Physics question thread

[Shrike]

All users may post questions about MCAT and OAT physics here. We will answer the questions as soon as we reasonably can. If you would like to know what physics topics appear on the MCAT, you should check the MCAT Student Manual (http://www.aamc.org/students/mcat/studentmanual/start.htm), though be warned, there are subjects listed there that are rarely tested, or that appear in passages only and need not be learned.

Be sure to check the Physics FAQs and Topic Writeups thread if you have a general question; eventually, many of your answers will be located there. Also, a request: to keep this thread at least somewhat neat, when replying to someone else's post please refrain from quoting anything more than what's necessary for clarity.

Acceptable topics:

• general, MCAT-level physics
• particular MCAT-level physics problems, whether your own or from study material
• what you need to know about physics for the MCAT
• how best to approach to MCAT physics passages
• how best to study MCAT physics
• how best to tackle the MCAT physical sciences section

Unacceptable topics:

• actual MCAT questions or passages, or close paraphrasings thereof
• anything you know to be beyond the scope of the MCAT

Side note: anyone who knows how to post subscripts and superscripts in this system, please PM me the method. I also wouldn't mind knowing how to post some obvious symbols, such as Greek letters and an infinty sign. Should be a matter of changing fonts, but I don't know what's available; again, a PM would be appreciated.

If you really know your physics, I can use your help. If you are willing to help answer questions on this thread, please let me know. Here are the current official contributors to the this thread -- a team to which I hope to add more people:

Thread moderated by: Shrike. Shrike is a full-time instructor for The Princeton Review; he has taken the MCAT twice for no good reason, scoring 14 on the physical sciences section each time. He majored in mathematics, minored in physics, and spent several years accumulating unused school experience (in economics and law).

Also answering questions: Xanthines, a Kaplan instructor. He scored 13 on the PS section of the MCAT and 34 overall.

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[lorelei]

Question: #310 in EK Physics 1001 Questions

Setup:

Two ropes hold a 10m-long board with a mass of 20kg. Rope 1 (T1) is at 7.5m, Rope 2 (T2) is at 5m (from right side). Board is at rotational equilibrium.

A 10kg mass hangs off of the left side. Find T2.

EK answers:

Choose point of rotation to be T2.

Counterclockwise torque = mg = 5kg(10m/s^2)
Clockwise torque = 2.5T1

My problem:
Setting them equal (b/c they are in rotational equilibrium): T1=20N

Since T1+T2 must equal the mass of the board + weight (20kg + 10kg):
T1+T2 = 30kg x 10m/s^2 = 300N

Substituting 20N into above gives T2 of 280N

The answer is 100N?

You're approaching the problem right. The mistake you're making is confusing mass with weight.

Center of rotation at T2.

CCW torque = T1 * 2.5m
CW torque = mass of weight * gravity * distance = 10kg * 10m/s^2 * 5 m

set those two equal, and you get T1 = 200 N.

T1 + T2 = weight of board + weight = (20kg * 10m/s^2) + (10kg * 10m/s^2)
T1 + T2 = 300 N

So T2 = 100 N.

I find that carrying units through on these problems can be really helpful in avoiding this type of mistake.

 

[seanth]

Can someone please briefly explain what "poiseuille flow" is? I saw it on the AAMC topic list.

Thanks!

 

[lorelei]

Can someone please briefly explain what "poiseuille flow" is? I saw it on the AAMC topic list.

Thanks!

It talks about flow in a tube. The fluid at the edges has a velocity of zero, while the fluid in the center moves the fastest. This is due to viscosity, or the resistance of fluid to flow (basically a measure of fluid friction - water has low viscosity, molasses has high viscosity).

This is often modeled by thinking of the fluid as a series of "lamina" (layers) in concentric cylinders, each of which has its velocity determined by its distance from the edge.

It turns out that, due to this type of flow, the effective resistance of a tube is inversely proportional to the fourth power of its radius. That comes up in cardiovascular physiology again.

You might find this Wikipedia article helpful:
http://en.wikipedia.org/wiki/Poiseuille's_law
Ignore the calculus; you don't need to know that detail for the MCAT. But there is a figure that does a good job of showing what is meant by laminar flow, and the text is pretty good.

 

[deleted106503]

Hi, I completely agree with you, but the answer is "the new pressure cannot be determined without more info" Perhaps I have briefed the question too much. So here is the actual Q:
An ideal fluid with pressure P flows through a horizontal pipe with a radis r. If the radius of the pipe is increased by a factor of 2, which of the following more likely gives the new pressure?
a.P
b.4P
c.16P
d."the new pressure cannot be determined without more info

So what's wrong with the way you described above in the last post about your approach to this question and why is the answer d? Thanks

You have to use this conservation equation: P + pgh + 1/2p(v^2) = K

You know v will decrease by a factor of 4 since the radius was doubled. The term 1/2pv^2 will decrease by a factor 16 (% wise), but you don't know the exact number so you don't know the exact number that P must change. P and 1/2pv^2 aren't proportional product-wise since you're adding the terms together, not multiplying.

Your equation. P = F/A, which also is P = pgy (where p is denisty, roe), is for a fluid AT REST with uniform density. Thus you cannot use it for a moving fluid.

 

[deleted106503]

15uC<---------10m------------>-1uC

-5uC charge of 3.5g released from rest 1.5m from negative charge, and moves towards the 15uC charge. What's its velocity at 3m from 15uC?

Help!

Do I use Ki+Ui=Kf+Uf or more classical kinematics?

I would use the change in KE = change in PE for this. I haven't ever had to find the PE of a charge between two charges. Below is my best guess at how you could find that. Please, someone correct me if I'm wrong.

For potential energy. Intial is (1)(5)/(1.5m) + (15)(5)/8.5 = 10/3 + 150/17 Since both forces are pointing to the left, I'm not worrying about +/- signs.

Final is (1)(5)/(7) + (15)(5)/3 = 5/7 + 25

Intial minus final is not something that the MCAT will ask you since you'd need a calculator because of the time limits. It comes out to -12.1289 for delta PE (U). Use .0035 kg instead of 3.5g. so KE = 1/2mv^2. (12.1289x2/.0035)^(1/2) = 83.25 m/s

Is this the correct answer? By the way, where did you come up with this question? Hopefully not an AAMC exam or practice tests...(really hoping!)

Edit: If they were to ask where the net force is 0, would you have to calculate where the PE is from either charge?

Like this:

(5)(15)/r = (5)(1)/(10-1) ???

 

[gridiron]

Can someone please briefly explain what "poiseuille flow" is? I saw it on the AAMC topic list.

Thanks!

That is surprising---poiseuille flow is something you learn in a fluids dynamics course and involves upper level calculus and differential equations to characterize flow. The classic problem of this type of flow involves the movement of a viscous laminar fluid between parallel plates. This type of problem assumes the length and width of the plates is much greater than the distance between them. The flow therefore is driven by a pressure gradient between the inlet and outlet of the flow. A velocity profile can then be derived to characterize the flow that takes into consideration the yield stress, density and pressure. If the plates are not moving, the velocity can be generalized to be zero at the plates due to the no slip condition--boundary conditions, and greatest as you move toward the middle of the plates. The yield stress profile is a little bit different. After this, it gets a little bit more complicated since profiles of laminar flow can be developed using Navier Stokes equation. However, the general conditon of such flow is when reynolds number is low and flow is laminar. Again, this is something you would encounter in uppler level fluids dynamics course--where you would also talk about newtonian and nonnewtonian fluids---but I think the MCAT would give you a passage from where you can figure out what most of the variables mean and a equation for you to manipulate. It is good to know what the variables viscosity and reynolds number mean. At low reynolds number, the fluid behaves as laminar and reynolds number is dimensionless. I hope this helps!

 

[MDavoodi_Co]

Hi guys, good topic.
Recently, i added a non calculus based physics. I was wondering if that would be a bad move for the mcats...... if it is, how bad is it???
I inted to finish the non-calculus based physics and take the mcats based on that knowledge. what do u guys think?

 

[Dr Durden]

Nothing calculus related is on the MCAT, even physics. The hardest math on the Physical Science section will involve some basic trig. As long as your non-calc physics is still a rigorous course, you'll be more than fine .

 

[RAD11]

Does the number of nodes correspond to what harmonic the wave is? For example, 1 node = 1st harmonic; 2 nodes = 2nd harmonic, 3 nodes = 3rd harmonic and so on?

 

[deleted106503]

Does the number of nodes correspond to what harmonic the wave is? For example, 1 node = 1st harmonic; 2 nodes = 2nd harmonic, 3 nodes = 3rd harmonic and so on?

The fewest number of nodes is 2. The first harmonic (fundamental) has 2 nodes. The length of the strings for a first harmoic is 1/2 of the wavelength.

So a node is a place where the vertical displacement is zero. For the first harmonic, the ends of the string connected to say...a wall, don't move. So 2 nodes. For the 2nd harmonic, the entire wavelenght is shown in the string. There are 3 points where the string doesn't move. The two ends, and the middle, so 3 nodes for a 2nd harmonic.

 

[RAD11]

The fewest number of nodes is 2. The first harmonic (fundamental) has 2 nodes. The length of the strings for a first harmoic is 1/2 of the wavelength.

So a node is a place where the vertical displacement is zero. For the first harmonic, the ends of the string connected to say...a wall, don't move. So 2 nodes. For the 2nd harmonic, the entire wavelenght is shown in the string. There are 3 points where the string doesn't move. The two ends, and the middle, so 3 nodes for a 2nd harmonic.

Hey thanks for the reply. Okay, I understood your explanation but I don't understand why the answers to these problems are the following:

1) 1st harmonic - since it only has 1 node
2) 3rd harmonic - has 3 nodes...shouldn't this be 2nd?
3) 1st harmonic - again only 1 node
4) 2nd harmonic - 2 nodes...shouldn't this be first?

Is this different because these are waves inside a pipe vs. to waves on a string?

 

[deleted106503]

Hey thanks for the reply. Okay, I understood your explanation but I don't understand why the answers to these problems are the following:

1) 1st harmonic - since it only has 1 node
2) 3rd harmonic - has 3 nodes...shouldn't this be 2nd?
3) 1st harmonic - again only 1 node
4) 2nd harmonic - 2 nodes...shouldn't this be first?

Is this different because these are waves inside a pipe vs. to waves on a string?

It is different for an open pipe and a string tied at one end, yes. My study books don't mention a difference in number of nodes for an open pipe. It just mentions that for this case, the harmonics are only odd numbers, 1,3,5, etc. It mentions that the open end is referred to as the anti-node, but it doesn't say how many nodes are in different harmonics. Hopefully someone else can chime in here and futher explain this.

Edit: My response a few posts up refers to: A string tied a both ends, a pipe closed on both ends, and a pipe open on both ends. I think the case your problem talks about is a string or pipe open on only one end, which I'm not sure at all.

 

[RAD11]

Hey WilliamsF1, are you using Examkrackers? That's what I'm primarily using. From EK, they have 2 formulas 1) L= n(lambda)/2 (n = # of harmonics; n = 1,2,3 ...). From what they say in the book, this eqn should be used for strings tied at both end as a node or where the end is loose creating an antinode, 2) L= n(lambda)/4 (n = 1,3,5,.....) and this eqn should be used for a string tied at only one end or a pipe only open at one end. But I'm not sure how I can use this formula to answer the types of harmonics in my question above since there are too many unknown variables in the formula (i.e. L and wavelength).

 

[go lakers]

On EK practice test 1-g there is a free standing question about torque and conservation of momentum.

A 100 kg man is standing on the left end of a 100 kg board 2 m long which is on a frictionless surface. The left end of the board is at point "0" and the right end of the board is at point "2." The question is where will the left end of the board come to rest if the man walks to the right end of the board and stops? The answer is -1 but I figured it would be -2. Any ideas? Thanks.

 

[deleted106503]

Hey WilliamsF1, are you using Examkrackers? That's what I'm primarily using. From EK, they have 2 formulas 1) L= n(lambda)/2 (n = # of harmonics; n = 1,2,3 ...). From what they say in the book, this eqn should be used for strings tied at both end as a node or where the end is loose creating an antinode, 2) L= n(lambda)/4 (n = 1,3,5,.....) and this eqn should be used for a string tied at only one end or a pipe only open at one end. But I'm not sure how I can use this formula to answer the types of harmonics in my question above since there are too many unknown variables in the formula (i.e. L and wavelength).

Holy crap, I am really sorry that I didn't look at your pdf file first. That would've gotten everything fixed at first.

I see how you got the nodes, which I agree with. 1, 3, 1, 2

Did the question say it's for an open pipe or not? Is this from Kaplan, EK, etc?

I just found this website for open-end air columns and harmonics. Keep in mind BOTH ends have to be open. For that, 1 node = 1 harmonic, go figure. Sucks EK didn't mention it, but now we have to refer to a K-12 webiste, wow: http://www.glenbrook.k12.il.us/GBSSCI/PHYS/CLASS/sound/u11l5c.html

Edit: For figure one, there appears to be a wall only on the left side, so I'm assuming that it's open ended on one side.

For that, we have to use the n = 1,3,5, etc like you said.

According to this website: http://id.mind.net/~zona/mstm/physi.../understandingSWDia3/UnderstandingSWDia3.html

1 node = 1st harmonic
2 nodes = 3rd harmonic
3 nodes = 5th harmonic

etc

Edit 2: A link with all 3. Just count the nodes on the pics and it will say what harmonic that graph is: http://library.thinkquest.org/C005705/English/sound/sound5.htm

I just wrote some notes in my EK book. Thanks for bringing this up!

 

[RAD11]

Hey WilliamsF1,

Thanks for the links...I will definitely go over them in detail. Definitely sucks that EK wanted us to assume that we would know what type of harmonics go with what. Their explanation was lacking (i.e. they just mention that the 1st harmonic start with 2 nodes, then you start adding nodes in order to figure out 2nd harmonic, 3rd harmonic and so on.

So I am assuming that we can't just use the formulas that I wrote in my above post. Time to memorize more stuff, fun (not)!

 

[SeventhSon]

Something very important to note is that yes, an integral is "needed" to solve the problem, if it were to be solved in terms of forces, since the forces are changing. HOwever, the idea of potential energy DOES THE INTEGRAL FOR US.

This goes by unnoticed a lot by students. Where do the expressions for U come from?

In each case, integrating the expression W = F *d and combining that with dw = -dU gives e.g.

Fg = mg GPE = mgh
Fk = -kx SPE = 1/2 kx^2
Fe = kqq/r^2 U = -kq/r

So yes... you use conservation of energy to solve these problems. The way I introduce electrical potential to students is that after talking about fields.... saying that they are insufficient in describing the velocity of a charge, if i were to put it down and let it interact with another charged particle.

Apologies...I misinformed you because I didn't read the problem correctly . I believe that you should use scalar quantities rather than vector quantities to solve this problem. I think, and somebody correct me if I am wrong, you can find the electric potential difference between the two points. When you multiply this value by the charge of the particle, you get the difference in potential energy. In this case, the answer should be negative because the particle is accelerating toward the other charge. The kinetic energy is the opposite the electric potential energy. Once you have that value, you can solve for the velocity. If I am incorrect, someone please correct me because I don't want to make the same mistake again

 

[deleted106503]

Hey WilliamsF1,

Thanks for the links...I will definitely go over them in detail. Definitely sucks that EK wanted us to assume that we would know what type of harmonics go with what. Their explanation was lacking (i.e. they just mention that the 1st harmonic start with 2 nodes, then you start adding nodes in order to figure out 2nd harmonic, 3rd harmonic and so on.

So I am assuming that we can't just use the formulas that I wrote in my above post. Time to memorize more stuff, fun (not)!

You can still use the forumulas.

L = nLambda/2 (for n=1,2,3,etc) for a string tied on both ends or loose on both ends. Also for a pipe open on two ends or closed on two ends.

L = nLambda/4 (for n=1.3.5,etc) for a string tied only on one end or a pipe open only on one end.

The only thing EK said was the number of nodes for each harmonic of a string tied at both ends. Maybe we don't have to know which harmonics something is for the MCAT. EK stresses that their book is all we need to know, so I'm not sure.

 

[gridiron]

15uC<---------10m------------>-1uC

-5uC charge of 3.5g released from rest 1.5m from negative charge, and moves towards the 15uC charge. What's its velocity at 3m from 15uC?

Help!

Do I use Ki+Ui=Kf+Uf or more classical kinematics?

Ok, after looking at this problem for a while (I'm weird like that ), I think I have finally come to a conclusion. You do need an integral to solve this problem if you use vectors--in fact you need two integrals because of the effects of two electrical fields. Seeing as there is no calculus on the MCAT, this is not the way to go about solving this problem. The best way is to use the scalar quantities of electric potential, kinetic and potential energy. You can use electric potential even though there are two charges--just be careful with your sign convention.

 

[tik-tik-clock]

Why is it that only reflection is a propertythat is indicative of both wave and particle theory , where as diffraction, interference and dispersion are not indicative of both the particle and the wave theory?

I kept thinking that both of these are the properties of light so they should all be indicative of both the wave and the particle theory?

 

[shantster]

Why is it that only reflection is a propertythat is indicative of both wave and particle theory , where as diffraction, interference and dispersion are not indicative of both the particle and the wave theory?

I kept thinking that both of these are the properties of light so they should all be indicative of both the wave and the particle theory?

If you have a strong beam of light pointing toward a wall, but there's an object (lets say a pole) in the way, with particles you should just get dark and light areas with no transition between them since the particles would bounce off and away from the wall. Instead, you see that there's a transition from light to dark, which is indicative of the wave theory since the light is bending around the object.

You could use the same reasoning for slit experiments with a laser. If you shine a laser through a tiny two slit surface, then instead of getting two distinct lines, you get diffraction patterns. Hopefully you've seen something like this in your physics course.

Hope that helps.

 

[shantster]

Ok, after looking at this problem for a while (I'm weird like that ), I think I have finally come to a conclusion. You do need an integral to solve this problem if you use vectors--in fact you need two integrals because of the effects of two electrical fields. Seeing as there is no calculus on the MCAT, this is not the way to go about solving this problem. The best way is to use the scalar quantities of electric potential, kinetic and potential energy. You can use electric potential even though there are two charges--just be careful with your sign convention.

Definitely don't need calculus for this. You can calculate the potential energy at the negative charge and the potential at the point 3m from the positive charge. The differece between the two is the kinetic energy and you can find v from there.

 

[gridiron]

Definitely don't need calculus for this. You can calculate the potential energy at the negative charge and the potential at the point 3m from the positive charge. The differece between the two is the kinetic energy and you can find v from there.

Umm....I say that in the rest of my explanation---i just didn't go through the steps, but I did in a earlier post

 

[abcxyz0123]

I have a quick question: If wave velocity is dictated by the medium through which the wave travels, and a change in frequency or wavelength does not change the velocity of a wave...how come all waves that travel in the air don't travel at the same speed? For example, how come sound waves travel slower than radio waves...shouldn't they travel at the same velocity if they are traveling through the same medium (air)?

 

[QofQuimica]

I have a quick question: If wave velocity is dictated by the medium through which the wave travels, and a change in frequency or wavelength does not change the velocity of a wave...how come all waves that travel in the air don't travel at the same speed? For example, how come sound waves travel slower than radio waves...shouldn't they travel at the same velocity if they are traveling through the same medium (air)?

Sound waves aren't electromagnetic radiation. They're longitudinal waves that require a medium so that you can have compressions and rarefactions of that medium. Radio waves are electromagnetic radiation like visible light or x-rays. They are transverse waves that do not require a medium, and in fact a medium slows them down.

 

[legobikes]

quick do-we-need-to-know questions from the kaplan book:

a) the ridiculous equations for light interference minima/maxima in the Kaplan book

and

b) the ridiculass expressions involved in standing waves (f=nv/2L where n=integers blah blah etc.)

and finally

c) the ricockulous equations relating to equipotential lines or electric dipoles in electrosatics?

thanks!

 

[gujuDoc]

quick do-we-need-to-know questions from the kaplan book:

a) the ridiculous equations for light interference minima/maxima in the Kaplan book

and

b) the ridiculass expressions involved in standing waves (f=nv/2L where n=integers blah blah etc.)

and finally

c) the ricockulous equations relating to equipotential lines or electric dipoles in electrosatics?

thanks!

Which equations in electrostatics are you talking about specifically???

As per the standing wave equations for both ends open and one end open, you should know both of those. I've seen it come up on practice tests.

 

[gridiron]

quick do-we-need-to-know questions from the kaplan book:

a) the ridiculous equations for light interference minima/maxima in the Kaplan book

and

b) the ridiculass expressions involved in standing waves (f=nv/2L where n=integers blah blah etc.)

and finally

c) the ricockulous equations relating to equipotential lines or electric dipoles in electrosatics?

thanks!

For the first question, diffraction is important. Although you don't need to know everything there is about what diffraction is, you should have a general feel for what is happening as well as concepts---equations as well. Believe it or not, diffraction and young's double slit experiment is actually a semester long class in my school, so don't worry too much if you don't understand everything.

 

[gridiron]

I have a quick question: If wave velocity is dictated by the medium through which the wave travels, and a change in frequency or wavelength does not change the velocity of a wave...how come all waves that travel in the air don't travel at the same speed? For example, how come sound waves travel slower than radio waves...shouldn't they travel at the same velocity if they are traveling through the same medium (air)?

The relationships between wave velocity and frequency are characteristic of the wave you are talking about. Velocity of a wave is dependent on tension and linear mass density. Therefore, for a given medium, a wave will always travel at the same velocity regardless of changes in frequency. What will change though is the wavelength.
Sound waves are mechanical waves and thus require a medium to transport its energy. Sound waves are caused by disturbances and since they are pressure waves cause compressions and rarefaction (not refractions) of the mediums molecules. Sound waves actually travel in faster in materials with a higher bulk modulus because of the pressure wave. Light on the other hand is a electromagnetic wave and doesn't require a specific medium for transport. Light actually travels faster in space than in glass because of the index of refraction of glass. Mechanical waves rely on particle interaction to transfer their energy.

 

[legobikes]

Which equations in electrostatics are you talking about specifically???

As per the standing wave equations for both ends open and one end open, you should know both of those. I've seen it come up on practice tests.

hmm well the electric dipole ones specifically;

v = k q d cos &#952; / r^2

E = 1/(4 &#960; &#949 p/r^3

v = k p cos &#952; / r^2

ugh.

 

[gridiron]

hmm well the electric dipole ones specifically;

v = k q d cos &#952; / r^2

E = 1/(4 &#960; &#949 p/r^3

v = k p cos &#952; / r^2

ugh.

I am usually told that those are not high yield on the exam, but you don't necessarily need to memorize them--you can derive them! Deriving them should take less than 30 secs---for the physical sciences, I practiced deriving certain equations and by the time I had to take the real thing it wasn't a time or a problem for me. The key though is to practice the technique before you actually use it!!

 

[Time Served]

Hey guys...I was wondering if someone can explain the difference between Young's 2 slit experiment and a simple 1 slit diffraction experiment? I understand that the experiment involves the wave nature of lights and constructive and destructive interference creating alternating bright and dark bands respectively. Anyone want to offer some insight into this commonly seen experiment/passage topic??

thanks

 

[gridiron]

Hey guys...I was wondering if someone can explain the difference between Young's 2 slit experiment and a simple 1 slit diffraction experiment? I understand that the experiment involves the wave nature of lights and constructive and destructive interference creating alternating bright and dark bands respectively. Anyone want to offer some insight into this commonly seen experiment/passage topic??

thanks

Young's interference experiment was as follows: light from a far monochromatic source illuminates one slit on a screen and the emerging light then spreads via diffraction to illuminate two slits. Diffraction of this light produces overlapping circular waves past the screen where waves from one slit interfere with the other. The basic principle behind this experiment is to determine the location of the fringes--alternative bright and dark spots. To do this you relate the path length to some angle theta so in actuality what appears at each point on the viewing screen is determinant of the path length difference of the rays reaching that particular point. Thus you get (a drawing here helps, I can't upload the file since it is too big):

change in L = d sin theta
which leads to the derivation of bright fringes at:
d sin theta =m lambda where m is some integer multiple and dark fringes at:

d sin theta = (m + 0.5) lambda
​

You can further derive equations for intensity by considering the electric field components of the light waves that are not in phase at a particular point. This however I believe is not tested on the MCAT.

As for diffraction by a single unit and locating the minima. If we examine the pattern of diffraction of waves incident on a slit of the length a, the mathematics becomes a little bit more tricky. I really can't go much more into detail without a drawing but I believe something of this nature will not be tested on the MCAT because of the different geometries involved. Good luck and I hope this helps!!

 

[gridiron]

Hey guys...I was wondering if someone can explain the difference between Young's 2 slit experiment and a simple 1 slit diffraction experiment? I understand that the experiment involves the wave nature of lights and constructive and destructive interference creating alternating bright and dark bands respectively. Anyone want to offer some insight into this commonly seen experiment/passage topic??

thanks

.

 

[legobikes]

Thanks for the previous answer..

Another question which is bothering me.

There is a problem where you have a capacitor and resistor in series. The question is what happens when you add another resistor in there. Now I thought that if resistors/capacitors are in series the voltage gets divided up between them while the current remains constant (cause Iin = Iout), and so the effect this would have on the voltage across the capacitor is that the voltage would decrease (because some of it was being used by the extra resistor in series, and the circuit uses up all the voltage put into it).

But the book says:

"By adding another resistor in series we have increased the overall resistance of the circuit. Qualitatively, the current through the circuit at any time t would be smaller because of the higher resistance. The flow of charge to the capacitor is therefore slowed, and so it will take longer for the capacitor to charge up to a particular level."

Does this mean that the number value of I is reduced because the current is slower, while conservation of charge still applies? And even if so, wouldn't the voltage across each component be smaller?

 

[kaybam20]

hey MCAT experts,
Quick question, how do you designate sign convention between work and potntial enegy.i.e. when do you do negative or positive work and how do you come to conclusion about who is doing the positive or negative work.


Also in electrostatics, how do you designate postive/negative electric potential.
specifically when using W = qV. In qusetins relting to mobement of postive and negative charges how do u determine postive/negative work and/or positive/ negative potential energy.



Thanks.

 

[gridiron]

hey MCAT experts,
Quick question, how do you designate sign convention between work and potntial enegy.i.e. when do you do negative or positive work and how do you come to conclusion about who is doing the positive or negative work.


Also in electrostatics, how do you designate postive/negative electric potential.
specifically when using W = qV. In qusetins relting to mobement of postive and negative charges how do u determine postive/negative work and/or positive/ negative potential energy.



Thanks.

For the first question, are you referring to thermodynamics? To answer the second question:

Electric potential by definition is the potential energy per unit charge. Two charges, either positive or negative, within a vicinity feel each other because of electric fields. When they move, work is required to move the charge with the electric field or against the electric field. Usually, when I teach this concept to students, I derive the expression for electric potential between two points from the electric field and I relate the concept at the macroscopic level. You won't need this for the MCAT, but the electric potential between two points can be derived as follows:

1.) First, a moving test charge feels a electric force so:

Work = Force dot distance
dW = qE dot dS (we relate to differential)
w = q integral Eds cos theta
If you divide by q you get W/q which is the electric potential

​

So for instance, if you place a negative test charge in the electric field of a positive charge what happens? The negative charge moves toward the positive charge. Since it is moving against the electric field, the work is positive and so is the electric potential, but the electric potential energy is negative (the work is positive because the theta is 180 degress--moving opposite of the electric field but the charge is negative as well). Now, what happends with a positive test charge in the electric field? Well, the positive charge will move away--in the direction of the electric field. Now, the work is again positive as is the electric potential but the electrical potential energy decreases again (W = -U). So what can we conclude? When charges do what they naturally want, two like charges repel or opposite charges attract, there will be positive work and their potential energy decreases. Why? Relate it at a macroscopic level. When you lift a book, you do negative work against the earth's gravitational field and the potential energy increases. When you drop the book, positive work is done, because the book is doing what it wants to do, and the potential energy decreases. So what kind of generalization can be formulated? A negative charge in a electric field of a positive charge will feel attraction. It will accelerate toward the positive charge and will convert potential energy to kinetic energy. If instead the negative charge was pushed away, negative work (external force) would be required to pull the charge away from the positive charge. However, since the charge is doing something it ordinarily wouldn't, the potential energy increases and is positive---much like lifting the book (external force is required to lift it against the earth gravitational field). I hope this helps and good luck!!!

 

[gridiron]

Thanks for the previous answer..

Another question which is bothering me.

There is a problem where you have a capacitor and resistor in series. The question is what happens when you add another resistor in there. Now I thought that if resistors/capacitors are in series the voltage gets divided up between them while the current remains constant (cause Iin = Iout), and so the effect this would have on the voltage across the capacitor is that the voltage would decrease (because some of it was being used by the extra resistor in series, and the circuit uses up all the voltage put into it).

But the book says:

"By adding another resistor in series we have increased the overall resistance of the circuit. Qualitatively, the current through the circuit at any time t would be smaller because of the higher resistance. The flow of charge to the capacitor is therefore slowed, and so it will take longer for the capacitor to charge up to a particular level."

Does this mean that the number value of I is reduced because the current is slower, while conservation of charge still applies? And even if so, wouldn't the voltage across each component be smaller?

Think about what resistors do. Here is an analogy: Consider a highway with only one lane in the downtown area after work is out. There is a lot of traffic. If the highway is longer in length the traffic will extend out. However, if it were maybe two, three of four lanes, the traffic will spread out. If more resistors are placed in series with a capacitor, current will encounter more resistance. By convention, current wants to travel a path with the least resistance. So if there is more resistance across any given length, less current will travel and thus the capacitor will take longer to charge. Current isn't slower---but the correct term to use is it encounters more resistance and thus fewer charges per time travel the length--the defintion of current is the flow of charge per unit of time. I hope this helps!

 

[Cloudcube]

Hi there,

I have a question whose solution doesn't make very much sense to me.

There's an incline and a 5 kg block takes 5 s to slide down a frictionless incline at an angle of 30 degrees. (It looks something like this > with the angle being at the point.) Find the normal force. The normal force should be. 42.4N. Could someone just let me know how they would go about attacking this problem? Thanks!

 

[DesiMcatAcer]

Hi there,

I have a question whose solution doesn't make very much sense to me.

There's an incline and a 5 kg block takes 5 s to slide down a frictionless incline at an angle of 30 degrees. (It looks something like this > with the angle being at the point.) Find the normal force. The normal force should be. 42.4N. Could someone just let me know how they would go about attacking this problem? Thanks!

Normal force on an incline is mg*cos(theta), the time given is irrelevant to this problem. Hence, (5kg)*(9.81 m/s^2)*cos(30)=42.4N. Hope it helps!

On a sidenote, I hope you mods don't mind if I answer questions at times?
-Desi

 

[QofQuimica]

Normal force on an incline is mg*cos(theta), the time given is irrelevant to this problem. Hence, (5kg)*(9.81 m/s^2)*cos(30)=42.4N. Hope it helps!

On a sidenote, I hope you mods don't mind if I answer questions at times?
-Desi

Anyone who has already taken the MCAT is welcome to help answer questions. We can always use more help in here.

 

[harrypotter]

I haven't had the lecture about light so I may be overthinking this problem.

There's light emitted and enters a different medium with a bigger n. They want to know what happens to wavelength.

The answer explains:When light enters a denser medium, its velocity will decrease. Since frequency is held constant, wavelength has to decrease as well.

During the test when I saw this problem, I imagined myself, standing at the edge of a swimming pool/ocean looking down. If the wavelength decreases or increases, wouldn't whatever I'm seeing look different colors to me? IN other words, would a fish that looks violet in air, look blue in water because the wavelength shortened?

Thanks in advance and sorry if it's a lame question!

 

[BruceWayne]

Hi everyone, I wanted to know how I can approach this problem and solve it.

Lets say theres a 1microC charge 1cm away from another charge that is negative 2microC. How much work is done when the distance between them is tripled?

Thanks!

 

[Creightonite]

Hi, I got a question. I was reading about speed of waves when moving from media to media. I understand that frequency does not change but wavelength is the one that changes affecting the speed of light in the media. Saw I was wondering if you were to arrahcnge this compund in terms of the speed of the sound wave (or any other type of wave) from having the lowest speed to the highest: water, metal, air, glycerol (a viscous compound (less dense than water) and kerosine (less dense than water)

Here is how I would arrange them (in the order of increasing speed of sound):

air -> kerosine --> glycerol --> water --> metal

Am I right?

 

[gridiron]

I do believe your trend is correct, but you need to know the densities of glycerol and kerosine to have a more accurate trend. Sound waves require a medium with particle interaction in order to transmit their energy. Media with a greater bulk modulus, or greater resistance to compression, will transmit sound waves faster. That is why metal transmits sound better than air. The speed of sound can be found as follows:

velocity = square root (bulk modulus/density)​

From this, you would assume that sound would travel faster in air than water since the density of water is greater than that of air. However, the bulk modulus is much greater for water than air---because water is much more incompressible than air. For the most part, metals are better mediums to transmit sound than air.

 

[BruceWayne]

Hi everyone, I wanted to know how I can approach this problem and solve it.

Lets say theres a 1microC charge 1cm away from another charge that is negative 2microC. How much work is done when the distance between them is tripled?

Thanks!

Never mind everyone, I found what I was doing wrong. Thanks anyway. If anyone else wanted to know how to solve the problem, you need to use:

changeWork=q*changeV
Where V would be V=kQ/r

 

[akinf]

Why does it require no work to move charge along an equipotential line even though the charge is moving?

I've never really understood the conceptual basis, but based on the mathematics, I can see why it is so.

 

[DrBowtie]

Why does it require no work to move charge along an equipotential line even though the charge is moving?

I've never really understood the conceptual basis, but based on the mathematics, I can see why it is so.

Because the equipotential lines run perpendicular to the E field lines. The E field isn't doing any work and ideally there shouldnt be any resistance so you can take a Newtons first law perspective and say no force is needed to move it at a constant velocity.

 

[akinf]

Thanks...that was a very quick response.

 

[legobikes]

I hope this helps!

Thanks!