A electric dipole is placed at the origin with the two opposite charges located

Question

A electric dipole is placed at the origin with the two opposite charges located on the x - axis. An electron is fired from far away along the x -axis towards the negative side of the dipole with a speed of 6.0×10^6 m/s and comes to a stop 2 cm from the center of the dipole. (You can assume the distance from the electron to the center of the dipole is much larger than the dipole separation.)

Using the concept of forces and fields, explain why the electron comes to a stop. Include a diagram showing the forces involved as part of your explanation.

Using the concept of energy, explain why the electron comes to a stop. Include an energy bar chart as part of your explanation. (In your bar chart, treat the interaction between the electron and each individual charge of the dipole as a separate interaction.)

What is the magnitude of the dipole moment?

Explanation / Answer

The negative side of the dipole is closer to the electron. Hence, the repulsive force is greater than the attractive force. Thus, it will eventually stop.

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Note that the potential energy increases as the electron goes closer to the dipole. Thus, by conservation of energy, the KE must decrease in return. Therefore, it will eventually stop.

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Note that the dipole moment p is

p = q d

where q = charge of one of the charges, and d = their distance of separation.

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