3. One of the following is not a source of magnetostatic fields:
   A dc current in a wire
   An electric field linearly changing with time
   A permanent magnet
   A charged disk rotating at uniform speed

Did you know that magnetostatic fields arise from steady currents or magnetic materials? A dc current in a wire and a permanent magnet both produce such fields, while a charged disk rotating at a uniform speed can indeed generate magnetostatic fields due to its movement. The real party crasher, however, is an electric field that changes linearly with time, which actually produces an electromagnetic field rather than a magnetostatic field!

Now, let’s talk about the quirky world of electromagnetism! While magnetostatic fields are all about static or steady influences, when those pesky electric fields change, they bring about their dynamic buddy, the electromagnetic wave. This interplay is what powers everything from your radio to mobile phones, making them hum along with all kinds of stuff you wouldn’t believe! So next time you switch on a device, you’re surfing the waves of electromagnetic fields!