Faraday’s law is a tool for calculating the strength of an induced current, but one important piece of the puzzle is still missing. What causes the current? That is, what force pushes the charges around the loop against the resistive forces of the metal? The agents that exert forces on charges are electric fields and magnetic fields. Magnetic forces are responsible for motional emfs, but magnetic forces cannot explain the current induced in a stationary loop by a changing magnetic field.

FIGURE 30.30a shows a conducting loop in an increasing magnetic field. According to Lenz’s law, there is an induced current in the ccw direction. Something has to act on the charge carriers to make them move, so we infer that there must be an electric field tangent to the loop at all points. This electric field is caused by the changing magnetic field and is called an induced electric field. The induced electric field is the mechanism that creates a current inside a stationary loop when there’s a changing magnetic field.

FIGURE 30.30 An induced electric field creates a current in the loop.

The conducting loop isn’t necessary. The space in which the magnetic field is changing is filled with the pinwheel pattern of induced electric fields shown in Figure 30.30b. Charges will move if a conducting path is present, but the induced electric field is there as a direct consequence of the changing magnetic field.

But this is a rather peculiar electric field. All the electric fields we have examined until now have been created by charges. Electric field vectors pointed away from positive charges and toward negative charges. An electric field created by charges is called a Coulomb electric field. The induced electric field of FIGURE 30.30b is caused not by charges but by a changing magnetic field. It is called a non-Coulomb electric field.

So it appears that there are two different ways to create an electric field:

1. A Coulomb electric field is created by positive and negative charges.

2. A non-Coulomb electric field is created by a changing magnetic field.

Both exert a force $\vec{F}=q\vec{E}$ on a charge, and both create a current in a conductor. However, the origins of the fields are very different. FIGURE 30.31 is a quick summary of the two ways to create an electric field.

FIGURE 30.31 Two ways to create an electric field.

We first introduced the idea of a field as a way of thinking about how two charges exert long-range forces on each other through the emptiness of space. The field may have seemed like a useful pictorial representation of charge interactions, but we had little evidence that fields are real, that they actually exist. Now we do. The electric field has shown up in a completely different context, independent of charges, as the explanation of the very real existence of induced currents.

The electric field is not just a pictorial representation; it is real.