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.