So far, we have largely been talking about what happens when the magnetic field is constant. We have talked about the sources of magnetic fields (including charges moving with constant speed, constant current, and permanent magnets ) and the magnetic force that acts on charged particles moving in the magnetic field. But what happens when you have a changing magnetic field? The following video demonstrates what happens when you move a permanent magnet towards a coil of wire. The coil is connected to a galvanometer, which is a device that measures small currents (on the order of $\mu A$), but it is not connected to a battery.

In the video, you should have seen the needle deflect on the galvanometer when the magnet was brought near the coil and when it was brought far away from the coil. This is a super exciting result! This gives us a means to create an electric current without relying on battery or chemical means to create a potential difference. But this observation begs the question: where does this electric current come from?

Earlier, we found that current (the charges in the wires that are moving) was the result of a constant electric field in the wire. This electric field produced an electric force on the charges that pushed the electrons along. This electric field was created by the gradient of the surface charges along the wire from one end of the battery to the other. Is this electric field we observe when we move a magnet a similar situation?

Add Figures 12.1-12.4

We could hypothesize that perhaps the magnet would push the surface charges to make a gradient just like before. The surface charges could then make an electric field that would drive the current in the wire loop. In that case, to get a constant current, there would have to be a constant charge gradient around the loop, which is all well and good until we return to the top of the loop. At the top of the loop, there would be a large change in the surface charges, which would create an electric field that points in the opposite direction! This is very problematic and would not result in a constant current. On top of that, the drastic change from negatives to positives would imply that there's a large change in voltage at that spot. We were ok with this before because there was a battery at that location to justify that change in voltage, but now there's only a wire. It doesn't make sense for there to be a drastic change in voltage in only one spot in the wire. This tells us that it is NOT surface charges that are creating the electric field in this case. Something else must be creating an electric field that is driving the current in the loop.

So far, the only other source of electric fields that we know about is charges, but we didn't charge the loop ahead of time to create this electric field only moved a magnet towards a loop of wire. Even if we had charged the loop (either positively or negatively), the electric field from those excess charges would point towards or away from the loop - not around the loop. This tells us that we are actually dealing with a new source of electric fields! Namely that a changing magnetic field will create a curly electric field. It is this curly electric field in the loop of wire that creates the current that we observe in the loop.

Notationally, because this is a curly electric field, which is very different than the other kinds of electric fields that we have talked about, we will write a curly electric field from a changing magnetic field as $\vec{E}_{nc}$ because it is this as the “non-coulombic” electric field, whereas we will keep $\vec{E}$ to be for an electric field from static charges.

Mathematically, we represent this relationship using a vector operation called curl; however, for this class, we will generally simplify the curl operation (using Stokes' Theorem) to be: $$-\int \vec{E}_{nc} \bullet d\vec{l} = \frac{d\Phi_{B}}{dt}$$ Where this equation (called Faraday's Law) says that the electric field around a loop is equal to the change in the magnetic flux through that loop. The next few pages of notes will go through each part of this equation in detail (including where the negative sign came from) and how to use it, but it's important to remember that the relationship behind this equation is that a changing magnetic field will create a curly electric field.