Medicine and metal spheres
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Medicine and metal spheres
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[post_date] => 2025-01-09 07:57:19
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[post_content] => Practice Passage (Question 1-5)
*This passage is the property of Khan Academy and has been reformatted into an AAMC-style interface in their entirety by MedLife Mastery. MedLife Mastery does not endorse and is not an affiliate of Khan Academy.
Electrostatics have an important role in the field of medicine. Anodes, cathodes, and electric potential sources are heavily relied upon in X-RAY machines, heart-rate monitors, electrostatic therapy, protein modeling, and a plethora of devices used in hospitals and clinics. The fundamental aspects of electrostatics are often applied to a variety of physician practices, and they are currently being explored by a biomedical engineering research team interested in developing a new device involved in pain management.
In a physics lab, this team takes two metal spheres A and B of identical mass and hang them from insulating strings of length L. The researchers can then electrically charge the spheres independently. After one such trial, it is seen that the two spheres attract each other so that they hang at rest with their centers a distance D apart and at an angle of θ from the vertical.
Figure 1. A diagram of the laboratory apparatus the team is using.
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[question] => Which of the following does NOT necessarily have to be true based solely on the description and observations made above?
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[answer] => 2
[description] => Reason for the Correct Answer:
Newton’s Third Law is always true.
Opposite charges definitely attract.
The spheres do not require the same charge, since the force will always be equal and opposite no matter what the charge is.
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[answers] => Array
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[0] => Array
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[each_answer] => A. Sphere A has the opposite sign of charge the Sphere B does since the spheres attract
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[1] => Array
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[each_answer] => B. Sphere A has the same magnitude of charge as Sphere B since the spheres hang at the same angle
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[2] => Array
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[each_answer] => C. Sphere A exerts the same magnitude of force on Sphere B as Sphere B does on Sphere A
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[each_answer] => D. Sphere A exerts a force on Sphere B in the opposite direction as the force that Sphere B exerts on Sphere A
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[1] => Array
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[quiz_unique_key] => 3873426850
[question] => What can be said about the electric field at the midway point along a line joining the two centers of the spheres?
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[answer] => 2
[description] => Reason for the Correct Answer:
Positive charges create electric fields that point away from them, negative charges create electric fields that point towards them.
The two spheres here must have opposite charges since they attract.
Since the two spheres have opposite charges they will both create an electric field that goes in the same direction at any point in between them.
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[each_answer] => A. The electric field must be zero
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[each_answer] => B. The electric field must not be zero
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[each_answer] => C. The electric field points up
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[each_answer] => D. The electric field points down
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[2] => Array
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[quiz_unique_key] => 83407773
[question] => Consider the electric potential at the midway point along a line joining the two centers of the spheres. Each sphere contributes to the total electric potential by virtue of the equation V=kq/r. What can be said for sure about the contribution to the electric potential at the midway point due to the two spheres?
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[answer] => 3
[description] => Reason for the Correct Answer:
The two charges do not have to have the same charge.
If the two charges attract they must have opposite signs.
The two spheres must have opposite signs but are not necessarily the same magnitude of charge.
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[0] => Array
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[each_answer] => A. The contribution of both spheres to the electric potential must cancel and be equal to zero since they are equally distance from the midway point
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[1] => Array
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[each_answer] => B. The contribution of both spheres to the electric potential must be equal but not necessarily different sign
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[each_answer] => C. The contribution of both spheres to the electric potential must be of opposite sign, but not necessarily equal
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[each_answer] => D. The contribution of both spheres to the electric potential must be of opposite signs and equal magnitude
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[quiz_unique_key] => 2261298308
[question] => Which of the following would decrease the angle θ?
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[answer] => 3
[description] => Reason for the Correct Answer:
Decreasing the mass of A would cause A to have less weight and rise higher, increasing the angle.
Increasing the charge on either sphere would cause the force to increase which would cause the spheres to rise, increasing the angle.
Increasing the mass of sphere A would cause the sphere to be heavier and therefore to hang at a smaller angle.
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[answers] => Array
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[each_answer] => A. Decrease the mass of the Sphere A
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[each_answer] => B. Increase the total amount charge on Sphere B
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[each_answer] => C. Increase the mass of Sphere A
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[each_answer] => D. Increase the total amount of charge on Sphere A
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[question] => If a large uncharged metal rod were touched to Sphere B and then removed without touching Sphere A, what would be observed to happen?
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[answer] => 1
[description] => Reason for the Correct Answer:
Touching a small charged piece of metal with a large piece of uncharged metal will uncharge the smaller piece of metal.
If sphere B has no charge the electric force is diminished and it will fall down.
If Sphere B has no charge then the electric force on A is also diminished and it will also fall down.
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[each_answer] => A. Both spheres would fall down
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[each_answer] => B. Only sphere B would fall down
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[each_answer] => C. Only sphere A would fall down
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[each_answer] => D. The spheres would move up, get closer together and hang at a larger angle
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