1. Apr 19, 2006

### linux kid

Hi everybody, it's nice to finally join the forum.

I'm a little...:uhh:.... Is electric current (the kind we use at home) a form of radio waves occilating at very low frequencies? What are the all the simmilarities and differences between electric current and radio waves?

2. Apr 19, 2006

### mimic

if you mean is the domestic mains a signwave, yes its a 50hz wave

RF is also a wave but it starts way higher up the frequency chart,the Radio frequency of a material(ie:a aerial) is all about its resonant frequency thus it tends to be at very least into the KHZ range.

Frequency is the amout of occilations a second, as with light diferent wave lengths are sperated into spectrums ie,IR,VISIBLE spectrum,UV an so on, radio is just the same as you increase frequency you enter diferent spectrums, rf,microwave,x-ray and so on, anything below rf frequency is just a wave, anything above shifts to the next spectrum up

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3. Apr 19, 2006

### linux kid

Ok, what about DC? What frequeny is the DC current?

4. Apr 19, 2006

### mimic

well that all depends, direct current from batterys is a constant 0hz, but if your taking rectified dc then you can have whats called Square or Pulse waves,which may vary in frequency.

the difference is, in dc the wave is just basicaly on/off in AC it actualy shifts in potential between -/+ volt

Last edited: Apr 19, 2006
5. Apr 19, 2006

### linux kid

Thanks to all of you for your help.

6. Apr 19, 2006

### pcxmac

think difference in potential

electricity can be measure in Volts or potential
RF signals can be measured in dB or decibels. which is a logarithmic scale, used to represent the strength (potential) of a signal.

RF is measured in dBmW or dBW (decibel Watts)

Watt = Current * Voltage

DC doesnt change potential like AC does, AC goes from 0(+)to 180(-) then back or from say +10V to -10V then back to +10V (there are many other ways to do this), AC changes or alternates its potential.

AC changes, DC doesnt.

RF frequencies are typically very high, because you require an antenna that is 1 or (1/2) or maybe (1/4) wave length to effeciently transmit or transduce a wire born signal in to space via {E & H fields}.

The wave length of 300 MHz is 1 meter, there for wave length =

(300*10^6) / Frequency .

The larger the frequency, the larger the antenna you need to properly propagate a signal.

One last thing potential, describes the difference between two sources.

say Ground = 1 V+ & Battery = 9 V+, the difference is 8V, which is what you would get if you measured the voltage in parallel, experienced by some arbitrary load (1 resister, but only 1 load item).

7. Apr 20, 2006

### Line

Electricity isn't wave it's a particle.

8. Apr 20, 2006

### vanesch

Staff Emeritus
First of all, the question here falls entirely within the scope of classical electrodynamics, so that's the framework in which to look upon this answer.

First of all, electric *current* is the MOTION of charges, mainly electrons in metallic wires.
Radiowaves are dynamical configurations of the electric and magnetic fields in free space (or in air) which tend to propagate through space at the speed of light. In fact, the dynamical configuration corresponds to a continuing alternation between electric fields and magnetic fields. In the case that the frequency of this alternation is below a certain limit, one calls this phenomenon a radiowave. If the alternation is faster, we give it different names, such as, microwaves, infrared radiation, visible light, UV light, X-rays, ... because the physical phenomena affected by it change in nature.

Now, what's the link between electric current and radiowaves ? Well, both are linked. An alternating electric current can generate a radiowave of the same frequency, and vice versa: a radiowave hitting a conductor can generate an alternating current in the conductor.
But they are not the same phenomena !

How do you make an electric current ? Simple: "pump" charges through a closed circuit of conductors. The "pump" can be a battery, a generator, you, with a small tool picking up electrons and pushing them into a wire (matter of speaking of course )...
How do you make an alternating current ? Well, force the current one way, then the other way, then again one way, ... with switches, or transistors, or any other technique.

With your conductor in which flows an alternating current, you can make radiowaves (of the same frequency). Simply put a line of conductor somewhere (straight, or in a loop...). It will generate some radiowaves, which will then propagate freely throughout the air and the vacuum, into space. Once they "leave" the wire, they propagate freely on their own.

When you slow down the frequency of an alternating current, you
generate less and less strong radiowaves with a given conductor in which your current flows. And when you stop the current alternation all together, and have a DC current, you do NOT generate a radiowave anymore. What you now generate is a (weak) magnetic field, "attached" to the wire, and which won't propagate. At least, when you turn your current SLOWLY on and off, because switching the current, itself, contains an AC part which WILL generate a radiowave.

cheers,
Patrick.

9. Apr 20, 2006

### pcxmac

look up transducer and antenna to see how a wire-line signal becomes radiated.

key items of interest are - E & H Fields, Transmission Line, Transducer, Antenna.

Another transducer of interest could be a microphone, taking the physical phenomenon of sound waves, and converting it in to an electrical potential, or current travelling along a wire / physical path.

http://www.st-andrews.ac.uk/~jcgl/Scots_Guide/audio/part6/page3.html

- interesting pics.

ps. Im not a big fan of particle physics, I think its an aproximation (IMO), but im not very knowledgeable, just my opinion.

10. Apr 20, 2006

### row

i ve a doubt,since electricity is dq/dt and and q=ne, and e are e~ with certain wave like properties(de broglie and stuff..)so over all wouldnt the statement be slightly baised towards particles??

11. Apr 21, 2006

### vanesch

Staff Emeritus
In the frame of the OP, this particle/wave discussion is misplaced, honestly. Electrical current is a flow of charges (and now it is up to you how you want to consider this: purely classically, it is the motion of charged mass points, say, electrons ; quantum mechanically, it is a quantity associated with the quantum state of the ensemble of electrons in the conductor, called, eh, the current density).

It seems to me that there's a big confusion here in this thread between "electrical current", "potential", "electical energy transport" and all that, but the OP mentioned *current* (though I think electrical energy transport was what was meant).

12. Jan 29, 2009

### Butterfly_grl

Many thanks for this amazing explaination.However here you have explained how radio waves are produced from a simple electric circuit , So i have a question. Can electricity be produced from radio waves? (the inverse)

Secondly, when you said "...slow down the frequency " I am not sure i can understand this part, How can you control frequency?

Thanks.

13. Jan 30, 2009

### Staff: Mentor

Yes. That's how a radio works. The radio waves produce small electric currents in an antenna, which the radio selects for a particular frequency, amplifies and uses to produce sound.

That depends on how you're producing the oscillating current. In a classic LC circuit using an inductor and capacitor, you change either the inductance L or the capacitance C.

14. May 4, 2010

### arjundahal

Yes,you got almost all the answers right from other friends.Radio waves are generated by electromagnetic oscillations,& have their wavelengths in the electromagnetic spectrum longer than infrared light.Electric current(mainly AC supply) has the sine wave oscillations,as the radio waves but it has lower frequency ranging from (50-60)Hz.Radio waves have extremely high frequency and can easily travel in vacuum.

15. May 4, 2010

### arjundahal

A dry cell provides power to electronics, and a capacitor stores charge in it,they both hold power in it.But how they differ in longitivity of power supply.When a capacitor's end is connected they totally discharge their power but not the dry cell.Please help me answer this,you'll be appreciated.

16. May 7, 2010

### ruko

Re: think difference in potential

If, in this case, "The larger the frequency" means higher frequency then the above statement is wrong. Increasing the frequency means a smaller or shorter antenna.