Sunday, March 19, 2017

Bonus day: question 36 from Physics GRE

This question comes from practice Physics GRE exam (2011 edition)

The capacitor in the circuit above is charged. If switch S is closed at time t = 0, which of the following represents the magnetic energy, U, in the inductor as a function of time? (Assume that the capacitor and inductor are ideal.)

That's a relatively simple question with P+=50%. Now let's get to the answer: Show answer
Correct answer is A

To get to it we might use process of elimination: find all answers that are NOT correct and see what's left. First of all, question says "Assume that the capacitor and inductor are ideal". What that means right off the bat is that there should be no dissipation of energy. So answer D is incorrect right away. Also, we should remember that inductor's energy depends on current (contrast it to the fact that capacitor's energy depends on voltage across its plates). In initial conditions there is no current going thorugh the circuit because switch S is open. Hence, correct answer should have plot start at 0. This eliminates answers B, C, D (again). We are left with either E or A answers. You can pick one by again referring to ideal-ness or ideality of the circuit. Indefinite oscillations (answer A) are the only possibility in that case. By the way, the fact that this quite simple question has P+ of just 50% should tell you that it is not that hard to score high on Physics GRE. You can do it!

Sample GRE Q55

Turns out this blog is still up and running :-) so here is a new one: Sample GRE biochem test, question 55 with P+ of 31% (from 2016 edition) [*P+ is percentage of test takers who gave correct answer.]

An amino acid transporter protein is responsible for the transport of a specific amino acid across a membrane. The KI values of several competitive inhibitors of the amino acid transporter are shown above. Based on these data, which of the following is most likely the amino acid transported by this protein?

Show answer
Correct answer is E And here is why. First, let's unwrap some of the language. "The inhibitor constant, Ki, is an indication of how potent an inhibitor is; it is the concentration required to produce half maximum inhibition." (here) So, the higher the Ki, more inhibitor concentration is necessary to prevent our transporter from moving amino-acid X through the membrane. It is also mentioned in the question that inhibitors are competitive. That means, to some degree, that inhibitors are similar in structure to that of the amino-acid X. Most similar inhibitor will then have lowest Ki, is est will occupy transporter's "slot" very nicely preventing amino-acid X from binding. In other words, inhibitor with lowest Ki will have most similar structure. Lowest Ki belongs to competitive inhibitor #3 (calledethyl benzene, by the way). It might be useful to notice second-best inhibitor as well, #5 (isoamyl alcohol). Both of these compounds are relatively not charged. Now let's look at structures of all possible answers:

As you can see, only tyrosine (E) carries benzyl group and also has less charge than other compounds. hence the answer is E