Part of the show BSE, Cervical Cancer and Toxoplasmosis
Arie in Northern Ireland asked:
I was listening to my radio one night and I noticed that when I turned on my desk light there was a little blip of noise on the radio. I repeated this and I noticed that when I was on the border of completing the circuit, such that when the bulb was flickering, I got severe interference on my radio. My radio was running on batteries. Why does this happen?
When your light switch is just on the border of making a circuit, you are creating a switch which is almost closed. At some point the switch will create a spark. When you create a spark you will create a little surge of current in the wire. The spark will then finish and break the circuit again. You will keep getting sparks, each creating it's own surge of current. Your light bulb is flickering because you have a changing current, and a changing current will produce a changing magnetic field. A changing magnetic field produces a changing electric current. So when we create a radio wave, what you've got is a transmitter which is applying a changing electric field to a piece of metal (the aerial). The changing current going up and down that piece of metal then induces a changing magnetic field around the metal and the changing magnetic field then creates a change in electrical current in the fabric of space around it. That electrical current in space time produce a wiggle of magnetic field, and so you get this wave that propagates as an alternating magnetic field and electrical field. This propagates through space at roughly the speed of light. That's a radio wave. So why did your interfere with your radio? Well when you're just on the verge of completing a circuit with your light, it's creating enormous amounts of changing current in the wire. This creates lots of funny frequencies of radio waves at a fairly low power. These then come out around the wire, spread out into the room and are picked up by the aerial of your radio. They interfere destructively. In other words they cancel out the radio waves of the station you were listening to. That's why you get the interference.