Impact of a suddenly reversed electric field

[Ascleoius]

Hello all,
Good evening!
Can anyone please help me clear my confusion about the following question from Kaplan:
A dipole is placed in an electric field and is allowed to come to equilibrium. How would the dipole react if the direction of the electric field is suddenly reversed?
A. It rotates to align with the new field.
B. It accelerates linearly along the field lines.
C. it experiences no rotational or linear movement.
D. It both rotates to align with the new field lines.
Answer: C. The dipole would experience no torque in the new electric field because the electric field and the dipole have an angle of 180 degree, and sine of 180 degree is 0.

I understand that the dipole won't experience any rotation because of zero torque, but i don't agree that the dipole won't experience any linear acceleration when the electric field is suddenly reversed.
If the direction of dipole at equilibrium in the original electric field points as the picture shown below (right one). Once the electric field is reversed, the dipole should accelerates towards the left. Therefore, I think answer B should be correct.

Can anyone help me with this question please?

Thank you so much in advance.

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[5words]

Is there a picture?

 

[Asteri]

Hey Ascleoius,

For this problem you have to use the equation F = qE
Since this system is a dipole, the negative charge and positive charge are equal here. Thus, there is a net zero charge on the overall dipole system. Thus, as net q = 0, net force F = qE = 0, so net acceleration (A = F/M) of the dipole system is also zero.

Another way to think about this is breaking the dipole down into parts. If you break the dipole down into parts, then the positive charge will be pulled in the direction of the electric field with the force of F = qE, while the negative charge will be pulled opposite of the direction of the electric field with the exact same force, making the net force equal zero. Thus, linear acceleration is zero in the system.

The short answer is in a system with a dipole and a uniform electric field, you can never get linear acceleration, only rotation. Since this system also is at 180 degrees, there will also be no torque (and so no rotation) as you noted. I hope this helps!

Edit: clarity

 

[aldol16]

If you have translational motion of a dipole with electric fields, then I think all matter would be a lot more unstable because dipoles are always instantaneously generated and ungenerated (basis of London dispersion forces).

 

[5words]

Hey Ascleoius,

For this problem you have to use the equation F = qE
Since this system is a dipole, the negative charge and positive charge are equal here. Thus, there is a net zero charge. Thus, as q = 0, F = qE = 0, so acceleration (A = F/M) is also zero.

Another way to think about this is breaking the dipole down into parts. If you break the dipole down into parts, then the positive charge will be pulled in the direction of the electric field with the force of F = qE, while the negative charge will be pulled opposite of the direction of the electric field with the exact same force, making the net force equal zero. Thus, linear acceleration is zero in the system.

The short answer is in a system with a dipole and a uniform electric field, you can never get linear acceleration, only rotation. Since this system also is at 180 degrees, there will also be no torque (and so no rotation) as you noted. I hope this helps!

So no accretion in a dipole? Nvm checked it..even if you rotate the field the charges will always move in opposite direction. . BTW you might want to change your first three line because it makes it seems as if q=0 when what's really 0 here is the net force as you pointed below... thanks for the explabation

 

[Asteri]

Just wanted to say. Your explanation doesn't hold.. even if the charges are equal F= product of the two charges/r squared. So there is no reason why q1 × q2 should be equal to 0. And object in electrostatic undergo Uniform circular motion, meaning that they have a centripetal acceleration. . So they ought to be moving .. wouldn't you agree?

1. This case is specifically a dipole, so the two charges are connected and cannot accelerate towards each other. F = q1*q2/r^2 deals with charges interacting, not charges in an electric field like in this problem. To determine how charges behave in an electric field, you need to use F = qE instead of F = q1*q2/r^2

2. Charges in a magnetic field undergo uniform circular motion. In an electric field, charges experience force either in the direction of the field (+ charges) or against the direction of the field (- charges) based on their charge

 

[5words]

1. This case is specifically a dipole, so the two charges are connected and cannot accelerate towards each other. F = q1*q2/r^2 deals with charges interacting, not charges in an electric field like in this problem. To determine how charges behave in an electric field, you need to use F = qE instead of F = q1*q2/r^2

2. Charges in a magnetic field undergo uniform circular motion. In an electric field, charges experience force either in the direction of the field (+ charges) or against the direction of the field (- charges) based on their charge

I ended up undertanding number 1... but thanks for number 2, the last thing I want it's to make a mistake on the mcat. I guess I thought the hyperbolic motion of a charge going through a capacitor with a uniform E was UCM...thanks for the clarification.

 

[Ascleoius]

Hey Ascleoius,

For this problem you have to use the equation F = qE
Since this system is a dipole, the negative charge and positive charge are equal here. Thus, there is a net zero charge on the overall dipole system. Thus, as net q = 0, net force F = qE = 0, so net acceleration (A = F/M) of the dipole system is also zero.

Another way to think about this is breaking the dipole down into parts. If you break the dipole down into parts, then the positive charge will be pulled in the direction of the electric field with the force of F = qE, while the negative charge will be pulled opposite of the direction of the electric field with the exact same force, making the net force equal zero. Thus, linear acceleration is zero in the system.

The short answer is in a system with a dipole and a uniform electric field, you can never get linear acceleration, only rotation. Since this system also is at 180 degrees, there will also be no torque (and so no rotation) as you noted. I hope this helps!

Edit: clarity

Thank you so much Asteri. I drew the diagram and understood how F=qE would act differently on positive and negative charges, and how those two forces would cancel out, resulting in no acceleration in the dipole even after the electric field is flipped. Your explanation really helped me and thanks again for your generous help.


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[howtomedicine]

Care to post that diagram?


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[Ascleoius]

Care to post that diagram?


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Yes of course. sorry. it's weird that i can see the picture on my screen.

the picture showed a dipole experienced torques in an uniform electric field to reach equilibrium. let me know if you can see it okay.

 

[howtomedicine]

Looks good, thanks!


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