Electric Dipole

[ilovemcat]

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Which answer would you consider to be correct? The answer said A (which I really don't understand how and what the heck it even means so I can't reason against it). But I thought the answer would be C. Increasing "y" by a factor of 2 results in a (2^3) or 8 Fold decrease in E-field (according to the equation they gave). Could anyone explain to me if this is an error or am I misunderstanding something here. Thanks.

 

[Rabolisk]

I think it's A. C has the quantifier "exact" which makes it untrue at distances close to the dipole. D is false, because only the vertical components cancel out. The horizontal components add together.

 

[SaintJude]

Umm, I hope I'm allowed to resurrect this post? Anyway, this electric dipole question is good & I've got a question about it.

Re: A.) Isn't A always true? As in the electric field is the sum of all charges contributing to it.

Re: B.) Is this choice false b/c the net field actually has a greater magnitude closer to the dipole?

D.) Is it true that he net field referred can be calculated by only considering the horizontal components of the electric field (since the vertical components cancel each other out)

Last, the dipole moment created by these two points will have rotational & translational motion, right?

 

[MedPR]

D should be true, since the question specifically asks about the y axis. So the net field on the y axis is less than the sum...

 

[milski]

Umm, I hope I'm allowed to resurrect this post? Anyway, this electric dipole question is good & I've got a question about it.

Re: A.) Isn't A always true? As in the electric field is the sum of all charges contributing to it.

Yes, that's correct.

Re: B.) Is this choice false b/c the net field actually has a greater magnitude closer to the dipole?

Correct again.

D.) Is it true that he net field referred can be calculated by only considering the horizontal components of the electric field (since the vertical components cancel each other out)

Yes, you could do that. That does not make D true - it is the sum of the two fields. Just because some parts cancel each other does not meant that it's less than that sum.

Last, the dipole moment created by these two points will have rotational & translational motion, right?

Not sure what you mean here - as long as the charges are not moving the dipole moment will stay the same. What motion are you thinking of?

 

[milski]

D should be true, since the question specifically asks about the y axis. So the net field on the y axis is less than the sum...

The net field at any point is the sum of all fields. Being on the y-axis does not change that.

 

[SaintJude]

Thank you milski! About rotational & translation motion.

Kaplan related these two motions to the dipole moment: In a constant field (such as one in a capacitor) , a dipole moment will have no translation motions, since the forces due to electric fields will effectively cancel each other out. There will be rotational motion of the dipole. In the picture below, the dipole will rotate clockwise.

So in the original thread problem, I noticed that the electric field was continually changing. And so I would think the dipole moment b/w the two charges would have rotational motion--as in the dipole would continually realign itself to the net electric field direction.

And the dipole would also experience a net direction (leading to translation motion) , b/c the forces due to the electric field form the different charges are not consistently equal and opposite.

 

[milski]

Your post is about the electrical field created by the dipole itself. The Kaplan illustration that you posted is about the same dipole in a uniform electrical field. These are two separate problems. The dipole charges on their own will have exactly one motion - translational towards each other. If you put then in a uniform field, you will have them act as described in the example above.

 

[SaintJude]

Yeah, I know that they are 2 conditions.

In the first scenario: uniform electric field
In second: non-uniform (why is why it should have rotational

 

[milski]

Yeah, I know that they are 2 conditions.

In the first scenario: uniform electric field
In second: non-uniform (why is why it should have rotational

You have only charges creating a field in the second case. What do you expect to have a rotational motion?