TBR Chapter 8 and 9

[dudewheresmymd]

Hey guys,
Was a bit unclear with a couple of questions after reading TBR explanations. Any clarification would be appreciated!

page 142, questions 4 and 6... BR Physics II

p.142 4) TBR says "electrons will be pushed towards the bottom of the rod. Current, by definition is the flow of positive charges. SInce electrons flow clockwise, the current will flow counterclockwise." To derive this, are they pointing their thumb to the right (in the direction of the v) and then fingers into the page, thus the force is up? This force would push protons up and electrons down right? So whenever they refer to current in the actual question, they are asking for the flow of protons?

p.142 6) "Magnetic flux does not tend to change..." Why is this? Also, they state "the rod is pushed either to the right or left depending on the direction of the current" How does one do the right hand rule for this question as compared to question 4?


p. 144 12) How does placing DNA in the sucrose density gradient lead to the conclusion that "viscous drag forces generally increase with increasing molecular size." The explanation the back was somewhat unclear. Anyone get this one?

p.145 18) "The positive charge on Sphere I will flow to sphere II, trying to get away from the glass rod?" I thought positive charges NEVER flow?? I was taught that electrons only flow, protons never flow? What the heck are they talking about here?


Thanks!

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[sdm33]

I don't have the passages but for 12) Drag force is nothing but resistance, so it is opposing DNA from moving.
p.145 18) "The positive charge on Sphere I will flow to sphere II, trying to get away from the glass rod?" I thought positive charges NEVER flow?? I was taught that electrons only flow, protons never flow? What the heck are they talking about here?
Of course, positive charges will not move out , may be they are taking about the net charges loss of negative charges between spheres. For example, when you are rubbing the glass rod with the silk cloth, electrons are stripped away from the atoms in the glass and transferred to the silk cloth. This leaves the glass rod with more positive than negative charge, so you get a net positive charge.

 

[milski]

Due to some arbitrary choices made centuries ago, the sign conventions in E&M are rather messed up. The real moving charges are electrons and they are negatively charged. On the other hand, current is defined to be in the direction of movement of positive charges. What makes things easier is that for most purposes of E&M (which you don't have to account for the mass of the charges), the movement of positive charges in one direction is equivalent to the movement of negative charges in the opposite direction. With this introduction:

142 4) Pretty much - it should follow directly from the intro above.
142 6) From Lenz's law. Current induced by a change in the flux will be in a direction which minimizes the change in flux. The reason behind that is self inductance which works like a brake on the flux change but that's outside of MCAT's scope - the qualitative idea of Lenz's law should be enough.
144 12) What they're trying to do is pull fragments of DNA with different force. Larger fragments have higher charge and will be pulled by stronger electrostatic force. Turns out that this does not work well in liquid solution since the drag that the fragments experience is also proportional to their size and the net result is that all fragment sizes move around together. I'm not sure how the gel gets around that problem.
145 18) See the intro - that's the same as saying that the negative charge flows the opposite way.

 

[dudewheresmymd]

Due to some arbitrary choices made centuries ago, the sign conventions in E&M are rather messed up. The real moving charges are electrons and they are negatively charged. On the other hand, current is defined to be in the direction of movement of positive charges. What makes things easier is that for most purposes of E&M (which you don't have to account for the mass of the charges), the movement of positive charges in one direction is equivalent to the movement of negative charges in the opposite direction. With this introduction:

142 4) Pretty much - it should follow directly from the intro above.
142 6) From Lenz's law. Current induced by a change in the flux will be in a direction which minimizes the change in flux. The reason behind that is self inductance which works like a brake on the flux change but that's outside of MCAT's scope - the qualitative idea of Lenz's law should be enough.
144 12) What they're trying to do is pull fragments of DNA with different force. Larger fragments have higher charge and will be pulled by stronger electrostatic force. Turns out that this does not work well in liquid solution since the drag that the fragments experience is also proportional to their size and the net result is that all fragment sizes move around together. I'm not sure how the gel gets around that problem.
145 18) See the intro - that's the same as saying that the negative charge flows the opposite way.

You are a godsend.

 

[swivelj]

Wanted to Bump this for question 12.

Is my thinking correct that because we have a GRADIENT of density in the liquid sucrose setup....we don't see breaks or separation in the bands. While if we were to use a constant density or viscosity a across a gel setup ...we would achieve separation or bands based off of mass/charge ratio and not due to some gradient in the media......or in other words regions were particular fragment sizes congregate?


I picture a funneling process in my mind for the Sucrose gradient setup, where we fill the lane completely down the gradient so we get poor resolution?

Not sure if I'm way off here or what.