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02:19
@JohnRennie I'm having trouble with the experiment.
I can't seem to have the waves for the voltages to have some phase difference to measure the phase angle.
02:55
@JohnRennie p/q=n means p=qn. That is right. But p=qn mean p/q=n only when we rule out q=0. How? Can you elaborate?
I can say it’s true for p, q other than 0.
 
1 hour later…
04:15
@Ajay Hi :-)
I'm around for a while if you want to discuss this.
@cOnnectOrTR12 Hi :-)
p = qn is troublesome when q = 0
For non-zero π‘ž we have only one value of 𝑛 for each 𝑝 e.g. if π‘ž = 2 then we could have 𝑛 = 3, 𝑝 = 6 or 𝑛 = 4, 𝑝 = 8 and so on.
But if π‘ž = 0 then for every value of 𝑛 we get the same value of 𝑝 i.e. zero.
So it isn't just the division by zero that is weird. Multiplication by zero is weird as well.
04:53
@JohnRennie Hi
Hi :-)
Did you get the equipment working i.e. can you see the waves on the oscilloscope and measure their voltage?
I can see the waves
but they are in phase
so I can't measure the phase angle?
It's possible the oscilloscope is adjusting its display so that the waves start at zero at the left side of the screen. That would result in the two waves always being displayed as if they were in phase even when they aren't.
However you can calculate the phase angle from the two voltages so you can still measure it, though it's an indirect measurement.
Suppose the resistor and capacitor voltages are Vr and Vc, then the phase angle is given by:
tan(ΞΈ) = Vc/√(Vr² + Vc²)
This might seem a little mysterious, but we can explain how this formula works with a diagram if you're interested.
Sure. But first let me send you a picture of what I am seeing.
05:04
Yes, it looks as if the scope is adjusting the horizontal position of the waves so they start at zero. There might be a setting to change that.
This is the mannual.
I've been looking through but I can't find a way to change it.
What resistor and capacitor are you using?
1K and 1 micro F
And frequency = 10Hz?
05:08
Yes
Give me a moment to look through the manual
Let me ask the lab technicians.
@Ajay on page 64 there is a section explaining how to control the trigger
If you set it to trigger off channel 1 it should adjust channel 1 so it starts at zero on the left side of the screen, then you should be able to see the phase offset of channel 2.
@Wolgwang Yes it worked :-)
@KavinIshwaran OK, so it was just a matter of working out what approximation they had used :-)
05:21
Yes :-)
@Ajay That looks to me like a 100Ξ© resistor not 1K
For a 1K resistor the colour bands should be brown, brown, red not brown, brown, brown.
Not that it matters - the experiment will work just as well with a 100Ξ© resisitor.
Also that looks like an electrolytic capacitor, and you shouldn't apply AC to an electrolytic capacitor.
@Ajay Also the amplitude of the oscillator seems far too high at 24V. 1V would be more suitable.
At 24V and a 100Ξ© resistor the current will get very high at high frequencies.
05:55
@JohnRennie It's still not working
You tried changing the trigger?
06:20
yes
but I don't think I did it properly
@JohnRennie I'm not too clear on what you mean.
@Ajay About the trigger? Or about one of the other things I mentioned above?
the trigger
Is it clear what "trigger" means with an oscilloscope i.e. what the trigger does and why we need it?
no
06:38
When the scope displays the wave it has to decide when to start the trace.
Suppose you have a voltage V = Vβ‚€ sin(Ο‰t) if we were graphing this we would normally start our graph at t = 0 so the voltage V = 0 at the origin.
@JohnRennie I think I need to do a different experiment.
But for the oscilloscope it just measures a voltage and it has no idea what the time t is supposed to be.
My Physics teacher said to abandon this one.
You should listen to your Physics teacher as they have a lot of experience and will know what projects do well for the IB.
So if they say abandon it then you should do as they say.
I think I should probably stop trying to recommend a project as I obviously don't know what projects work well for the IB.
No, the experiment was fine.
It was a problem with the oscilloscope.
He gave me two other ideas
I can't remember what he said exactly
but this is what he drew
06:43
OK, that looks like the timing circuits that we talked about before.
I can explain the theory if you want.
Sure
Is it very different from before?
Just to be clear - are the left and right diagrams the two experiments he suggested?
Yes
If so the right diagram is very close to what you already have.
Yes
06:46
We talked about tone controls yesterday, and you could change your experiment very slightly to measure the behaviour of the circuit as a filter instead of measuring the phase angle.
Ok
It's exactly the same experimental setup, but you measure Vc and Vr as a function of frequency rather than measuring the phase angle.
Then you get the two graphs I added to the spreadsheet yesterday showing how the voltage Vc decreases with frequency and Vr increases with frequency.
Ok
I think he's calculating the frequency at which R = Xc i.e. the frequency at which the output voltage is half the input voltage.
@JohnRennie Are you Free ?
06:51
@JohnRennie I think I should straighten out the details with my teacher so we can move forward with certainty.
It would be better to pause for now. I'll get back to you later.
Ping me if you need anything more. Maybe show the teacher the spreadsheet so they know what you've already done.
@KavinIshwaran Hi, yes I'm free :-)
ok.
@JohnRennie What is weak and strong conjugate Base and Acid ?
06:56
How much do you already know about this?
Except the point that Weak acid/base have strong conjugate base/acid and vice versa
I know nothing about conjugate base and acid
Let's take a reaction like:
HCl + NH₃ ⟢ NH₄⁺ + Cl⁻
In this case HCl is the acid and NH₃ is the base because HCl donates the proton and NH₃ accepts the proton. Yes?
07:01
And HCl is a very strong acid because it donates protons very strongly.
But ...
All reactions are reversible, so we could also write:
NH₄⁺ + Cl⁻ ⟢ HCl + NH₃
Yes?
But this reaction hardly goes at all. If you started with high initial concentrations of NH₄⁺ and Cl⁻ you'd only get tiny amounts of NH₃ and HCl. Yes?
Yes
So since after dissociation the Cl hardly gets back any H+ ion, it is a weak conjugate base ?
(Just posting my question) JRS, Anytrick comes in mind?
A prism mounted over a triangle
@KavinIshwaran That's because NH₄⁺ is a very weak acid and Cl⁻ is very weak base i.e. NH₄⁺ does not donate protons very strongly and Cl⁻ does not accept protons very strongly.
07:05
Oh OK Got it :-)
@KavinIshwaran Yes. HCl is a strong acid and its conjugate Cl⁻ is a weak base
Likewise NH₃ is a strong base but its conjugate acid NH₄⁺ is a weak acid.
@KavinIshwaran OK :-)
@Wolgwang Good grief!!
Was that really asked in the JEE?
AITS
(You might have heard name)
I actually took quiet a time just to visualise the structure.
I have no idea how to do that. From the symmetry the current in the vertical wire will be zero so you can remove it. Also the circuit is symmetrical about its mid point so that relates the currents in the left and right halves.
Just knowing the approach is enough. I ain't going to try that. ;)
@JohnRennie Can you help me with geometry?
It's not my strongest subject, but I can try ...
07:17
The triangle is equilateral and the radius of circle is R.
> An infinite long hollow metallic cylinder of radius R and surface charge density
Sigma is placed symmetrically with an imaginary surface of the shape of a prism. The length of prism is R and its three sides are all equal to 3R. The flux through the prism is
Actually a physics question.
So you just need to find the lengths of the arcs that are inside the triangle?
Yes
The radius of circumcircle is sqrt3 R
I can have a go, but at the moment I have another student asking me about phonons!
Ok. I am not in a hurry :)
07:59
@Wolgwang For an equilateral triangle of side π‘Ž the height of the centroid is a√3/6 so the height of the centroid here is R√³β„β‚‚ = R cos30
That means the angles marked have to be 30° and therefore that the angle subtended by the part of the circle outside the triangle is 60°.
And that means the total angle outside the triangle is 180° i.e. half the circle is outside and half is inside.
So half of the charge on the cylinder is inside the triangular prism.
Ah
@JohnRennie Why are we taking n=6 for 4th ring?
08:15
Hint, 6 = 10 - 4 :-)
Consider the central bright spot. The path difference for this spot is 10Ξ»
Yes?
Yes
Such setup is not much in JEE syllabus.
Why are there only 10 dark rings?
So for the first ring away from the central spot the path difference is 9Ξ»
Then the second ring has a path difference of 8Ξ»
And so on
Hi @JohnRennie
The last bright ring would have a path difference of zero, but that can only happen if the light is emitted parallel to the screen, so the 10th bright ring appears at infinity on the screen.
OK so far?
08:22
And each dark ring precedes the bright ring, so the 10th dark ring appears in between the 9th and 10th bright rings.
That's why there can only be 10 dark rings on the screen.
@cOnnectOrTR12 Hi :-)
And 0=0.5, 0/0=5, 0=0.10 , 0/0=10 which means is 0/0=5 or 10. So p=qn doesn’t mean p/q=n . Correct?
Yes. Basically 0 is weird :-)
Is there a mistake or something is not clear?
p=qn does mean p/q=n if and only if we exclude q = 0
I worry my proof is not exactly correct.
08:34
What you have proved is that the value of 0/0 is indeterminate.
From a mathematical point of view we say division works only for non-zero numbers.
So what’s the proof?
@JohnRennie o yes.
OK :-)
What’s the proof?
Proof of what? You have correctly proved 0/0 is indeterminate.
You mean if 0=0.5 then 0/0= indeterminate not 5.
08:42
You have done what we call a proof by contradiction.
To do this we start by assuming 0/0 is determinate. If this is true then 0/0 must have a single value.
But you have shown that 0/0 does not have a single value because you have shown it could be both 5 and 10.
And that means our initial assuming is wrong i.e. 0/0 is not determinate.
Yeah so if 0=0.5 then 0/0 not equal to 5.
@JohnRennie Hi !
Hi :-)
Gauss law is applicable only for symmetric closed surface ?
08:50
It applies to any closed surface, but we usually try to choose a closed surface that is symmetrical as it makes the calculations easier.
So we choose symmetric closed surface so that we can simply make use of the formula
$q$/$\epsilon_0$
Yes. Regardless of the shape of the surface the total flux though it is always equal to q/Ξ΅β‚€
Yes
Because we always consider a spherical Gaussian surface over the closed surface
@KavinIshwaran No, we actually pick cylindrical more often than spherical.
08:59
@KavinIshwaran We choose a surface with the same symmetry as the system of charges we are looking at.
@Wolgwang I will get different answer if I use $d(1-\frac{y^2}{2D^2}=6\lambda$
When to use what?
@JohnRennie Yes
If we are looking at a point charge we use a sphere with the point its centre, or if we are looking at a line charge we use a cylinder with the axis along the line.
@JohnRennie Thank you for the clarifications :-)
You're welcome :-)

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