Problem Solving Strategies

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NOTE Book

05:22

@JohnRennie I am late but happy belated birthday😅❤️.

It should be 3 S=O right?

John Rennie

@NOTEBook Thanks :-)

NOTE Book

oh maybe it doesn't account for formal charges

John Rennie

It's an odd question because I have never heard of non-polar being part of the definition of a planar molecule.

NOTE Book

HCl is also a planar molecule but a polar molecule

John Rennie

But if the S-O and/or S=O bonds are even slightly polarised it's clear that (3) will have a non-zero dipole moment due to the bond polarisation.

@NOTEBook Yes

NOTE Book

05:27

@JohnRennie Yes

When we charge a cell does current flows from negative terminal of cell to positive terminal?

I am not able to understand how to get Terminal voltage= EMF+Ir

John Rennie

@NOTEBook Yes, when charging the cell the current flows in the opposite direction.

So when charging the conventional current flows from negative to positive.

NOTE Book

John Rennie

05:42

If we are charging with some current I then we go through the battery backwards. That means as we go through the battery we get a voltage drop of E instead of a voltage increase of E.

And we get a voltage drop of Ir due to the internal resistance.

So the total voltage drop is E + Ir.

Does this make sense so far?

NOTE Book

Yes

We use different cell to charge

John Rennie

Yes

NOTE Book

So terminal voltage is always potential difference across cell and its internal resistance whether they are in combination?

John Rennie

Yes. If we are using the cell as normal then as we go through the cell (from -ve to +ve i.e. in the direction of the current) we get a voltage rise of E and a voltage drop of Ir.

So now the total voltage change is E - Ir

NOTE Book

yes

John Rennie

05:48

We commonly think of the internal resistance and the cell in series.

It isn't really like that, but the series description works because voltages in series add.

NOTE Book

John Rennie

You'll see diagrams like this where everything inside the dashed line is the cell.

NOTE Book

yes

thank you

i understood it

John Rennie

OK :-)

NOTE Book

BTW what is your typing speed?

John Rennie

05:55

I've never measured it. It's nowhere near as fast a trained typist.

NOTE Book

I see

I am stuck around 35-40 words per minute.

John Rennie

i think you need proper training to get much faster than that. I would guess I'm around the same or maybe slightly faster.

Is that on a full size keyboard? Or a laptop?

NOTE Book

laptop

John Rennie

I find laptops harder to type on. I use a full size keyboard connected to my laptop.

NOTE Book

oh i see

After the exams are over i will work on excel and my typing speed

John Rennie

06:01

I have never found my limited typing speed to be a problem.

Usually the speed of my thinking is slower than the speed of my typing :-)

NOTE Book

hehe

anyways thank you

John Rennie

You're welcome :-)

1 hour later…

NOTE Book

07:09

@JohnRennie Hi

John Rennie

Hi :-)

NOTE Book

This is the graph for terminal voltage and I

Shouldn't it follow ohms law graph?

John Rennie

We know the terminal voltage of a battery is the EMF, E, minus the voltage drop due to the internal resistance. Yes?

NOTE Book

yes

John Rennie

The voltage drop does follows Ohm's law i.e. the drop ΔV = Ir

And the terminal voltage is:
V = E - Ir

OK so far?

NOTE Book

07:14

yes

John Rennie

So if we graph V against I we get a straight line with gradient -r and y intercept E.

Yes?

NOTE Book

hm yes

John Rennie

And that's what your graph is showing.

NOTE Book

It is for constant emf and we are changing external resistance to vary I?

John Rennie

Yes. The cell is connected to some external resistance like in the diagram I posted earlier.

NOTE Book

07:17

ohh ok

John Rennie

1 hour ago, by John Rennie

NOTE Book

yes

John Rennie

where R can be varied from ∞ to zero.

NOTE Book

Thank you

John Rennie

You're welcome :-)

NOTE Book

07:19

I was confusing different things

4 hours later…

TPL

11:35

Hey, Could anyone please help me with this ?

Q: Confusion regarding Angular Velocity of a Rigid Body about a point outside the the body { on the fixed axis }

My Understanding: If a rigid body rotates about a fixed axis with angular velocity $\omega$, every point in the body has velocity $\mathbf{v} = \omega \times \mathbf{r}$. and "All particles are moving with the same $\omega$ about any point on the rigid body or it's axis." Now suppose the followin...

Any clarification would be appreciated

5 hours later…

F. Zer

16:16

@JohnRennie Thank you sir. But shouldn't (2) match the octet rule ? S has three bonds with two electrons each = 6 electrons total.

John Rennie

17:08

@F.Zer The octet rule is more of a guideline than a firm rule. Molecules like sulphur trioxide have complex structures that aren't easily explained with the octet rule.

F. Zer

17:27

Thank you John :)

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