Can physicists do any physics problem from a university physics textbook?

[MetalManuel]

I'm a second year college student right now and I have decided to major in physics, it is the only subject I have taken so far that has provided me with a challenge (this semester).

I am currently taking Physics 2 right now and I have got to say this class is really challenging, well not the class, but the professor. I thought I was really good at Physics last semester, until I got to this guy. My Physics 1 professor gave us basic problems of blocks sliding down ramps with a certain friction, etc etc and were relatively easy; which i think is the way most professors are. My current professor seems to love physics and the way he teaches it to us is just using variables (not the hard part, actually makes it simpler). The hard part is he gives us situations and wants us to describe them with mathematical formulas.

That's the hard part, coming up with the proper formulas to describe that certain situation. We have done this so much I don't even know the numerical value for some things, like epsilon 0, only the units.
Like this for example(took me hours to solve by myself with no help):

Since taking this professor I have gotten all his homework from his Physics 1 class and decided to try to start solving it, but like this semester's homework, it's really difficult and it's like relearning the class again.

What I was wondering is if I were to present a problem like this to the average physicist, would he be able to solve it? What I mean is, is having a really strong understanding of all of the basic Physics classes (mechanics, electricity, magnetism, optics, thermodynamics, waves/acoustics, and modern physics) a must?

I feel like I got jipped last semester by taking that professor's class.

 

[Phyisab****]

Yes, the average person with a PhD in physics would be able to solve that no problem.

 

[flyingpig]

You are suppose to show your work first.

This is just my intuition, but you are going to need to use $$\epsilon \frac{A}{4\pi d}$$

A = area, d distance between the plates.

 

[MetalManuel]

You are suppose to show your work first.

This is just my intuition, but you are going to need to use $$\epsilon \frac{A}{4\pi d}$$

A = area, d distance between the plates.

No no, that's the problem itself. He wanted us to prove it (he provides us with the answer he wants us to come up with).

Yes it's something like that, but it's different for square plate capacitors. The problem is trickier than it looks.

 

[Redbelly98]

You are suppose to show your work first.

The OP has solved the problem. His actual question was:

What I was wondering is if I were to present a problem like this to the average physicist, would he be able to solve it?

Yes. Though in my own case I would have to go back and review dielectrics and electric displacement fields, since I am more ex-physicist than physicist these days.

 

[AlephZero]

What I mean is, is having a really strong understanding of all of the basic Physics classes (mechanics, electricity, magnetism, optics, thermodynamics, waves/acoustics, and modern physics) a must?

The answer depends on whether you want to learn physics, or just get a degree in it. (And the same applies to any other subject, IMO)

 

[MetalManuel]

The OP has solved the problem. His actual question was:

Yes. Though in my own case I would have to go back and review dielectrics and electric displacement fields, since I am more ex-physicist than physicist these days.

I was just wondering if in order to understand the higher levels of physics a very strong understanding of these is required. For example, you can't do basic single variable Calculus until you have a strong understanding of basic algebra and trigonometry. Is it the same for physics?

The answer depends on whether you want to learn physics, or just get a degree in it. (And the same applies to any other subject, IMO)

I want to do both. If I could spend as much time as I wanted learning physics, I would try and master every single page of my textbook before I went onto the upper division classes for physics, but i don't know if I have that time. We got lives and have to get that piece of paper that validates that we can do something as well so we can survive.

I have noticed that most students taking all these classes only seem to care about their grade and not actually learning. They want to keep that GPA up. Which is all fine and dandy, but then I think about it. Some of these students go on to get their masters and PhD's, but they never truly mastered these basic classes. I'm just unsure of what to do right now.

 

[Redbelly98]

If I could spend as much time as I wanted learning physics, I would try and master every single page of my textbook before I went onto the upper division classes for physics, but i don't know if I have that time.

With a lot of things, mastering the basics comes from seeing them a 2nd or 3rd time again in the advanced classes, and seeing them applied to more advanced material. Hammering away in detail with a basic textbook won't necessarily get you there -- it's fine if you have the extra time, but as you said, "We got lives and have to get that piece of paper..."

For me, I remember (30 years ago now) how Fourier series and transforms came up in many different advanced topics. Seeing those advanced applications are what really made the concept start hitting home for me.

 

[brocks]

I have noticed that most students taking all these classes only seem to care about their grade and not actually learning. They want to keep that GPA up. Which is all fine and dandy, but then I think about it. Some of these students go on to get their masters and PhD's, but they never truly mastered these basic classes.

I guess there are some places where you can get a Masters just by squeaking through some more classes, but most if not all PhD programs have a preliminary exam, which is a very tough test, sometimes taking two full days, and covering pretty much all of undergraduate physics. Typically you get two or three chances to pass it, and if you don't, you are out of the program.

Here's a description of Berkeley's physics prelim:
http://physics.berkeley.edu/academics/grad/prelim-description.pdf

And not to insult your intelligence, but you may not be far enough along to realize that some problems in, say, mechanics, that would be a nightmare to solve by the techniques you learn in freshman physics, are much easier to solve with methods you learn in later classes, e.g. Lagrangian and Hamiltonian dynamics.

If it's OK with your prof, could you post a link to his Physics 1 problems? I have a nephew who is looking for exactly the type of problems you describe.

 

[Mépris]

Yeah, I was thinking about something very similar as of late and my plan is to learn more maths (try go through Linear Algebra, Analysis and PDEs - or at least, go as far as I can) and then move on to working through the Halliday & Resnick book. Or some other, more advanced book. I can be a bit of an extremist. It sounds like a lot of hard work but I get annoyed too quickly and I just *have* to do it the proper way. I don't like things being watered down.

If it's OK with your prof, could you post a link to his Physics 1 problems? I have a nephew who is looking for exactly the type of problems you describe.

I, too, would be interested in seeing some of these problems. :)

These could be of interest to your nephew:

http://www.cmi.ac.in/admissions/sample-qp/ugphysics.pdf
http://www.cmi.ac.in/admissions/sample-qp/ugphysics2010.pdf

 

[MetalManuel]

I guess there are some places where you can get a Masters just by squeaking through some more classes, but most if not all PhD programs have a preliminary exam, which is a very tough test, sometimes taking two full days, and covering pretty much all of undergraduate physics. Typically you get two or three chances to pass it, and if you don't, you are out of the program.

Here's a description of Berkeley's physics prelim:
http://physics.berkeley.edu/academics/grad/prelim-description.pdf

And not to insult your intelligence, but you may not be far enough along to realize that some problems in, say, mechanics, that would be a nightmare to solve by the techniques you learn in freshman physics, are much easier to solve with methods you learn in later classes, e.g. Lagrangian and Hamiltonian dynamics.

If it's OK with your prof, could you post a link to his Physics 1 problems? I have a nephew who is looking for exactly the type of problems you describe.

Yea I was exaggerating about the PhD, but I know that the difficulty level of getting a PhD and a master's is much different. It makes sense that there are easier methods to solve these problems when you get to higher levels.

I'm pretty sure my professor is ok with me posting his homework up since it is online and accessible to everyone. He teaches/has taught a lot of other classes as well. He was going to teach introduction to astrophysics this semester, but not enough students were interested in it.
http://www.swccd.edu/~jveal/phys270/hw.htm
These aren't problems from a textbook by the way.

Here's the other classes he teaches, you can access all the homework through here if you're interested. The physics classes are at the bottom.
http://www.swccd.edu/~jveal/classes.htm

Yeah, I was thinking about something very similar as of late and my plan is to learn more maths (try go through Linear Algebra, Analysis and PDEs - or at least, go as far as I can) and then move on to working through the Halliday & Resnick book. Or some other, more advanced book. I can be a bit of an extremist. It sounds like a lot of hard work but I get annoyed too quickly and I just *have* to do it the proper way. I don't like things being watered down.

I, too, would be interested in seeing some of these problems. :)

These could be of interest to your nephew:

http://www.cmi.ac.in/admissions/sample-qp/ugphysics.pdf
http://www.cmi.ac.in/admissions/sample-qp/ugphysics2010.pdf

Yea I understand. Personally I just feel that if I am going to be a physicist that if I open up my textbook to any problem I should be able to solve it.

 

[brocks]

Thanks for the links, and good luck with your classes.

 

[Phyisab****]

I think you're looking at this problem the wrong way MetalManuel. It's much less about knowing basic physics and much more about being able to write statements to describe mathematical relationships.

You really don't need to know anything about physics except for the equation for a parallel plate capacitor. The crux of the problem is being able to state mathematically the relationship between volume, thickness, and area of the dielectric that has been described.

 

[CNS92]

Yeah, I was thinking about something very similar as of late and my plan is to learn more maths (try go through Linear Algebra, Analysis and PDEs - or at least, go as far as I can) and then move on to working through the Halliday & Resnick book. Or some other, more advanced book. I can be a bit of an extremist. It sounds like a lot of hard work but I get annoyed too quickly and I just *have* to do it the proper way. I don't like things being watered down.



I, too, would be interested in seeing some of these problems. :)

These could be of interest to your nephew:

http://www.cmi.ac.in/admissions/sample-qp/ugphysics.pdf
http://www.cmi.ac.in/admissions/sample-qp/ugphysics2010.pdf

Out of curiosity, in what year would you study those topics on that paper? How old would the students taking that exam be?

 

[Phyisab****]

Out of curiosity, in what year would you study those topics on that paper? How old would the students taking that exam be?

Looks like freshman physics. They look a little harder than the class I took as a freshman, which explains why it's an honors class. It doesn't look like a junior level class to me.

 

[CNS92]

Looks like freshman physics. They look a little harder than the class I took as a freshman, which explains why it's an honors class. It doesn't look like a junior level class to me.

Sorry, I'm a Brit - is that 18-19 year old students?

 

[Phyisab****]

Sorry, I'm a Brit - is that 18-19 year old students?

Yea, special class for the smart ones.

 

[CheckMate]

What if it is an engineering problem rather than a physics one ?

 

[jeebs]

I find that when you first learn a topic, it's hard, but once you've done a semester of it and you've sat an exam on it, it's much easier a second time round if you need to go back over it. Stuff is always much harder when it's less familiar.

 

[Mépris]

A couple of weeks ago, I was learning magnetism and some concepts left me confuse and some others took more time to sink in. I talked about my doubts with my teacher and I think I kept thinking about the topic at random times during the day. I didn't make a conscious effort to but I just was and things got progressively clearer.

Out of curiosity, in what year would you study those topics on that paper? How old would the students taking that exam be?

Entrance exam for a selective Indian university. (or Mathematical Institute, as they call themselves) The students taking the exam are generally 17-18. They take it around April, then enroll by August. (or at least, around that time)

 

[twofish-quant]

Yes, the average person with a PhD in physics would be able to solve that no problem.

I'd have to spend an hour remembering some formulas, but I think I can do it easily if it was open book.

 

[G01]

If you asked me right after I passed my quals and was still a qual problem solving zombie, I think I would have been able to solve it 5 different ways and give you a lecture on the benefits of one method versus the other!

Now, I may need to look up a few formulas, but otherwise, I think I could solve it no problem.

 

[krome]

I know this is almost a year later, but I thought I would state what I think the professor was trying to get the student to understand, or at least what I think is the most important part of the problem.

Let C1 be the capacitance just of the region directly above the dielectric, C2 the capacitance just of the dielectric region, and C3 that of the remaining region of the capacitor. Then, the problem is equivalent to C1 and C2 in series, then this pair connected in parallel to C3. That is: C = (1/((1/C1)+(1/C2)))+C3.

Each of C1, C2 and C3 is given by its own appropriate version of the parallel-plate capacitance. The derivation of the parallel-plate capacitance is a standard result (c.f. Eqn. 2.54 page 105 of Griffiths).