TBR Physics, Section 8, Review Questions, Passage 1, Question 6

[sillyjoe]

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I am very weak in this subject so before I even ask the question I will say that I am having trouble wrapping my head around Figure 2 and exactly what is going on. With that said, the question reads:

If the rod in Figure 2 is released with zero velocity, and if the rails on which the rod sits are frictionless, the rod will:

A. Not move when a current is running through the resistor.
B. Move to the left, to minimize the area of the circuit loop.
C. Move to the right, to maximize the magnetic flux through the circuit loop.
D. Remain stationary, to keep the magnetic flux through the circuit loop constant.

Choice D is the best answer. Let's start by considering A. Using the right-hand rule on a current running through the wire in Figure 2 shows you that the rod is pushed either to the right or to the left (depending upon the direction of the current). This invalidates choice A. Magnetic flux does not tend to change. That is why, as stated in the passage, the current generated in a wire loop will orient itself so that its magnetic field opposes the induced change in flux~a condition described by Lenz's law. This invalidates choices B and C, and favors choice D. The best answer is choice D.

Can someone please explain to me how they are using the right hand rule to show that the rod is pushed towards the left or right? Also, can you please explain this experimental set up? I just don't understand it at all.

Thanks!

 

[orangetea]

I would watch this into like 2 Min..

then at 5 min he explains this exact situation(with the right hand rule and the forces and everything)


🙂

 

[sillyjoe]

I would watch this into like 2 Min..

then at 5 min he explains this exact situation(with the right hand rule and the forces and everything)


🙂

This is great. It really clarified this problem. I guess some of my confusion comes from the application of Lenz's law. In what scenario do you apply Lenz's law where you use a magnetic field in opposition to the applied magnetic field?

 

[orangetea]

This is great. It really clarified this problem. I guess some of my confusion comes from the application of Lenz's law. In what scenario do you apply Lenz's law where you use a magnetic field in opposition to the applied magnetic field?

So we are using Lenz's law in this problem. One of the questions in the passage asks you what direction is the current going did you have time to answer that one?

If you notice the current is going in a direction to create it's own magnetic field which OPPOSES the external magnetic field.

Now ask yourself which direction is the rod's current pointing in? and also what is the direction of the magnetic field it creates?

 

[orangetea]

So pretty much when the question asks what is the Rod's magnetic field and current apply Lenz's law. Hope that made sense 🙂

 

[sillyjoe]

So pretty much when the question asks what is the Rod's magnetic field and current apply Lenz's law. Hope that made sense 🙂

Just went through it again. Makes complete sense thanks to you!