmitted and the duration, we could calculate the current. The duration alone is insufficient.

(D) Distortion.Other information is required, as described above.



I said A, because you can find I via V = IR, IF you know the internal resistance to find the resistance of the parallel resistors. Their answer doesn't seem right to me. Any thoughts or help? Something I didn't see? Thanks!

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The question is just asking to find the voltage drop across the speaker which is correctly explained by Kaplan.
If you tried to find voltage with V = IR, how would you know what the current through the speaker is? - here you require 2 pieces of info.
The speaker is parallel to the resistor, so you just need to find voltage across the resistor, whose resistance you do know. - here you require just 1 piece of info.
 
What do they mean by voltage drop? Meaning the difference in V that went into the speaker vs the voltage that when through the speaker? What i'm saying is that you can determine the voltage, and therefore the current if you know the resistance in the speaker. Or is resistance and "internal resistance" something different in this circumstance?
 
Yes, you can find the current through the speaker like the way you said, but the question is just asking for the voltage drop, so why do extra calculations or require extra information? (Or was that something you were just wondering about outside the question?)
voltage drop means exactly that: the drop in voltage experienced by the current as it moves through a resistor; current leaving from a battery has a high voltage but once that current has passed through a resistor the resulting voltage is lower (because the initial electrical energy dissipates in the resistor) - hence you "drop" in voltage
you don't need to know the resistance of the speaker. the fact that it's parallel to the resistor means both will have the same voltage

internal resistance refers to the inherent resistance of the apparatus; e.g. wire, battery, etc. The problem is giving the speaker as an actual resistor
 
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Okay, thanks for the in depth explanation. My confusion lay was founded in the "internal resistance" vs the speaker acting as a resistor. I thought the internal resistance of something acting as a resistor was the same as an actual resistor's resistant measurement given.
 
What is the purpose of metal cylinder in this question?
I would have chosen D, because V provided by the battery would be the same as V going through speaker and R. What am I missing?
 
The wire and the metal cylinder mean nothing in the question. The voltage is the same, but the current for each is different. Since the current is different, their resistances are different. This questions relies on us knowing that different resistances cause different voltage drops, based on the current going through. Personally, I think the question is poorly worded... oh wait its the MCAT...
 
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