Im using BR for physics...i find it hard to adapt to thinking the same way they use to answer many questions. They answer physics questions on pure logic and a part of me always wants to stick to equations to work things out...is it best to use a balance of both equations and their logic...or stick to what works best?
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berkeley review logic
Started by Yoyomama88
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If it's a calculation question I would try to follow their method as you will need all the time you can spare on the MCAT. For conceptual based questions you can probably use equations to answer some problems, but you really need to grasp the physics behind the question being asked. The equations are a very nice descriptive way of telling the reader how a system behaves, but try to understand those equations on a conceptual level.
On top of this suggestion I would like to add you should use their logic because the best way to solve a MCAT or any physics problem is to always go from general knowledge to specific knowledge (such as equations). When many people on here harp out conceptual understanding this is what they mean. Start with what you can determine and figure out from the problem on a general knowledge basis and get more and more specific until you can solve the problem. Heavy equation usage comes in handy when you don't see another way of figuring out the question conceptually in about 10-15 seconds of thinking and you just need to answer the question and move on. This is just my opinion though.
-LIS
Logic. I know it's hard. However, if you use logic, then you can check your numerical answer. To illustrate. If you increase volume while holding pressure constant and moles as well, what happens to temperature? If you blindly plugged in you would find it increases. This is incorrect, the gas is doing work since it is expanding against a constant pressure so it's temperature actually decreases. Also, you must practice this because what if you get a primarily conceptual physics test. Equations only tell part of the story. Do the calculations but also think to see if they make sense.
Yeah I agree make sure you check to make sure your answer makes sense, although this works if you use the equations correctly as well.
P1V1/nT1 = P2V2/nT2
Hold pressure and moles constant and you have Charles Law which equals,
V1/T1 = V2/T2
Volume and Temperature are inversely related so you should come up with the same answer it'll probably just be a waste of time however since this is easy to imagine conceptually.
For the most part you will not have time on the MCAT to plug in values and work things out. It's very important to have a conceptual understanding of the forumals, why they work, and what they mean. Proportionality ("If X doubles, Y doubles") is a central theme. There might be one or two random questions where you actually have to calculate things like the velocity of a sliding block or something, but in general it's much faster to look at the answers and derive the most logical or only possible choice.
TPR is still a very very good book though, especially for PS. Look it up in the 30+ thread, a lot of people do really well using only the TPR books. I'm using both for my PS review (TBR and TPR) and TPR just has a great way of explaining things even if you haven't had the course in a long time.
Content wise they're about equal with TBR being maybe only slightly better but TBR blows it away with their practice. However, the TPR science workbook is great too, so you wouldn't be at a disadvantage if you used TPR for PS only.
And of course EK is just bad in physics, unless you just finished the course, but their G. Chem book is surprisingly better than most people rate it on here.
Just my thoughts,
-LIS
I'm confused, V and T are not inversely related, they are directly related. That is in the equation PV=nRT, because they are on opposite sides of =
If the relationship is V1/T1 = V2/T2, V2 > V1, then for the relationship to hold true T2 > T1. If you want to exert the same pressure with the same number of moles you must increase the temperature to get to that volume. The molecules are not going to fight it out themselves until they get to a new volume, unless you change external pressure.
However, if you artificially increased the volume, meaning the molecules didn't push there way out themselves, then wouldn't temperature stay the same?
If the relationship is V1/T1 = V2/T2, V2 > V1, then for the relationship to hold true T2 > T1. If you want to exert the same pressure with the same number of moles you must increase the temperature to get to that volume. The molecules are not going to fight it out themselves until they get to a new volume, unless you change external pressure.
However, if you artificially increased the volume, meaning the molecules didn't push there way out themselves, then wouldn't temperature stay the same?
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I see the relationship but I think the answer lies in how the volume was increased, as well as what the external pressure is, did it stay constant, decrease etc.
Yeah, it's one of those weird idiosyncrasies. If the temperature of the gas was increased, then the volume would increase everything else being equal; but the reverse isn't necessarily true.
so yeah, they are directly related. Will Hunting could you break down the logic of that answer.
I think Berkeley Review tends to be VERY heavy on subject matter and calculations for PS and BS, excessive in my opinion. I bought 5 of their practice MCATs but felt they were not nearly as indicative of the real thing as the AAMC practice tests, better than kaplan though...
Yes, but the recent tests have been which is why you should get a good handle. You may get a conceptual test or you may get a calculation intensive one. So, you should be ready.
PV=nrt. I see why you guys are confused. I wasn't very clear. I should have stated the gas is expanding against constant pressure so perhaps atmospheric pressure. This is necessary to calculate work =P*Volume change. Work=F*D. Here, F=Pressure and the volume is the distance. PV=nrt indicates that there is a DIRECT correlation between T and V. Why? They are on opposite sides mathematically. Secondly, PV/nr=T So if V increases then so should temperature assuming you're not holding temperature constant. So this is why I say be careful and think.
However, in the question I posed, it is a THERMO question. Many students will blindly use the ideal gas law and make a mistake. My point is that you can't just do this.
Let's say we are at STP. We have a diatomic gas in a piston. Let's say there is 22.4 Liters of the gas at 1 atmosphere. PV=nrt. What if we decide to compress the gas so it's volume is half. Pressure would be predicted to double and volume is .5 so 2P*.5V=nrT. The number of moles is constant and so is R, so T shouldn't change if you go BY the ideal gas formula. wHY? The left side stays the same.
In this adiabatic (constant external pressure) compression, P, V, and T change. So, you can't use the ideal gas law. Do you see? Temperature is decreasing, Volume is increasing, and pressure is decreasing? In ideal gas law one wouldn't see a volume decrease with an increasing temperature. You would expect the opposite.
However, if I had stated under isobaric (constant systemic pressure), then the ideal gas Law works fine. V1/nt=V2/nt So If we increases volume then we must increase temperature and visa verse. So, the key to doing well is to READ carefully for the conditions and then use an equation.
In closing, I should have stated under adiabatic conditions ideal doesn't work because you must think in thermodynamic terms. Under isobaric, isochoric, or when moles are held constant it works well.
However, when gases expand (increasing volume) they do work and their temperature decreases which the ideal gas would not show to be the case because we have 3 changing variables: decreasing internal pressure, decreasing temperature, and increasing volume.
