Hertz measuring the speed of radio waves. (1 Viewer)

[theodore0307]

How did Hertz measure the speed of radio waves.

Thanks

 

[anomalousdecay]

Check out this thread:

http://community.boredofstudies.org/18/physics/329559/hertz-experiment-ac-dc-power-source.html

I outlined in the post below a brief answer of how.

However for Hertz' experiment:

Yeah, I do it with the whole standing waves stuff (assuming I have had the class in Year 11 -- as I teach standing waves in World Communicates), but even the concept of standing waves is beyond the scope of what HSC wants us to say -- keep in mind it is merely an outline question.

Key things needing to be known:
- Simple diagram of set up (including parabolic (or other) reflectors)
- Frequency of spark in transmitter was known due to characteristics of the circuit (dont need to EXPLAIN how, just outline that it was known)
- Frequency of spark is receiver was the same (he actually used multiple receivers, each 'tuned' to different frequencies)
- Wavelength of EMR was determined by changing position of reflectors and analyzing interference patterns (this is all you need to say -- merely sketching in general terms)
- speed calculated using universal wave equation ~ 3x108ms-1

In the post below I have an outline of how everything is calculated.

DC does not oscillate at all. The results are due to effects of induction in motors, hence why you get sinusoidal shapes.

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Ok so Hertz did use a DC source. He uses the induction coil as an inductor which produces an emf. Inductors by nature will attempt to avoid any change in current through them. Capacitors conversely prevent a change in the voltage going through them as they induce electric fields along their plates.

Now in the induction coil used for Hertz' experiment, we have an interrupter which breaks this DC input. Also, a capacitor is used. With this LCR (inductor, capacitor, resistor) circuit, we obtain a sinusoidal output. In the end, this causes oscillations which can be calculated. Some more information about the circuit used:

http://en.wikipedia.org/wiki/Induction_coil

Note that none of this is in the HSC syllabus.

In terms as to how we know the frequency as asked by a few people here:

Well the circuit is made so that the inductor and capacitor used provide an alternating current with constant amplitude. You can calculate values for which this occurs. In the end, the frequency can be calculated by knowing the resistor, capacitor and inductor values. The wavelength can be found by reflecting the waves in such a manner to create a standing wave, which then represents a node for half a wavelength. As a result, the speed of light was calculated.



So really, by choosing a suitable frequency to measure half a wavelength by creating standing waves, Hertz was not too far off the actual speed of light.

___________________________

Ok now 90% of what I said above is out of the scope of the syllabus. Long story short for HSC:



Other thing I would add is that he knew the wavelength due to the standing waves he set up and finding the interference pattern and also he knew the frequency by knowing the frequency of the AC produced from the circuit. In HSC I think they don't specify that he knew this due to the components deliberately picked in his circuit.

/thread

Disclaimer: Fizzy_Cyst may know or not if there is a HSC SEZ case to this. Really I don't know if there is one or not. I just know what you really do if you want to do Hertz experiment from scratch in your own lab using basic components.

 

[Quantum College]

To measure the speed of any electromagnetic radiation/wave we need two things:

- The frequency (f)
- The wavelength (lambda)

This is shown in the equation v=f(lambda).

From his experimental set-up Hertz was able to find the frequency of the apparatus.

To find the wavelength he placed his radio wave transmitter away from a mirror. Then he began moving it back. The times that the spark gap didnt spark was when the radio waves destructively interfered. These are called standing waves. From this he PHYSICALLY MEASURED the distance between these standing waves using a ruler and got a measurement in metres. This would be the wavelength.

From these two numbers he multiplied it and found out that in fact the waves traveled at the speed of light.