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Question Video: Representing the Magnetic Field in a Moving-Coil Galvanometer Physics

The diagram shows a cross section of a moving-coil galvanometer. Which of the following diagrams most correctly represents the magnetic field around the galvanometer core? [A] Diagram A [B] Diagram B [C] Diagram C [D] Diagram D [E] Diagram E

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Video Transcript

The diagram shows a cross section of a moving-coil galvanometer. Which of the following diagrams most correctly represents the magnetic field around the galvanometer core?

Before we begin, it’s important to remember that the core is made of iron and is represented in the diagram by this circular part in the center. These big sections to the left and right of the core are the north and south poles of a permanent magnet. And for simplicity, the entire magnet isn’t shown in the diagrams, just the two polar pieces. Now, our job here is to recognize the general trends of the magnetic field produced by this setup. So we should recall that a magnetic field always points from the north pole to the south pole.

Just knowing this, we can see that something is wrong with answer options (D) and (E). They represent the magnetic field with closed concentric circles. So the lines have no start and no end, which means they don’t point from north to south. So we should eliminate these two choices.

Now let’s look at answer choice (A). These field lines do indeed point north to south. But because they’re uniform and parallel, they would better represent the system if there was no iron core between the north and south poles. The iron has certain magnetic properties that alter the field produced by the magnet. And by design, this makes the magnetic field point approximately radially. Therefore, we can eliminate (A) and check out the remaining choices (B) and (C), which both do show the magnetic field pointing radially about the iron core.

These two options look pretty similar, but notice that all of the lines in option (C) point inward toward the core. This goes against what we established earlier that the field points away from the north pole and toward the south pole. Option (C) shows the field pointing away from the north pole and also away from the south pole, so it’s incorrect. Now, option (B) agrees with both of the points written over here. The field points from north to south and is shaped radially because of the iron core. Therefore, answer choice (B) best represents the magnetic field around the galvanometer core.

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