0 - 2020-03-17

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Knight

07:24

@JohnRennie You there?

John Rennie

07:35

@Knight hi :-)

Knight

08:00

@JohnRennie Are you free to help me?

John Rennie

@Knight I need to wok for about 15 minutes

Knight

@JohnRennie No problem sir!

Just ping me when you get free

John Rennie

08:14

@Knight hi.

Knight

08:41

@JohnRennie Hello

John Rennie

@Knight hi.

Are you still interested in the stuff from Griffiths about the field ina coil?

Knight

@JohnRennie Yeah

Who else other than you can solve my doubts :-)

John Rennie

I did a diagram to show what Griffiths means, or at least what I think he means. Give me a moment and I'll upload it.

Knight

Okay

John Rennie

Consider the simplest coil possible i.e. just a single loop.

Suppose there is some radial component of the field shown by the red arrow.

Knight

08:46

Okay

John Rennie

What Griffiths is saying is that if we reverse the current the field should reverse as shown at the top right.

Knight

Yes

John Rennie

But, if we just take the coil and rotate it 180° about the dashed line in the bottom right we shouldn't reverse the field.

But the two diagrams on the right have the current flowing in the same direction but have opposite fields, and this creates a contradiction.

So the conclusion has to be that there is no radial component of the field.

Knight

“We shouldn’t reverse the field” why? Isn’t turning gonna change the field?

John Rennie

If you look where I've drawn the dashed line in the bottom right diagram, then rotating about this axis cannot change the field because the field lies on the axis.

Knight

08:50

Sir I don’t why but I’m unable to convince myself, if we rotate the loop by 180 degrees we do so by flipping the axis (dashed line) so why wouldn’t the direction of red arrow gonna change?

John Rennie

Because a rotation about an axis does not change any points on that axis. It only changes points away from the axis.

Knight

Can you please explain me “rotation about the axis does not change any point on the axis” through some other example so that I too find it obvious (like you). I apologise if I’m too demanding

If I draw an arrow on a paper and then rotate it through the arrow by 180 degrees

Yeah it’s not changing :-)

John Rennie

@Knight :-)

Knight

@JohnRennie You know I was thinking that we had to rotate that arrow by 180 degrees

But I was wrong, it was the whole system that we had to rotate, and by this simple experiment by drawing an arrow on a paper and rotating it has done the job

Knight

09:13

@JohnRennie Something for you sir to amuse yourself. South African President is trying to say “in the beginning”