Hertz's Experiment (1 Viewer)

[nml10]

Describe an experiment Hertz carried out that enabled him to determine the speed of the waves he had produced.

The question is worth 3 marks. How would you approach the question? I am so lost..

 

[Fizzy_Cyst]

Describe an experiment Hertz carried out that enabled him to determine the speed of the waves he had produced.

The question is worth 3 marks. How would you approach the question? I am so lost..

• Hertz used a loop of wire with a tiny gap in it to detect the waves. The electromagnetic waves generated a voltage across the gap that caused a spark to jump across the gap in the detector loop.
• To find the speed of the waves, Hertz set up standing waves to measure the wavelength of the waves he generated and then used the wave equation (v = f * lambda) to find the speed. (Hertz used an induction (Rumford) coil connected to a spark gap to generate high frequency sparks that produced radio frequency electromagnetic waves and used a Lloyd’s mirror set-up to measure the wavelength of the waves and determined the frequency of the waves by calculating the resonance frequency of his circuit. He then used the wave equation to find the speed.)

 

[imZerroo]

Hey thanks for the answer Fizzy, really appreciate it but just a couple of questions more...

1. Why was there a spark on the detecting coil? I get that the EMR emmitted from the spark of induction coil hit the detecting coil, then due to P.E electrons started moving in detecting coil causing a spark? Have I answered my own question? :L

2. Do we really have to know how he found speed, wavelength, frequency etc.? Some textbooks say it, some don't and it sounds really hard to understand. :s

Thanks again for your help, really appreciate it.

 

[Fizzy_Cyst]

Hey thanks for the answer Fizzy, really appreciate it but just a couple of questions more...

1. Why was there a spark on the detecting coil? I get that the EMR emmitted from the spark of induction coil hit the detecting coil, then due to P.E electrons started moving in detecting coil causing a spark? Have I answered my own question? :L

Hertz created different transmitter/receiver combinations which were tuned to different frequencies. The spark jumping across the gap in the transmitter causes electrical current to oscillate back and forth across the gap and along the surface of the rods. The frequency of this oscillation depends on the measurements of the rods and this is the frequency of the emitted EM wave.
The EM wave emitted by the transmitter was detected by the receiver. The natural oscillating frequency of this detector loop must match the transmitted frequency of the EM wave produced in the transmitter, therefore when the EM wave hits the receiver, it will cause the electrons to oscillate and will produce a spark.
So, even when it was placed many metres away (away from the electrostatic influence of the induction coil) a spark was produced, indicating that an EM wave was being transmitted through the air.

2. Do we really have to know how he found speed, wavelength, frequency etc.? Some textbooks say it, some don't and it sounds really hard to understand. :s

It is a definitely a good idea to understand this, the more you can say about Hertz' experiments, the better =) But is often hard to understand, particularly if you didnt look at the concept of standing waves in Year 11 "World Communicates". If you would like me to explain, let me know =)

Thanks again for your help, really appreciate it.

For sure =)

 

[superstar12]

Sorry to bump this tread again, but I have a question.

Does the induction coil produce DC or AC? If it produces DC, how did he measure the frequency of the EMR?

 

[photastic]

Sorry to bump this tread again, but I have a question.

Does the induction coil produce DC or AC? If it produces DC, how did he measure the frequency of the EMR?

AC

 

[superstar12]

AC

mmm, Physics in Focus said he used a DC induction coil, and I googled induction coils and they seem to be all DC... Can you please explain this? Thanks!

 

[photastic]

mmm, Physics in Focus said he used a DC induction coil, and I googled induction coils and they seem to be all DC... Can you please explain this? Thanks!

Whoops, was thinking of the induction coils for motors. Hertz did use a DC induction coil. He carried out experiments to show that
• It could be reflected by metal plates
• It could be refracted by pitch or asphalt blocks
• It could be diffracted around obstructions
• It could be polarised (when he rotated the receiving coil he found that the sparks were stronger at certain angles compared to others)
• It travelled at the speed of light. Hertz connected the two loops together with a wire, so that there was interference between the AC wave in the wire and the wave caused by EMR transmission.
From this, he was able to calculate the wavelength of the radio waves, and knowing the frequency of his wave generator he was able to show that the radio waves travelled at the speed of light. So essentially, he used interference to find the the distance between the peaks of the EMR, which is the wavelength. Hence using v = fλ, he calculated the frequency.

 

[superstar12]

Whoops, was thinking of the induction coils for motors. Hertz did use a DC induction coil. He carried out experiments to show that
• It could be reflected by metal plates
• It could be refracted by pitch or asphalt blocks
• It could be diffracted around obstructions
• It could be polarised (when he rotated the receiving coil he found that the sparks were stronger at certain angles compared to others)
• It travelled at the speed of light. Hertz connected the two loops together with a wire, so that there was interference between the AC wave in the wire and the wave caused by EMR transmission.
From this, he was able to calculate the wavelength of the radio waves, and knowing the frequency of his wave generator he was able to show that the radio waves travelled at the speed of light. So essentially, he used interference to find the the distance between the peaks of the EMR, which is the wavelength. Hence using v = fλ, he calculated the frequency.

How did he create an AC wave in the wire if he used a DC induction coil?

 

[anomalousdecay]

Have a read through this thread: http://community.boredofstudies.org/18/physics/329559/hertz-experiment-ac-dc-power-source.html

You can create AC characteristics using a DC source with the appropriate interrupter circuit here. For example in the Hertz experiment, a particular circuit is used to produce a sinusoidal output.

This isn't mentioned in HSC as circuit analysis like this is beyond the scope to even mention and also requires calculus and Fourier series which are far beyond the scope to mention either.

So what do you need to remember? DC source was used with an appropriate circuit to produce the appropriate output and this:

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

 

[Fizzy_Cyst]

Induction coils are DC, but they contain an interrupter, this basically switches the current on and off --> creates change in flux --> induces current in secondary circuit.

I tell my students that there are little Russian people in the induction coil switching the flow of current on and off really quickly. Dunno why they are Russian.

 

[anomalousdecay]

Induction coils are DC, but they contain an interrupter, this basically switches the current on and off --> creates change in flux --> induces current in secondary circuit.

I tell my students that there are little Russian people in the induction coil switching the flow of current on and off really quickly. Dunno why they are Russian.

Tbf it's an RLC circuit with resonance voltage rise coming into play. Russian-Lithuanian-Canadian?

Also the change in flux is referring to an inductor, which isn't covered in HSC.