Problem Solving Strategies

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Kevin N

4:44 AM

@JohnRennie Hi :)

John Rennie

@KevinN hi :-)

Kevin N

When we say potential difference between A and B, what does it actually mean?

I'm kinda confused

I mean the electric potential between A and B keep changing

John Rennie

Potential difference is just a work done per unit charge.

The graph shows a field strength as a function of distance, so we can write this as $E(x)$. OK so far?

Kevin N

So when we say between A and B, what does it really indicate?

The workdone in moving charge B to A?

or vice versa?

@JohnRennie ok

John Rennie

And the force on a charge $q$ in an electric field $E$ is $F = qE$.

shelton Benjamin

4:49 AM

Sir please check this physics.stackexchange.com/q/570977/267970

Kevin N

@JohnRennie Yes

John Rennie

So if we move a small distance $dx$ the work done is $dW = qE(x)dx$. Yes?

@sheltonBenjamin I'll have a look at that in a bit ...

Kevin N

@JohnRennie yes :)

John Rennie

And the total work done if we move from the point $x=A$ to $x=B$ is calculated by integrating this:

$$ W_{ab} = \int_a^b q E(x) dx $$

And if we set the charge $q$ to unity we get the work per unit charge, which is just the potential.

$$ V_{ab} = \int_a^b E(x)dx $$

Kevin N

@JohnRennie What I'm a bit puzzled about is, let's say, the electric potential at a certain point is the work done needed to bring that test charge to that point from infinity. When we say the electric potential between A and B, which point is it actually referring to? Is it the work done needed to move a test charge from A to B?

John Rennie

4:54 AM

Potential doesn't have an absolute value. The only thing we can ever measure is a potential difference.

Kevin N

So potential difference between A and B is basically the work done needed per unit charge to move it from point A to B?

John Rennie

It is often convenient to choose the potential to be zero at infinity. The potential at some point A is the difference between the potential at A and the potential at infinity, but choosing the potential to be zero at infinity makes this difference just equal to $V_a$.

@KevinN yes. It is that simple.

Kevin N

@JohnRennie Ah alright thanks :)

@JohnRennie When we say the potential is zero at infinity, is it really zero or approaching zero?

John Rennie

"Infinity" is a risky thing to mess around with. What we would actually say is that the limiting value of the potential as the distance increases is zero.

Typically we'd get $V = -kq/r$ which is never actually zero but tends to zero as $r \to \infty$.

Kevin N

Ah I see :)

shelton Benjamin

5:00 AM

Sir i want to learn infinite potential well for excercise in ie irodov chapter atomic model do i have to learn quantum mechanics also to learn the concept?

John Rennie

@sheltonBenjamin the maths behind the infinite potential well is easy so you can certainly learn how to do it without needing to fully understand quantum mechanics.

I would say go for it.

shelton Benjamin

Ok sir can you now check this physics.stackexchange.com/q/570977/267970

John Rennie

5:59 AM

@sheltonBenjamin this is my FBD:

4 hours later…

Guru Vishnu

9:37 AM

@satan29: Hi! It seems I was bit too late to see this message of yours. I was online but I didn't hear your ping as I had muted the speakers and was on a different tab. Now coming to your question, I don't have anything valuable than this message by John Rennie sir.

On looking at the main site, the following question showed up:

Q: Why does an increase in pressure raise melting point?

I've heard that if you increase pressure in a system that has ice in it, then its melting point will increase. However, I would've thought that it would decrease the melting point as the ice would heat up more easily due to the air molecules colliding more frequently with the surface of the ice, ...

Might be useful. I thought I would find some other reasons there. But they also describe this based on the anomalous expansion of water on freezing.

By the way, there seems to be a similar outcome for an increase in mechanical pressure. See: Regelation

You might find this video by Veritasium to be interesting too. This is with reference to the second kind of pressure.

1 hour later…

RishiNandha_M

10:48 AM

Chemistry told me Anamolous expansion is because water gets into Hydrogen bonding ands gets arrange in a very spacious lattice (Each water molecule is tetrahedrally attached to other)

Not sure if Im right in this part, most probably wrong, but In Clausius Clapeyron Relation delta S/ delta V must be +ve though delta V is negative since final entropy is lesser. Ice's possible microstates for the given state variables is much lesser than Water because of the spacius lattice arrangement by H bonds