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Deriving electromagnetic fieldfor point charge

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nos
#1
Feb21-13, 10:31 AM
P: 32
Hi all,

I was going through the derivation for the electromagnetic field of point charges by Griffith(Introduction to electrodynamics page 437).

I'm missing minus sign somewhere:

The book says that:[itex]\nabla(\vec{n}\cdot\vec{v})=\vec{a}(\vec{n}\cdot \nabla tr)+\vec{v}-\vec{v}(\vec{v}\cdot\nabla tr)-\vec{n}\times(\vec{a}\times \nabla tr)+\vec{v}\times (\vec{v}\times \nabla tr)[/itex]

Using the rule for triple cross products gives:

[itex]\vec{v}+(v^{2}-\vec{n}\cdot\vec{a})\nabla tr[/itex]

However is should be:

[itex]\vec{v}+(-v^{2}+\vec{n}\cdot\vec{a})\nabla tr[/itex]


I'm sure Im missing something.

Thanks
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nos
#2
Feb21-13, 11:00 AM
P: 32
oh I've got it. Forgot the minus sign in the triple cross product rule :$


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