Describing energy process

[SaintJude]

Noticed that MCAT test makers love this Q : Which of the following best describes the energy conversion process that occurs during [X,Y, Z ] ?

So here's a q via EK scenario:
Imagine a pendulum made from a conducting material swinging in and out of a magnetic field pointing into the page. Current eddies develop in the pendulum due to its electrons swinging through the magnetic field. Which of the following best describes the energy conversion process that occurs as the pendulum swings in and out of the field?

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a. heat to kinetic energy
b. mechanical energy into kinetic energy
c. mechanical energy into heat
d. mechanical energy to gravitational energy to internal energy

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[MedPR]

Noticed that MCAT test makers love this Q : Which of the following best describes the energy conversion process that occurs during [X,Y, Z ] ?

So here's a q via EK scenario:
Imagine a pendulum made from a conducting material swinging in and out of a magnetic field pointing into the page. Current eddies develop in the pendulum due to its electrons swinging through the magnetic field. Which of the following best describes the energy conversion process that occurs as the pendulum swings in and out of the field?

View attachment 18717

a. heat to kinetic energy
b. mechanical energy into kinetic energy
c. mechanical energy into heat
d. mechanical energy to gravitational energy to internal energy

Since mechanical energy is PE + KE, I'm going to go with C. You start at some height with PE, then as the pendulum swings it loses PE and gains KE, so mechanical energy is conserved. Gravitational energy is just potential energy, and kinetic energy is kinetic energy. I don't see how the relationship between PE and KE can change without heat being involved.

???

 

[SaintJude]

Yeah, it's C.

 

[MedPR]

Yeah, it's C.

Can someone shed some light on this?

I guessed C only because I thought of a regular, non electricity related, pendulum in which PE + KE are conserved and constantly being converted into one another. No energy is gained or lost assuming no resistance or any of that complicating stuff.

What makes A, B, and D wrong?

 

[milski]

Something that you need to consider before answering the question is that the eddy currents are going to end up generating heat. They are virtually random so you're not going to get the energy that created them back as some sort of KE when the pendulum is swinging through the magnetic field.

a - there is no heat being transformed into motion here, so this is not true
b - that's the sum of the kinetic and potential energy. For about half of the motion of the pendulum that is not true - the potential energy increases and the kinetic decreases.
d - the internal energy is the sum of the potential and the kinetic energy of the particles of the object. That might have been a good candidate if they said that the conversion was from kinetic to internal or from mechanical to internal energy. Gravitational energy which is just a case of potential energy is not turned directly into internal energy which makes this the wrong answer.

 

[MedPR]

Something that you need to consider before answering the question is that the eddy currents are going to end up generating heat. They are virtually random so you're not going to get the energy that created them back as some sort of KE when the pendulum is swinging through the magnetic field.

a - there is no heat being transformed into motion here, so this is not true
b - that's the sum of the kinetic and potential energy. For about half of the motion of the pendulum that is not true - the potential energy increases and the kinetic decreases.
d - the internal energy is the sum of the potential and the kinetic energy of the particles of the object. That might have been a good candidate if they said that the conversion was from kinetic to internal or from mechanical to internal energy. Gravitational energy which is just a case of potential energy is not turned directly into internal energy which makes this the wrong answer.

Is there ever a case where "mechanical energy to kinetic energy" or "mechanical energy to potential energy" would be correct?

 

[milski]

Is there ever a case where "mechanical energy to kinetic energy" or "mechanical energy to potential energy" would be correct?

That would depend to some extend on who interprets the text. If you are converting kinetic to potential energy (for example, in a fall), you could argue that more of your mechanical energy becomes kinetic. One is a subset of the other, so it's really more of a philosophy question than physics.

 

[MedPR]

That would depend to some extend on who interprets the text. If you are converting kinetic to potential energy (for example, in a fall), you could argue that more of your mechanical energy becomes kinetic. One is a subset of the other, so it's really more of a philosophy question than physics.

Ok that's part of the reason why I immediately threw out those answers. Since kinetic and potential are both part of mechanical, saying mechanical is converted to kinetic or potential seems ambiguous.

 

[milski]

Ok that's part of the reason why I immediately threw out those answers. Since kinetic and potential are both part of mechanical, saying mechanical is converted to kinetic or potential seems ambiguous.

I'm with you there, especially when there is a better worded answer which has no other flaws.