14.8. So What Gets an EM Wave Going?#
I’m glad you asked. Imagine a positive charge just sitting still in front of you minding its own business. Think about the field that results from that stationary (relative to you) charge: it’s just our now familiar E field pointing radially away.
Now let’s imagine that the charge is moving by you at a constant speed. What are the fields now? Well, you’d still see the E field, but now that moving charge is a current and so there’s be the familiar circular B field around it as it moves.
Now: apply a force to that charge so that it accelerates. Its changing speed–the definition of acceleration after all–causes a kind of kink in the fields. That kink propagates outwards and is in fact, an EM wave.
How to make an electromagnetic wave:
An accelerated electrical charge creates an Electromagnetic wave.
What’s a broadcast antenna? Well, it’s nothing more than a wire in which electrons flow, driven by a source of energy that causes those electrons to accelerate up and down. Those accelerating electrons produce EM waves that propagate outwards: that’s your favorite radio station.
Meanwhile somewhere in your car or your home, you’ve got another wire which is sitting there with its electrons idling their motors waiting to be tickled. When that EM wave from the radio broadcast antenna pass by, the electric fields in them apply forces to those electrons in your receiving antenna and the electronics in your radio transform those oscillations into Lady Gaga’s voice.
14.8.1. Energy#
It takes energy to apply the forces to the electrons in the broadcast antenna and energy is clearly applied to the electrons in your radio’s receiving antenna. So obviously, the EM wave that’s in-between carries energy from one place to another. Faraday anticipated this and Maxwell modeled it in his elastic “molecules” but today we know that EM waves contain and deliver energy.
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