potential energy is zero when electric dipole is making 90 degrees angle...


Joined Jul 3, 2008

Could you please help me with the query included in the attachment? For higher resolution copy of the attachment, please use this link: http://img824.imageshack.us/img824/5431/dipole5.jpg

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Thank you for the help.

Your question relates to the simple fact that energy scales can always be shifted by a constant value. It is energy differences that have physical meaning. Notice how the text book very carefully explains where the potential energy is most positive and where it is most negative. Then they point out that you could shift the energy, but this definition seems most convenient.

We see the same thing with voltage (which is just potential energy per unit charge) since the reference voltage can be any point in the circuit, or even an external ground, or voltage at infinity. We normally call ground zero voltage for convenience, but ground could be called -1156 V and as long as all voltage differences are maintained, there is no physical difference.

Think back to potential energy in gravity at the surface of the earth, here U=gh where h is the height above the ground. At h=0, there is still potential energy because we can dig a whole and let the mass fall further and do work. Unless there is a force to stop it, systems generally move towards lower potential energy, when possible. Hence, a ball rolls down the hill and the dipole rotates all the way to zero angle.