I'm starting to get cold feet and wondering if I should push back to April. Whew. I'm still strugging a lot with chem and physics and it shows in my practice
There is still a month and a half to go, which is plenty of time. Chemistry and physics are amongst the easiest to climb the bell curve in, mostly because the majority of test takers struggle with this section. So, small improvements can equate to big gains. Just be very deliberate when you review questions. One of the things I push on our students is to follow the evolution of a question. Take a question like the following:
A researcher studies the flow rate of a liquid through three identical pinholes in a 10-cm vertical pipe filled to the top with the liquid. One pinhole (labeled a) is 1.25 cm below the upper rim, a second pinhole (labeled b) is located in the middle, and a third pinhole (labeled c) is located 1.25 cm above the base. What is observed for the three pinholes during the experiment?
A) The liquid shoots vertically out of each pinhole at the same speed.
B) The volume flow rate, Q, is greatest from pinhole a.
C) According to Bernoulli's principle, the pressure against the internal wall at each pinhole elevation is the same.
D) Pinhole a would have to be made larger to have the same amount of liquid escape per unit time as pinhole c.
At first read, this question is overwhelming. So you need to look at the evolution of the question and break it down to something simpler. This often starts with a visualization or analogy to a system you understand. You can think of any iced tea dispenser or coffee urn you have seen. Where is the valve located? It's at the bottom. Why? Because you want to get the fluid out. As it empties over time, it fills your cup or glass more slowly, so the height of the liquid clearly has some impact. You can also think of bleeding, and why we elevate wounds. If the wound is left at a lower elevation, the person will bleed out faster. We raise the wound to slow the bleeding, fitting with our tea/coffee dispensing example.
So we have evolved their question from a generic lab experiment question into a real life scenario.
We know choice A does not fit our visual, so it is eliminated. The same can be said for choice B, as the greatest exit speed and volume of fluid per second would be at the bottom, pinhole c. We have eliminated the visual choices.
We now need to evolve into considering physics equations. Bernoulli's principle has to do with pressure exerted against a wall. It takes into account height (rho·g·h) and movement (1/2·rho·v^2), both of which reduce the pressure against the inside walls of a pipe. The fluid is moving down the pipe (as it drains) at essentially the same speed (close to 0) as it slowly leaks, so we will ignore the 1/2·rho·v^2 term. But the heights of the pinholes are notably different. As h increases, according to the equation, pressure against the walls is reduced. So the pressure against the walls is less at the top and greater at the bottom, which is the reason behind the differences in the escape speeds we see. Choice C is out.
For choice D, we need to consider Poiseuille's equation, where volume flow rate, Q, is proportional to both P and r^4. At a higher point in the pipe, the pressure against the internal walls pushing the liquid out is less than at a lower point, so in order to make the volume flow rates equal, the higher elevated pinhole (pinhole a) must have an increase in r. This supports choice D.
Along our path to finding the answer, we evolved from a visualization and/or an analogy to something we understand into using the equations to compare the variables.
Being cognizant of your thought process and seeing how it evolves as you go about employing the process of elimination is critical in developing into a better test taker. You have to be able to quickly translate between concepts, simple examples, and equations for many topics. As you get better and better at doing this on questions, you will see both your confidence and your scores improve.
