Hertz' experiment: AC or DC power source? (2 Viewers)

[emilios]

All the sources tell me that Hertz used a DC power source in his experiment to detect radio waves, which was then stepped up to high voltages using an induction coil. This is so the electrons can jump across the air gap from the primary coil to the receiving coil.

However, when they go on to talk about how Hertz proved these waves travelled at the speed of light they all say the same thing:

c= wavelength * frequency where the wavelength is known from measuring the interference pattern on a metal sheet, and (here's the thing), that the frequency is known from the oscillation of the current.

... but it's a DC power source? There shouldn't be any oscillation?

 

[tomnomnom696]

its an induction coil so it switches the voltage on and off very rapdilt to induce emf. when this occurs it alternates the spark. as voltage rises the spark goes one way according to lenzs law and then when emf decreases there is an opposite change in flux so the spark jumps the other way

 

[mreditor16]

All the sources tell me that Hertz used a DC power source in his experiment to detect radio waves, which was then stepped up to high voltages using an induction coil. This is so the electrons can jump across the air gap from the primary coil to the receiving coil.

However, when they go on to talk about how Hertz proved these waves travelled at the speed of light they all say the same thing:

c= wavelength * frequency where the wavelength is known from measuring the interference pattern on a metal sheet, and (here's the thing), that the frequency is known from the oscillation of the current.

... but it's a DC power source? There shouldn't be any oscillation?

yeh that's something that has annoyed me as well.

also emilios and others, I have an associated Q - how did hertz know that the oscillation of the current was the frequency of the radio waves? because different waves on the EM spectrum would have been emitted - thus, of different frequencies. so how did he know that the frequency of the source was the frequency of the radio waves he was focusing on??? :O

 

[tomnomnom696]

if my above explanation doesnt make sense, graph the flux of a rapidly changing on and off DC current (it will have successive spikes) __/\___/\___/\ etc. if you then graph the derivative youll notice it changes positive negative periodically. this is the induced emf which constitutes the rapidly changing direction of the spark across the spark gap (pos to neg rapidly changing)

 

[emilios]

if my above explanation doesnt make sense, graph the flux of a rapidly changing on and off DC current (it will have successive spikes) __/\___/\___/\ etc. if you then graph the derivative youll notice it changes positive negative periodically. this is the induced emf which constitutes the rapidly changing direction of the spark across the spark gap (pos to neg rapidly changing)

ooh ok so switching the induction coil off actually causes the current to reverse direction? that's a bit counter-intuitive but the graphs make sense

 

[tomnomnom696]

yeah cos the change in flux is actually in opposition (first increasing flux then decreasing flux)

 

[mreditor16]

yeh that's something that has annoyed me as well.

also emilios and others, I have an associated Q - how did hertz know that the oscillation of the current was the frequency of the radio waves? because different waves on the EM spectrum would have been emitted - thus, of different frequencies. so how did he know that the frequency of the source was the frequency of the radio waves he was focusing on??? :O

anyone??

 

[emilios]

anyone??

hmmm i'm not too sure. my guess would be this: since charged particles emit emr when accelerating, a stream of electrons that periodically reverses direction emits emr every time it changes direction. this means the EMR is as 'frequent' as the current, hence the same frequency?

that was probs a terrible explanation

 

[QZP]

anyone??

It's just the way the EMR is generated - by oscillating charges. Frequency of oscillation would then dictate the frequency of the EMR

Edit: A more technical explanation would be something like EMR consists of oscillating electric and magnetic fields. In the Hertz experiment, the oscillating charge is what produces the oscillating electric field (here you can see how the oscillating frequency dictates EMR frequency) which induces a perpendicular oscillating magnetic field around it, thus generating EMR.

 

[harrypotterfan]

Idk guys Jacaranda says "high frequency AC supply"

 

[tomnomnom696]

Idk guys Jacaranda says "high frequency AC supply"

no it doesnt

 

[harrypotterfan]

no it doesnt

lol nws I just double checked, the old and new versions say different things gg

 

[xc1408]

All the sources tell me that Hertz used a DC power source in his experiment to detect radio waves, which was then stepped up to high voltages using an induction coil. This is so the electrons can jump across the air gap from the primary coil to the receiving coil.

However, when they go on to talk about how Hertz proved these waves travelled at the speed of light they all say the same thing:

c= wavelength * frequency where the wavelength is known from measuring the interference pattern on a metal sheet, and (here's the thing), that the frequency is known from the oscillation of the current.

... but it's a DC power source? There shouldn't be any oscillation?

DC does oscillate (recall motors and generators voltage graphs) - it's the "true DC" that doesn't oscillate

 

[invalidvalidity]

hmm but in Andrew Harvey's notes it says Hertz used high voltage AC current in the sparking wire? Don't know if the information is highly reliable tho. Would've thought AC current would facilitate the production of the propagated waves

 

[mirachael]

Hertz fed a low voltage DC power source into an induction coil to produce a high voltage, high frequency AC which he then fed into the transmitter.

 

[invalidvalidity]

Hertz fed a low voltage DC power source into an induction coil to produce a high voltage, high frequency AC which he then fed into the transmitter.

Ah okay that makes more sense. Thanks!

 

[anomalousdecay]

DC does oscillate (recall motors and generators voltage graphs) - it's the "true DC" that doesn't oscillate

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

_________________________________

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:

Hertz fed a low voltage DC power source into an induction coil to produce a high voltage, high frequency AC which he then fed into the transmitter.

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.

 

[Fizzy_Cyst]

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.

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

 

[anomalousdecay]

Well yeah to be honest going off what Fizzy_Cyst said, that is all I really did know back in HSC. All this other stuff I didn't understand until I figured it out after learning some uni content or after being briefly shown in lectures. Once you have the right tool kit, you begin to understand what those circuit diagrams represent.