Moved on...

I assume that you've found this blog because you are taking Physics 102 at McGill University (or maybe another similar course elsewhere). I used to be a teaching assistant for this course and we experimented with using a blog format to interact with students. I have since moved to a new institution and will no longer be maintaining the blog. This means that I will not answer new questions. However, I will leave the blog here as a reference. You are welcome to browse around and look at previous questions that students posed and what answers I posted. You are free to post comments (unless things get unruly, or spammed) and discuss concepts with each other (to an extent which would not be seen as "cheating", if I notice this, I will simply shut down the comments sections). I encourage you to make use of the webCT discussion boards and the T.A.'s at your tutorials.

Best of luck!

TA strike called

As of 12:01AM Tues April 8, 2008, the TA union (AGSEM) called a strike.

For more information see the announcement sent by McGill Strike by Teaching Assistants and Demonstrators at McGill,
Or the AGSEM website.

This means I must (by law) cease my TA work. I will not be able post to the blog, webCT, or answer emails.

I wish you the best of luck.

Topic open: AC Circuits

The following topic is now open for questions: RC, RL, LC, LRC circuits (and any AC circuits)

To pose a question, please post a comment to this post by clicking on the comments link below.

Your TA's

Hi all,

This is a "head's up" notice to let you know that AGSEM, the union for all of your TA's at McGill has passed a strike mandate. This does not mean that a strike will necessarily occur, but it does mean that one is possible in the next few weeks.

If it should occur, all of your TA's will be required (by law) to stop working as teaching assistants, including myself. This means that no additional postings will be made to the webTA sites or webCT. I will leave all existing material in place for you to access. I see no reason to take it down. If a strike begins, I will post another notice here to inform you of the situation.

I hope it does not come to a strike, but if it does your support would be appreciated. We do what we can to help our students, and it is unfortunate that nearly any action we take will affect you. I don't wish to make this a soapbox, so if you are interested in finding out more, please visit the AGSEM website.

Thank-you for your understanding.

Sarah

Magnetic fields: clearing up some confusion

difference between magnetic field and B-field? What 's the difference between those two?

why the direction of magnetic field of a loop is a circle and the professor use the word" B field" to refer to the direction at a certain point which is tegant to the circle?

Magnetic field and "B-field" are the same. Since we usually use "B" to denote magnetic field, we sometimes refer to the magnetic field as the "B-field". Consider it physics slang. As confusing to the uninitiated as OMG, LOL. ;)


Now, for you second question, if we consider the magnetic field around a current carrying wire, we know that the magnetic field forms a closed loop around the wire. But the magnetic field is a vector, right? So those vectors are tangential to these magnetic field lines and represent the vector field. Those lines just make it a little easier to visualize (without too many crazy arrows all over the place, because I drew 3 and that doesn't look too bad, but to truly represent it we need one for every point along that circle as finely as we are interested in).

to clarify the meaning of "perpendicular distance" when talking about B-fields.

perpendicular distance is the distance from the wire to the point in the magnetic field we are interested in (the angle between this line and the wire does not have to be 90).

Please correct if needed.

The perpendicular distance does indeed need to be perpendicular. I think you are still referring to the magnetic field from a straight segment of wire. In this case, the magnetic field produced is uniform along the wire, and varies radially away from it (~1/r). Since the only variation is radially, you only need this radial distance (which is always perpendicular to the wire).

Please let me know if you are concerned about a different situation.

Force of B-Field of one current-carrying wire on another.

Example: one horizontal wire (I1) has a current flowing to the right. Another, perpendicular to it, is above it with a current flowing upwards. The direction of the force of I1 is downward, by RHR. But, the direction of the force acting on I2 is to the right, because, though the direction of B1 is taken into consideration, the direction of the force is based on the direction of I2 and not I1. Why?

Ahh! But you have already taken into account the direction of I₁ when determining the direction of the magnetic field (B₁) produced by it. You then consider the force exerted by this magnetic field on the second wire, so you must then consider the direction of I₂.

I hope that clears up some confusion concerning magnetic fields.

Question: clarification of lecture notes on magnetic fields

I would like to have clarified a number of points regarding the Magnetism slides.

1) Regarding B and decrease of B as distance from I increases.

The relationship between B and distance from I (r) is B ~ 1/r (re: Capa 8 q 1a). However, in the class slides (slide 1 p. 4) it says

" delta B ~ 1/r2 (r squared)"

What r is this? I notice this is not the perpendicular r, but in the following delta B equation, sin theta is multiplied, and thus becomes the perpendicular r. Confusion?

2) Slide 2 p. 5, for a circular clockwise I, P1 within the circle points into the page, while P2 outside the circle points out of the page. Is it that B's outside a circular I are always in the opposite direction as that in the I?

3) slide 4 p. 3. The current loops in the diagram show a B opposite to RHR. Is it because the loops are electron orbitals, and electrons produce B's opposite to RHR?

4) similar question as previous. slide 1 p. 6. why is the B opposite to the RHR?

Thank you.

Regarding your first question, while there is an apparent inconsistency here, both are correct. The problem is one expression is for "B" and one is for "ΔB". To get B~1/r you have to integrate "ΔB" over the particular geometry appropriate to the problem. This is why you get different expressions for the B-field generated by current in a straight wire, a loop of wire, etc.

The "r" being referred to for "ΔB" is the distance from the point on the wire generating the field when integrating. Since all parts of the wire contribute to the magnetic fields in all parts of space you have to consider "off-axis" contributions as well as those perpendicular. Once the expression is integrated, for example in the case of a straight current carrying wire, the r will refer to the perpendicular distance away since the field will be symmetric and constant along the length of the wire anyhow. Otherwise, it might be important!

For your second question, I assume you are using a right hand rule where you curl your fingers in the direction of the current and your thumb tells you the direction of the magnetic field inside the loop. You can also point your thumb in the direction of the current and curl your fingers around to determine the direction of the magnetic field around the wire. If you do this you will quickly see that your fingers point up on the outside of the loop and down on the inside of the loop no matter what part of the loop you look at. And yes, the field should point in the opposite direction on the outside of the coil compared to the inside. This is because magnetic field lines must always close (the field lines would look like rings around the loop). It might help you to take a look at The "Right-hand Rules" and Magnetic fields.

For the last two questions, so far as I can tell, the directions are as given by the RHR. Pg. 3 slide 1, I get B pointing right, and for slide 4, I get B pointing left. It's not so easy to see which way the loops are supposed to be coming out of the page... look for where the breaks in lines are, indicating that that part of the line is passing behind another line.

Hope that helps!

Topic open: Magnetostatics

The following topic is now open for questions: Magnetic fields.

To pose a question, please post a comment to this post by clicking on the comments link below.

Not related to electromagnetism, but some good clean Physics Phun

Professor of Physics at MIT Peter Fisher helps Conan O'Brien with the ring spin record...





I just had to share when I saw this.

Edit: Why do you think too much vaseline would be a bad thing for the ring spin? Discuss...

found via physicsknits blog

Topic open: DC Circuits

The following topic is now open for questions: Circuits. Anything about resistors, capacitors, currents, batteries, etc.

To pose a question, please post a comment to this post by clicking on the comments link below.

Question: Electric PE could be 0? This is very distrurbing...

I know we have down tons of questions where we have to find where V is zero (or similar like it..)

I never made the connection until now that V =0 means the electric potential energy at that point is 0 too! This is deeply disturbing.

Does this mean that if say a charge move between two other charges.. suddenly, middle of the way, it losts all of its potential energy? It is certain possible for V to be zero, which would imply electric PE be 0 as a consequence.

I am so used to the potential energy in a gravitational sense, where potential energy is 0 only when you hit the bottom.. This is like jumping off the a building then middle of the way, finding all of your potential energy all gone of a sudden. Where did it go?

Disturbing indeed.

Rest assured, physics is not broken, but there are two interesting points to be made here.

Firstly, while the analogy to gravitational energy is a good one, it is incomplete due to the fact that there is no negative mass, but there are indeed negative charges. This means that the potential surface can dip below zero such that a local minimum in potential energy may very well be "beyond zero". Really what the universe seeks is minima, not "zero", especially since we can arbitrarily choose where zero is (think about gravitational energy and choosing the top of a cliff to be at zero PE... one still loses PE if one falls to the bottom).

Secondly, what matters in terms of motion is force, not PE, and that is related to the slope of the potential surface, not the "value" of it.

Let's look at a potential map:



I started this off with some small rectangular regions at set voltage (these could be metal plates held at a potential for example) and then calculated the potential in between. We can see that there are some "hills" (white, or lighter yellow) and some "valleys" (darker shades) and a gradient in between representing a smooth potential surface (ignore the pixellation, I didn't calculate this on a very fine mesh).

In particular, take a look at the region between the highest positive potential and the lowest negative potential. We can see that there is an equipotential line running between them at V=0. But, if we think of this like an elevation map that V=0 crossing is on the middle of a hill! A positive charge (we would invert the map for a negative charge) moving across this potential surface would act like a marble rolling on an elevation map. If we put it on top of the white hill of +2V it would roll down into the -5V valley, still experiencing a force due to the electric field as it passed through 0V.

So, "zero potential" is a somewhat arbitrary thing, and because of negative charges it comes up naturally more often for electrostatic potential that you might be used to from gravitation. Also, the force, the thing which drives motion, is related to the slope (really a directional kind of slope which senses the steepest change).

The very useful thing about electrostatic "potential" is that it gives a convenient way to map out the landscape that a charge will "see" without reference to that charge, just as electric field maps out the force a charge would experience without reference to that charge, and the two are related by slope. Such a map can give an intuitive feel for what will happen when a charge is introduced even for complex arrangements of charges or plates.

I hope that helps. This is far from a complete explanation, so feel free to stimulate further discussion in the comments here!

Topic open: Electric fields, potentials and capacitors

The following topic is now open for discussion: Electric fields, potentials, capacitors and parallel plates

If you have a question regarding this topic, please post a comment to this post by clicking on the comment link below.

Electric fields and electrostatic potential

I've had an interesting discussion regarding Electric field and Electric potential and thought I would repost it here so it wasn't buried in the comments of an old post...

In relation to the post Question: E=V/d or V=-Ed? (to minus or not to minus), Peter asked:

Hey, I was just reading Cutnell & Johnson Physics 7th edition (Competitor textbook to Giancoli) and on pg 585 it had some comments about this. I am not sure if it is applicable?

They first derived the -form of the equation. So they said
W = Fd =qEd
But W = -PE

qEd = -PE

qEd/q = -PE/q

Ed = -V

V = -Ed

Ok so far so good.

then they had some comments about the form of the equation where V = Ed (no negative sign)

"When applied strictly to a parallel plate capacitor, however, this expression is often used in a slight different form. In figure 19.16, the metal plates of the capacitor are marked A( higher potential) and B (lower potential). Traditionally, in discussions of such a capacitor, the potential difference between the plates is referred to by using the symbol V to denote the amount by which the higher potential exceeds the lower potential. V = Va-Vb.

Thus,

E = -V/d = - (Vb-Va)/d = (Va-Vb)/d = V/d.

Would this be a valid explanation as well since in this course we are mostly dealing with parallel plate capacitors? Thanks.

Hi Peter,

Excellent idea to check out another text book, sometimes a different perspective is all you need...

However, I disagree with Cutnell's approach here. They are essentially taking advantage of two negatives which cancel, and I think it is confusing. It is true that, as a sort of shorthand, the potential difference between the plates is simply given as the amount by which the higher potential exceeds the lower. The problem I see with using this "no minus" version of the equation is that it does not represent the true relationship between E and V (ie. that the electric field points from the high potential to the low potential). This takes a relationship and turns it into an equation, which I dislike. Equations are limited in scope and easy to misapply, relationships can give you better understanding which is a much more solid basis.

I'd rather see you sketch a diagram showing the high and low potential and where the positive and negative charges are separated on the two plates and applying the "V" given as the magnitude of the potential difference. It will be much harder to go wrong with this picture in front of you, and the whole "-" issue more or less disappears.

I hope that helps.

Gee you are totally right. I posted this early this morning and now I already have a different idea, which you have already hinted here but I want to make sure I got this down 100%.

This just came to me.



Is this way of thinking correct?

Thanks!

Which is exactly right. In fact, the "full" form (using vector calculus) is given by:



which essentially says to add up (integrate) the components of the electric field parallel to the path taken (the dl). If the electric field is constant and the path is straight, then everything reduces to what we have above.

Conversely, the electric field is proportional to a sort of directional slope of the potential (called the gradient) such that the greatest forces will be felt by charges in the "steepest" regions of electrostatic potential. More on that later perhaps...

Topic open: Electric Potential and Potential Energy

The following topic is now open for discussion: Electric Potential and Potential Energy.

If you have a question regarding this topic, please post a comment to this post by clicking on the comment link below.

Topic open: Forces on charges and Electric fields

The following topic is now open for discussion: Forces on charges and Electric fields.

If you have a question regarding this topic, please post a comment to this post by clicking on the comment link below.

New Example! Chapter 16 #15

Now posted on the main WebTA site:

Chapter 16, #15

Before heading over to check it out, I suggest you try it yourself and then take a look.