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Physics32. Electromagnetic WavesWhat is an Electromagnetic Wave?

In an electromagnetic wave traveling west, The B field oscillates up and down vertically and has a frequency of 93 kilohertz and an rms strength of 7.15 times 10 to the minus 19 teslas. Assume that the wave travels in free space. Okay, and part A says, "What is the frequency of the electric field?" Okay so let's start with Part A. Okay, we have an electromagnetic wave and it says that we're gonna describe the B field. And the B field that we drew before was going in and out of the board so let's draw the whole thing again. Here was our E field going up, going down, going up and down. And the B field was coming in and out of the board. Okay, E going up. B, like that. So the first thing they tell us is the B field oscillates with a frequency of 93 kilohertz. So if the B field is oscillating at 93 kilohertz, what do you think the frequency of the electric field is? What do you think? >> (student speaking) 93. >> Got to be the same, right? I mean look at the picture. E is going up and down at exactly the same rate as the B is going. So this has to be 93 kilohertz as well. All right, let's try it. 93. Let's see if it'll take a kHz for our kilohertz. And in fact it does. And it says we are correct. All right, Part B says, "What is the rms strength of the electric field?" And what they gave us was the strength of the B field rms. And they told us it was 7.15 times 10 to the minus 9 tesla. And so now we're looking for E rms. Hmm. Well we're not really sure how E relates to B yet. But if we push forward a little bit in the discussion, there is a very nice and simple relationship between E and B. And let me pull it up here. And this is going to be a little bit ahead of ourselves but we'll get back to it in a second. What we can say is the following. The energy in the E field is in fact equal to the energy in the B field. And those things relate to each other by the speed of light. So what is the electric field? It's equal to c, the speed of light, times the B field. Okay? So, E rms is just equal to C times B rms. So if I take this value of B and I multiply it by C, 3 times 10 to the 8 meters per second, we're gonna get the right answer. Okay, and if somebody wants to try that in your calculator. Tell me what you get. Should be about, what's that? 3 times 7 is 21. And then we've got 3 of those 15s, which is the 0.45, and then we've got a 10 to the minus 1. So I'm gonna say this thing is 2.145. And it's the value of electric field which is, of course, volts per meter.