# Electromagnets and waves

## Main Question or Discussion Point

if you create an electromagnet and move it up and down, wouldn't that be creating electromagnetic waves? if it is, wouldn't it be possible to create x-ray, light, and radio waves by moving it at the correct speed?

thinking this over i get the feeling this is not true. with this new feeling in mind, how do you get an electromagnetic field to pulse/create waves?

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Integral
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Gold Member
You are correct, by moving a magnet, either an electro magnet or a ferro magnet, you create Electromagnetic waves. The amplitude, frequency and, wavelength are determined by the motion of the magnet. The frequency will be determined by the number of oscillations per second of the magnet. To generate a 1MHz radio wave the magnet would have to oscillate 1 million times in a second. Do you see a problem here? I do not believe that any physical magnet (much bigger then an atom) that could with stand the accelerations necessary to vibrate at even 1Mhz. The frequency of Xrays is several orders of magnitude Higher!

Your computer uses a crystal do create its operating frequency, essentially atoms oscillating.

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how do radio stations and microwaves gain the velocity to generate such waves?

Moe
They dont.
They use a coil and a capacitor in what is called a resonant circuit. The inductivity of the coil and the capacity of the capacitor determine the resonance frequency. It can be adjusted by modifying either of those two values.

Integral
Staff Emeritus
Gold Member
Electromagnetic waves are also generated by moving electric fields. Radio stations generate rapidly changing currents (or electron motion) to generate a changing electric field which is the source of electromagnetic waves. So radio waves are generated by moving electrons.

I believe that Hertz generated the first radio waves with a mechanical vibrator and a spark gap.

i don't understand. how does the coil and capacitor work? how do they get the electrons to move?
i have not taken physics (in case you will now have you modify your explanations).

I haven't much of a background in electromagnetic theory yet, but as an engineer i can tell you that a rapidly changing voltage on a wire (longer than 1/2 the wavelength) will produce a radio wave. That is why AM radio towers are so large compared to something like a cell phone antenna.

Are you familiar with inductors and capcitors? Well, if you put them in parrallel you'll get a tank circuit. Basically there will be one frequency which be stable, and that's a resonant frequency. I can't remember off the top of my head, but i believe that the solution to that is a 2nd - order differential equation with complex numbers.

What i do remember off the top of my head is that

i = c * dv/dt
and
v = 1/L * di/dt

those are equations for current accross capacitor and voltage across inductor respectively.

sorry. im not familiar with inductors or capacitors. with the producing waves, your saying you just raise and lower the amount/power of electricity rapidly and that creates the waves? how do they fluctuate it so quickly?

(on this subject you kind of have to speak to me as if im dumb. if thats too much work you can just post in your terms and ill do my best or you can post a website or something)

chroot
Staff Emeritus
Gold Member
If you apply a changing voltage to a piece of wire, the electrons in the wire will move back and forth. If you move them back and forth fast enough -- by making the voltage change fast enough -- you will be radiating radio waves.

- Warren

Okay in plain english:

A capacitor is simply two metal plates with a substance in between.
A capacitor will charge up and as it charges up less and less current will flow.

An inductor resists current changes so you can change voltage really rapidly and it will maintain a relatively steady current.

So basically, inductors and capacitors have different coefficients which determine how good they are at doing their thing. So basically there is 1 frequency (with a certain bandwidth around it) that will be an equilibrium point.

I can't really explain more than that without showing you an oscilloscope or math or whatever.

you need different inductors and capacitors for different frequencies? how is the voltage changed quickly enough to form radio waves?

you don't need to use "dumb" phrases. you just pretty much have to explain every detail (at least enough to make sense).

chroot
Staff Emeritus
Gold Member
relativelyslow:

Forget entirely about capacitors and inductors. They are not required to make radio waves, and they are not the components that are actually used to produce radio waves. While they do release some unwanted radiation, they are not used as antennas.

All you need to know is that if you apply a rapidly oscillating electric field to a wire, the electrons will rapidly oscillate back and forth. As they oscillate, they radiate away energy in the form of EM radiation.

- Warren

ok. how is the electric field made to oscillate rapidly?

chroot
Staff Emeritus
Gold Member
There are many ways to do it. The easiest is to exploit resonances in reactive components like capacitors and inductors. You can also exploit the natural vibrational modes of a small crystal of quartz. You can use an over-stable amplifier. There are literally hundreds of ways to make oscillators. The actual mechanism by which the field is made to oscillate is not relevant to the production of radio waves, however.

- Warren

so in the case of capacitors and inductors, the current running between them has a frequency. by applying this current to a wire, it produces radio waves?

chroot
Staff Emeritus
Gold Member
First, imagine a swing set: a swing is essentially a pendulum. Every pendulum has a natural frequency defined by the length of its rope or chain. When you push on a child in a swing with a frequency close to the natural frequency, the child keeps getting pushed higher and higher -- until your arms can't push any harder. If you try pushing at any other frequency, you'll find that the child won't go very high (and you'll probably get yelled at).

Essentially, a capacitor stores energy in the form of an electric field. An inductor stores energy in the form of a magnetic field. When you connect the two together, energy can slosh back and forth between them, much like a pendulum swinging. The amount of capacitance and inductance determines how fast this sloshing takes place -- just like how the length of a pendulum's rope determines how fast it naturally wants to oscillate. By themselves, a capacitor and inductor do not produce any energy -- they only serve to produce a resonance so that frequencies other than their natural resonant frequency are attenuated. This is the basis of any oscillator: make one frequency "preferred" while others are attenuated. If the system is very sensitive, it will begin oscillating on its own, reinforcing itself.

- Warren

so the capacitor and inductor, slushing back and forth in their natural frequency, do not emit radio waves. only when they are applied to a wire so the electrons fluctuate are radio waves emitted. correct?

Basically ya.

awesome. physics is so cool. i love this stuff. thank you for seeing this out to the end. i appreciate it.

by aquiring capacitors and inductors could you create your own waves? or are these things in reality (not my mind) mammoth in size? i imagine little hand held things.

chroot
Staff Emeritus
Gold Member
Capacitors and inductors range in size from smaller than a grain of rice, up to objects as big as a computer monitor. Rather than spending your money on some components that you won't know how to use, I suggest that you spend your money on a good book on electronics. One of my favorites is "The Art of Electronics" by Horowitz and Hill. You might be able to find some good hobby books on radio projects at Radio Shack, of all places.

- Warren

Well the rule of thumb is the bigger you want your C value to be, the bigger the cap has to be. The faster the frequency the smaller the cap. Typically you'll want a tuning capacitor though. Inductors can be all kinds of shapes and sizes.

I agree with Chroot, in order to build a radio frequency generator or even a crystal radio, you'll need lots more theory. If you can get your hands on a dc-power supply, a function generator, and multi-meter, and an oscilloscope i really suggest playing around with those. Theory only gets you so much understanding.

what could i do with radio waves? whats the C value? i think i will have to get some stuff to play around with. this stuff sounds cool

Moe
Well, with radio waves you can basically do a lot, the problem is that it requires more than the simple things described in this thread to broadcast a song, for instance. I don't see a whole lot you can do besides maybe creating and detecting them, although even that has a lot of applications (e.g. a self-made remote control for something).

There are two approaches you can take:

-the lego approach
-the engineering approach

When i say lego, i really mean to say assemble and tweak through experimentation and intuition.

If you want the lego approach look for some info on crystal radios and try building an AM crystal radio first. There's lots and lots of documentations.

If you want the engineering approach get a textbook and start from square 1. Read about resistive networks, delta-Y, thevinin + norton, and then get onto AC circuits and phasors. If you take this approach you will first need to learn calculus and linear algebra, and depending on how you mix circuits, possibly 1st and 2nd order differential equations. Typically engineering students begin to learn about microelectronics and semiconductor physics here.

So the engineering approach isn't easy and you'll have to be patient, but in the end you will be able to design all kinds of things.

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