A Visual Demonstration of the Electric Field of a Coil Carrying a Time-Varying Current

The nature of the electromagnetic fields of a helically wound coil carrying a time-varying current is examined. It is shown how simple applications of Faraday´s and Ampere´s laws yield the dominant field components. It is pointed out that within the coil, both magnetic and electric fields are predominantly axial and uniform. The axial electric fi´eld is many times greater than the radially dependent circumferential electric field usually associated with the time-varying axial magnetic field within the coil. A method is described that uses heat-sensitive cholesteric liquid crystals and resistive paper to provide a visual display in color of the electric field of the coil.