Got it ! So when the spheres are brought too close to each other, the charge distribution on the spheres changes right? Therefore the voltage method fails?

@PandaScientist Yes

@JohnRennie hello sir !

Hi :-)

Electric field lines go from +ve to -ve charge. Is there any condition that electric field lines go from higher potential to lower potential?

It's just a convention.

The lines show the direction of the force on a positive test charge.

Since two positive charges repel, the lines point away from positive charges and towards negative charges.

But you could use a negative test charge instead and you'd get the same lines but pointing in the opposite direction.

Okk . Sir I've a q: say you've an isolated system containing three conductors. Is it possible for all of them to have both + and - charge distribution on their surfaces? Assume no external field or charge. The conductors may have cavities.

Do you mean on their outer surfaces? Or are you including the surfaces of the cavities?

Surface of the cavities as well(if there are any)

I don't know. That seems a tough question.

The solution was based on the fact that the highest potential conductor cannot contain negative charge

Can you post a link to the solution?

Umm...it's in Hindi

I can translate it for you

Post it anyway

Watching now ...

The whole solution was based on the fact that. Field lines have to exist . i) they move from +ve to -ve charge (convention) ii) they move from higher to lower potential

@PandaScientist Ok, yes what he is saying makes sense.

If B is the highest potential then no field lines can start somewhere else and end on B. And if no field lines end on B then B cannot have any negative charge on its surface.

But don't negative charges have negative potential? Are we only considering magnitude

Meanwhile I watched the video again to see if I missed something. This was it^^

Remember that the potential at any point is the sum of the potentials from all three bodies.

So a negative charge on the surface of a body is not necessarily at a negative potential.

So what he did in the video was, change the potential of different conductors (for comparison purpose) by changing the charge densities on other conductors and the conductor itself right?

Suppose you take a conducting sphere and put it in an external field, then the sphere becomes polarised so it has positive charges at one side and negative charges at the other side. However all the charges on the surface must be at the same potential because the potential is the same everywhere in a conductor.

So even though we have both positive and negative charges they are all at the same potential.

This seems weird at first, but it's because we have to add the potential from the external field as well.

For example it means that a field line cannot start and end on the same conductor.

Ah! Got it!

OK :-)

@JohnRennie this should mean that the potential from field is greater at the negative end right?

Correct. And we can easily see this if we draw a quick diagram.

Shall I draw the diagram or is it obvious?

I guess it makes sense because say we've the +ve charge causing the external field. And -ve charge is nearer to it

So greater potential right?

@JohnRennie diagram pls !

OK, give me a few mins ..

@PandaScientist There

If you look at the two fields on the right, the external field decreases the potential in the downwards direction, but the dipole field of the sphere decreases the potential in the upwards direction.

When you combine the two they cancel and the potential everywhere on the surface of the sphere is constant.

My drawing of a dipole field is rubbish :-)

@JohnRennie seems like I got it

Just one doubt, in the rightmost diagram, you've drawn fields emerging within the sphere and going into the sphere

So if we isolate a sphere (having both + and - charges) it creates a dipole right? So in an isolated sphere, the potential is not same everywhere?

The field I've drawn on the right is the field we get from the polarised charges, but in the absence of an external field there wouldn't be any polarised charges.

So what I've drawn is not physically realistic - it's just supposed to show that the total fied can be written as the sum of the two separate fields.

Oh , so in the absence of any external field, the charges get distributes evenly so there's a net 0 charge right?(Without polarisation)

Correct

This single question cleared a lot of concepts

Thank you sir :-)

OK :-)

@JohnRennie still suprised how you did that. auto translated captions?

@PandaScientist The guy was speaking a mixture of English and Hindi. There was enough English that with the aid of the diagrams I could work out what he meant.