# Electric Dipole in a Magnetic Field: Conservation of energy

1. May 4, 2012

### sparkle123

I don't understand how you get from the first line to the second line in (2). (I'm not too familiar with working with cross products and vectors).

Also, I don't know how you get (5) from (1) and (2).

Any help would be much appreciated. Thanks! :)

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2. May 4, 2012

### dikmikkel

(2) the equality Comes from $\vec{v}_{cm} = \dfrac{1}{2}(\vec{v}_1+\vec{v}_2)$
(5) Try to wite down the full expression $E = \dfrac{1}{2}mv_{cm}^2+\dfrac{1}{2}I\omega^2$
Using (1) and (2)
And take the time derivitative(here dE/dt is indicated with a small dot above). Then you would see that the energy is conserved.

3. May 6, 2012

### sparkle123

Thanks dikmikkel! :)