Video Transcript
Each of the following diagrams shows some iron filings in the presence of a magnetic field. In which case do the iron filings reveal the magnetic field produced by a single, current-carrying wire going into or out of the screen?
Here, we see five candidate diagrams. We know that these diagrams show us the positions of iron filings. These are very small cylinders made of iron that align with external magnetic fields. The filings can’t tell us the exact direction of a magnetic field but, based on their position and orientation, can show us the overall shape of magnetic field lines. In this example, the magnetic field mapped out by the iron filings is created by current in a wire that either points into or out of the screen. The magnetic field created by a straight current-carrying wire appears as concentric rings around that wire.
Depending on whether the current in the wire points into the screen or out of the screen, we’ll determine the exact direction of these field lines around the wire. But as we mentioned, iron filings don’t indicate exact magnetic field direction. They simply align along the overall field line shape. We would expect then that the pattern of iron filings for a single current-carrying wire carrying current into or out of the screen would look roughly like a set of concentric rings. It’s worth noting that an iron filing doesn’t need to lie along any one of these rings as we’ve drawn them. Rather, for a filing that was any given distance away from the current-carrying wire, that filing would simply align in the direction of these rings.
Knowing this, let’s consider our answer options, starting with option (A). In this option, we see a few examples of what looks like sets of concentric rings of filings. There seems to be a center of such rings located right about here, here, and here. Since there’s more than one center, we know that this corresponds not to one single current-carrying wire. Perhaps three such wires would create the magnetic field shown by these iron filings. We won’t choose answer option (A) for our final answer then.
Answer choice (B) shows us a field that does not appear in concentric rings. In fact, answer choice (B) looks like the magnetic field created by a solenoid. A cross section of the loops of such a solenoid that look like this would create such a magnetic field. This then is not the field created by a single current-carrying wire pointing into or out of the screen. Looking at answer option (C), we see that here the filings don’t seem to be arranged according to any pattern. It appears as though they’re not lined up with any external magnetic field. This is not the way that filings would look in the presence of a field created by a single current-carrying wire.
In answer choice (D), like in answer choice (A), we do see concentric ring patterns, but there’s more than one of them. Answer choice (D) shows us four such patterns, which might be produced by a set of four current-carrying wires. Since in our scenario we just have one wire, we know that this will not represent the field created by that wire. This finally brings us to answer choice (E). And here we indeed see what looks to be a field created by a single current-carrying wire. The iron filings here generally align in concentric rings around a central point. That point could represent where our current-carrying wire is located, carrying current into or out of the screen.
Answer option (E) then is our final answer. This shows us a diagram of iron filings that indicate a magnetic field formed by a current-carrying wire pointing into or out of the screen.
