Anderson Video - Electromagnetic Waves

Professor Anderson
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<font color="#ffffff">So, the idea is this</font> <font color="#ffffff">a changing E</font> <font color="#ffffff">generates a B.</font> <font color="#ffffff">And, a changing B</font> <font color="#ffffff">generates an E.</font> <font color="#ffffff">I change B; I can generate E.</font> <font color="#ffffff">But if I change E, I can generate B.</font> <font color="#ffffff">And maybe, these things can balance out just right</font> <font color="#ffffff">that you can generate electromagnetic waves.</font> <font color="#ffffff">And, that is indeed what happens,</font> <font color="#ffffff">okay? An electromagnetic wave looks like this:</font> <font color="#ffffff">I've got an E field</font> <font color="#ffffff">that is going up and down like so.</font> <font color="#ffffff">And, this means that it's pointing up in this region</font> <font color="#ffffff">and then it's pointing down in this region,</font> <font color="#ffffff">and so forth: up, go through zero, pointing down</font> <font color="#ffffff">and just keeps oscillating back and forth.</font> <font color="#ffffff">But,</font> <font color="#ffffff">that changing a field can generate a changing B field.</font> <font color="#ffffff">And, the B field is</font> <font color="#ffffff">orthogonal to that.</font> <font color="#ffffff">It's at a 90-degree angle to it.</font> <font color="#ffffff">So if E is up-and-down in the plane of the board,</font> <font color="#ffffff">then B is in fact going in and out of the board.</font> <font color="#ffffff">And this is a little hard to draw,</font> <font color="#ffffff">but the idea is B is pointing this way,</font> <font color="#ffffff">and then it's pointing into the board,</font> <font color="#ffffff">and then it's pointing out of the board,</font> <font color="#ffffff">and then into the board and so on.</font> <font color="#ffffff">Okay?</font> <font color="#ffffff">So, these things are doing something like this:</font> <font color="#ffffff">E field is going up and down.</font> <font color="#ffffff">B field is going in and out and so they're sort of doing this:</font> <font color="#ffffff">Okay?</font> <font color="#ffffff">It's like two fish swimming along one going up and down one going left and right.</font> <font color="#ffffff">As this happens this entire thing propagates to the right.</font> <font color="#ffffff">So,</font> <font color="#ffffff">what is weird about this?</font> <font color="#ffffff">What's weird about this is there's no charges anymore.</font> <font color="#ffffff">There's no wires of current anymore.</font> <font color="#ffffff">These things can just propagate on their own</font> <font color="#ffffff">because the changing E field, creates a B field changing B field creates a D field.</font> <font color="#ffffff">And, they just keep going like that</font> <font color="#ffffff">-- sloshing back and forth.</font> <font color="#ffffff">And, this is called an electromagnetic wave.</font> <font color="#ffffff">So, how fast did these things propagate?</font> <font color="#ffffff">We said that it's gonna propagate to the right.</font> <font color="#ffffff">It is called a transverse wave</font> <font color="#ffffff">because the E field and the B field</font> <font color="#ffffff">are perpendicular to the propagation direction.</font> <font color="#ffffff">Okay?</font> <font color="#ffffff">So even though the wave is moving this way, the E field is oscillating up and down.</font> <font color="#ffffff">The B field is oscillating in and out those are both perpendicular</font> <font color="#ffffff">to the propagation direction.</font> <font color="#ffffff">How fast are these things moving?</font> <font color="#ffffff">How fast does an electromagnetic wave propagate?</font> <font color="#ffffff">Well, I think we know the answer to that, right?</font> <font color="#ffffff">They propagate at the speed of light</font> <font color="#ffffff">3 times 10 to the 8th meters per second.</font> <font color="#ffffff">So,</font> <font color="#ffffff">This stuff, electromagnetic waves, came about from something called Maxwell's equations.</font> <font color="#ffffff">And, up until Maxwell,</font> <font color="#ffffff">we had independent quantities.</font> <font color="#ffffff">We had electricity.</font> <font color="#ffffff">And then, we had magnetism.</font> <font color="#ffffff">Electricity was governed by things like Coulomb's law.</font> <font color="#ffffff">Magnetism was governed by things called Ampère's law.</font> <font color="#ffffff">Faraday's law started to tie those two together, </font> <font color="#ffffff">but it wasn't until Maxwell came along and combined all of those things into one quantity,</font> <font color="#ffffff">that we ended up knowing that light itself is made up of electromagnetic waves</font> <font color="#ffffff">People knew a whole bunch about light.</font> <font color="#ffffff">People knew a whole bunch about electricity and magnetism.</font> <font color="#ffffff">But, what they didn't know was that light itself was electromagnetic waves. </font> <font color="#ffffff">And, Maxwell put all those equations together -- combined them into four neat equations.</font> <font color="#ffffff">And if you happen to continue in physics and take my</font> <font color="#ffffff">upper-division electricity and magnetism course, it's all about Maxwell's equations.</font> <font color="#ffffff">Start with those four equations, and spend an entire year trying to uncover all their interesting phenomena.</font> <font color="#ffffff">Okay, it's really very fascinating stuff.</font> <font color="#ffffff">All right.</font> <font color="#ffffff">If we have these changing electromagnetic waves that are flying around the universe</font> <font color="#ffffff">how do we detect them?</font> <font color="#ffffff">Well, let's think about this wave right here.</font>
<font color="#ffffff">So, the idea is this</font> <font color="#ffffff">a changing E</font> <font color="#ffffff">generates a B.</font> <font color="#ffffff">And, a changing B</font> <font color="#ffffff">generates an E.</font> <font color="#ffffff">I change B; I can generate E.</font> <font color="#ffffff">But if I change E, I can generate B.</font> <font color="#ffffff">And maybe, these things can balance out just right</font> <font color="#ffffff">that you can generate electromagnetic waves.</font> <font color="#ffffff">And, that is indeed what happens,</font> <font color="#ffffff">okay? An electromagnetic wave looks like this:</font> <font color="#ffffff">I've got an E field</font> <font color="#ffffff">that is going up and down like so.</font> <font color="#ffffff">And, this means that it's pointing up in this region</font> <font color="#ffffff">and then it's pointing down in this region,</font> <font color="#ffffff">and so forth: up, go through zero, pointing down</font> <font color="#ffffff">and just keeps oscillating back and forth.</font> <font color="#ffffff">But,</font> <font color="#ffffff">that changing a field can generate a changing B field.</font> <font color="#ffffff">And, the B field is</font> <font color="#ffffff">orthogonal to that.</font> <font color="#ffffff">It's at a 90-degree angle to it.</font> <font color="#ffffff">So if E is up-and-down in the plane of the board,</font> <font color="#ffffff">then B is in fact going in and out of the board.</font> <font color="#ffffff">And this is a little hard to draw,</font> <font color="#ffffff">but the idea is B is pointing this way,</font> <font color="#ffffff">and then it's pointing into the board,</font> <font color="#ffffff">and then it's pointing out of the board,</font> <font color="#ffffff">and then into the board and so on.</font> <font color="#ffffff">Okay?</font> <font color="#ffffff">So, these things are doing something like this:</font> <font color="#ffffff">E field is going up and down.</font> <font color="#ffffff">B field is going in and out and so they're sort of doing this:</font> <font color="#ffffff">Okay?</font> <font color="#ffffff">It's like two fish swimming along one going up and down one going left and right.</font> <font color="#ffffff">As this happens this entire thing propagates to the right.</font> <font color="#ffffff">So,</font> <font color="#ffffff">what is weird about this?</font> <font color="#ffffff">What's weird about this is there's no charges anymore.</font> <font color="#ffffff">There's no wires of current anymore.</font> <font color="#ffffff">These things can just propagate on their own</font> <font color="#ffffff">because the changing E field, creates a B field changing B field creates a D field.</font> <font color="#ffffff">And, they just keep going like that</font> <font color="#ffffff">-- sloshing back and forth.</font> <font color="#ffffff">And, this is called an electromagnetic wave.</font> <font color="#ffffff">So, how fast did these things propagate?</font> <font color="#ffffff">We said that it's gonna propagate to the right.</font> <font color="#ffffff">It is called a transverse wave</font> <font color="#ffffff">because the E field and the B field</font> <font color="#ffffff">are perpendicular to the propagation direction.</font> <font color="#ffffff">Okay?</font> <font color="#ffffff">So even though the wave is moving this way, the E field is oscillating up and down.</font> <font color="#ffffff">The B field is oscillating in and out those are both perpendicular</font> <font color="#ffffff">to the propagation direction.</font> <font color="#ffffff">How fast are these things moving?</font> <font color="#ffffff">How fast does an electromagnetic wave propagate?</font> <font color="#ffffff">Well, I think we know the answer to that, right?</font> <font color="#ffffff">They propagate at the speed of light</font> <font color="#ffffff">3 times 10 to the 8th meters per second.</font> <font color="#ffffff">So,</font> <font color="#ffffff">This stuff, electromagnetic waves, came about from something called Maxwell's equations.</font> <font color="#ffffff">And, up until Maxwell,</font> <font color="#ffffff">we had independent quantities.</font> <font color="#ffffff">We had electricity.</font> <font color="#ffffff">And then, we had magnetism.</font> <font color="#ffffff">Electricity was governed by things like Coulomb's law.</font> <font color="#ffffff">Magnetism was governed by things called Ampère's law.</font> <font color="#ffffff">Faraday's law started to tie those two together, </font> <font color="#ffffff">but it wasn't until Maxwell came along and combined all of those things into one quantity,</font> <font color="#ffffff">that we ended up knowing that light itself is made up of electromagnetic waves</font> <font color="#ffffff">People knew a whole bunch about light.</font> <font color="#ffffff">People knew a whole bunch about electricity and magnetism.</font> <font color="#ffffff">But, what they didn't know was that light itself was electromagnetic waves. </font> <font color="#ffffff">And, Maxwell put all those equations together -- combined them into four neat equations.</font> <font color="#ffffff">And if you happen to continue in physics and take my</font> <font color="#ffffff">upper-division electricity and magnetism course, it's all about Maxwell's equations.</font> <font color="#ffffff">Start with those four equations, and spend an entire year trying to uncover all their interesting phenomena.</font> <font color="#ffffff">Okay, it's really very fascinating stuff.</font> <font color="#ffffff">All right.</font> <font color="#ffffff">If we have these changing electromagnetic waves that are flying around the universe</font> <font color="#ffffff">how do we detect them?</font> <font color="#ffffff">Well, let's think about this wave right here.</font>