Section Bank Chem/Phys #8

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Section Bank Chem/Phys #8

Post by Lionroar » Sun Dec 31, 2017 1:08 pm

In the Section Bank Chem/Phys section, question 8, the AAMC explains that the energy meter should read 0 because all the energy from Em should be converting to either deltaHu or deltaHnr. I don't understand why the meter should not be reading the deltaHnr, since, as the passage said, deltaHnr is the "heat detected", and I assumed that meant the heat detected by the energy meter.
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Re: Section Bank Chem/Phys #8

Post by NS_Tutor_Andrew » Tue Jan 02, 2018 2:42 pm

Hi Lionroar,

Excellent question. There are two ways to approach it: why ΔHnr is wrong, and why 0 is right.

Let me start with why ΔHnr is wrong, because that seems like the most immediate point of confusion. The key here is that we have to read the passage very carefully to understand what the energy meter actually records. We're told in the passage that there are basically two recording systems: the oscilloscope, which records a sound signal whose amplitude is proportional to ΔHnr (that is, ΔHnr is recorded indirectly), and a separate "energy meter" that uses the photoelectric effect. The fact that we're told that the energy meter uses the photoelectric effect means that it must measure the energy of incident photons, not heat. Therefore, it cannot measure ΔHnr (or ΔHu, for that matter) directly, so we can eliminate those answers.

Instead, what the energy detector measures is essentially the laser energy that is left over after being pulsed through the sample. The question gives us a scenario where the energy of the laser beam is exactly the amount needed to break the bond, so nothing will be left over -- hence the reading of 0.

Hope this helps clarify things! This is definitely a confusing point, and very careful reading is needed to figure out exactly what's going on here.
Andrew D.
Content Manager, Next Step Test Prep.
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