Can You Solve Physics GRE Problem #28 Using an Oscilloscope?

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Discussion Overview

The discussion revolves around solving a specific problem from the Physics GRE that involves analyzing a signal displayed on an oscilloscope. Participants explore the characteristics of the signal, including its frequency components and amplitudes, as well as the challenges of interpreting the blended waveforms.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant expresses difficulty in solving the problem and mentions using the concept of beats without success.
  • Another participant describes the signal as a combination of a low-frequency sinusoid and a smaller, higher-frequency sinusoid, providing specific measurements for peak-to-peak voltage and suggesting choice D as the answer.
  • The second participant also estimates the frequency of the low-frequency component to be around 400 Hz and discusses the ratio of high to low frequencies.
  • A third participant acknowledges the explanation provided and reflects on the challenge of distinguishing between the low and high-frequency components in the blended signal.
  • A later reply affirms the reasoning of the third participant without introducing new information.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the solution to the problem, as some express uncertainty about distinguishing the components of the signal while others provide specific interpretations and calculations.

Contextual Notes

Participants rely on visual analysis of the oscilloscope display and measurements, which may depend on subjective interpretation of the blended waveforms. There are unresolved aspects regarding the accuracy of the frequency estimates and the identification of the components.

Who May Find This Useful

Individuals preparing for the Physics GRE, particularly those interested in experimental physics and signal analysis using oscilloscopes.

quantumworld
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GRE GRE GRE... I need some help :blushing:

this question is kinda of experimental question, dealing with oscilloscope, I tried to work it by calculating the beat, but it didn't work out...
may I provide the link to the question, because there is figure that follows it,
it is page 26, number 28.
http://phys.columbia.edu/~hbar/Physics-GRE.pdf
I am really stuck at it... thought about it for a long time.

Many thanks for all efforts
 
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You can tell the signal is one low-frequency sinusoid with another, smaller, higher-frequency sinusoid added to it. The minimum of the low-frequency component is right around 2 cm, at about 1.5 divisions (3V), while the maximum is right around 5 cm, at about 4 divisions (8V). That means the signal is 5V peak-to-peak, or has an amplitude of 5/2 = 2.5V. There is only one answer which fits this, choice D.

You can double check this answer easily. The second component certainly does have an amplitude of about 1.25V, 2.5V peak-to-peak, or a little over one division peak-to-peak. Also, there seem to be about six cycles of the high frequency component for every one cycle of the low-frequency component, so 83:500 Hz seems like the correct ratio.

You could go a step further and calculate the frequency of the low-frequency component. It appears to be about 5 cm per cycle, or about 2.5 milliseconds per cycle, or about 1/0.0025 = 400 Hz. The frequency given in choice D, 500 Hz, is probably close enough for comfort. (I wouldn't do all these checks on the test to avoid wasting time, but, if you're really not confident in your first answer, they will help.)

- Warren
 
Thank you Warren for your reply,
I could see your explanation, if I thought of the low frequency sinusoid as the average of the given graph, and the high frequency sinuoid as the wiggling of it. My problem was trying to distinguish which is which, because they are already blended.

THanks again, and please correct me if I was wrong.
 
That's a reasonable way to think of it.

- Warren
 

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