# Search results for query: *

1. ### I Electric arc length in relation to voltage

I don't think the capacitance is anything to do with it. My understanding is that an air gap tends to break down at approx 20 to 30 kV/cm. But it is only approximate, and in his experiments with radio waves, Hertz investigated spark lengths and found that a spark will never occur below about 300...
2. ### I Oscillating charged particles and E.M waves

The motion of the two is presumed to be far below the speed of light so relativity does not come into it. In this case the radiation from one, when it reaches the other, will be detectable.
3. ### B To measure light's frequency, do we interact with each crest + trough?

Light frequency is too high to measure electronically, so the usual method is to find the wavelength and then calculate the frequency from knowledge of the speed of light. Wavelength is most easily found by creating standing waves using mirrors or by measuring the interference pattern created by...
4. ### I Optics: infinite light source illusion question - can you help?

I think a good illusion to use for this is Pepper's Ghost, used on stage, and described in this Wiki article. It uses a sheet of glass at an angle, which is itself invisible, but reflects the image when it is illuminated. https://en.wikipedia.org/wiki/Pepper%27s_ghost
5. ### I Why do we use low-coherence light in Optical Coherence Tomography?

As far as I can see, it is a radar, looking at the reflections from the various layers of the retina. If we use a pure coherent source, we have in effect a CW radar. This does not allow distance determination; the source must be modulated to obtain distance information. An ordinary radar uses a...
6. ### B Should I use True North or Magnetic North to align my sundial?

You need to use True North for a sun dial. It should be sufficient to use a magnetic compass and apply Magnetic Deviation (called Magnetic Variation in UK) for you location, which should be on maps. If setting up a sun dial using the Sun, its motion is not uniform over the year. Clocks are based...
7. ### [Mystery Solved] What household objects can pick up radio signals?

Have a look at this video explanation of such a spying bug, recently re-incarnated for TV.
8. ### Measuring the Speed of Sound in Water, Using Faraday's Law

Not sure about the requirement for plane waves. The energy from a point source will spread out spherically at the speed of sound until it reaches shell of a given radius. I can't see an error here.
9. ### B How does wind affect a turn for an aircraft?

Agree. Once clear of the ground, an aeroplane does not know if the wind is blowing or not. Similarly, a boat does not know if there is a current.
10. ### Measuring the Speed of Sound in Water, Using Faraday's Law

Further to my post #5, I would also like to mention that I have myself had problems demonstrating the speed of sound using air filled telescopic tubes. The effect is that the sound delay does not vary progressivly with increasing distance, due to the effect of standing waves. If we study...
11. ### Measuring the Speed of Sound in Water, Using Faraday's Law

The pipe is a transmission line not a free space medium. So propagation is described by the various modes of vibration which are possible. In particular, notice that a transmission line having standing waves does not have a propagation time as such, because it is an energy store, not a...
12. ### [Mystery Solved] What household objects can pick up radio signals?

It is known that people have heard music coming from a cooker, for instance. It is very easy for pick-up on power lines to amount to a few volts, and detection can happen with loose contacts, rusty metal etc.
13. ### Receiving High Freq Signals via Passive Downconverter

For satellite reception I think you need a low noise amplifier ahead of the down converter so that the signal is raised above the receiver noise. Otherwise the mixer has losses will take the signal below the noise floor of the IF amplifier. In a typical case using the mixer alone may provide a...
14. ### A Measurement uncertainty due to thermal noise

It appears to me that the noise power which is jiggling the electron arises from the warm resistance in the tuned circuit. If the tuned circuit also has damped oscillation that is separate and unrelated. The current and voltage from the thermal noise and the damped oscillation just add but do...
15. ### Help with Tektronix differential probes

Is the BNC connection lead grounding successfully??
16. ### A Measurement uncertainty due to thermal noise

May I suggest a simplistic answer? Assuming a series RLC circuit, the noise power Pn will be 2kTB, where k is Boltzmann's Constant, and the noise current will be sqrt (Pn/R). Bandwidth B = centre frequency/Q. And Q=Xl / R. As this current is entirely carried by the motion of the electron, which...

Agree.
18. ### I Polarization of Light -- Please help me with some basic questions

For an unpolarised source, such as the Sun, the vectors of any two orthogonal polarisations are incoherent, so we cannot add them. We cannot add vectors which are varying in amplitude in a random way, but we can add the two powers.
19. ### I Polarization of Light -- Please help me with some basic questions

If we place the polariser normal to a beam of unpolarised light this is how to analyse what gets through. The source in this case can be considered as two sources having orthogonal (opposite) polarisation. One polarisation (as defined by the E-field) is lined up with the slits and the other is...
20. ### I Transmission Line EM Wave vs EM Wave in Free Space

I think in classical terms we can only describe the properties of free space by observing the behaviour of "massless" charges subjected to an EM wave. Such charges have the mechanical properties of springiness (due to their attraction) and inertia (due to the creation of a magnetic field when...
21. ### I Transmission Line EM Wave vs EM Wave in Free Space

But the question is why does a vacuum, which classically contains nothing, impose that ratio upon the fields?
22. ### The voltage of a voltaic cell

The Daniel Cell gives 1 volt approx and uses copper and zinc. I think the Volt was originally defined from this type of cell.
23. ### B Archimedes Principle - A Rock and Water Magic Trick

But the mass of water and hence its gravitational attraction is unchanged. A tall cylinder weighs the same as a squat cylinder.
24. ### I Surface charge of neutral solids

I am not an expert on this, but my supposition is that the negative bias on the surface of a conductor is the Schottky Effect (https://en.wikipedia.org/wiki/Schottky_effect). At a small distance we will see the field arising from the negative charge, whilst at long distances, by superposition...
25. ### I How is combustion energy released?

Once we start the reaction, by providing the initiation energy, using a match, for instance, the reaction is self sustaining. This is because, for a fire, the energy released by bonds being made is greater than the energy absorbed by bonds breaking.
26. ### I How is combustion energy released?

This is called the initiation energy. We have to break a few bonds to initiate the reaction, as when lighting a fire.
27. ### I How is combustion energy released?

The atoms in a compound attract one another. If you pull them apart you are putting energy in; if you let them combine you obtain energy out. Combustion of carbon is a chemical reaction involving the making of bonds, so that we have energy out - it is exothermic. On the other hand the combustion...
28. ### How can vinegar attenuate the pain of a bee sting? (acid base reaction)

I found that Dock leaves also help with the itching of eczema.
29. ### I Measuring Low Resistance: Why Meter Bridge is the Preferred Choice

I think the measurement range will be limited by the available ratio arms and standard resistances. In addition, a bridge does not make best use of the meter sensitivity because it is shunted by the bridge components. There is a sensitive method for low resistances where we first pass a current...
30. ### I Polarization of Light -- Please help me with some basic questions

Consider the type of polariser which consists of metal rods forming a grid. The wave must not have an E-field acting along the metal rods, which would cause it to be reflected back as for a sheet of metal, so it should be at right angles to the grids in order to pass through. On the other hand...
31. ### I Pickleball Physics -- Player reaction time vs aging

I am rather old and can stop a falling ruler in about 10 cm. From s=0.5 a t^2 we find that t is 141 ms. My theoretical reaction time from Post #1 should, however, be 366 ms. At that rate I would take 67cm to stop the ruler, which is definitely incorrect.
32. ### How many ppm of O2 does H2O2 add to water?

I wanted to work this out just for fun as I am not a real chemist. Molar Mass of hydrogen peroxide is 34, and of oxygen 32. The reaction is 2 H2O2 gives O2. So 68g of hydrogen peroxide give 32g of oxygen, a yield of 32/68 x100=47%. To provide 10ppm in 1 litre of water will require 1000 x 10^-5...
33. ### Looking for a 12VDC motor with low RPM

A motor from a computer printer is quiet in operation. Alternatively, a cassette motor is very quiet and reliable. You can make a motor turn slowly by driving it with a square wave having a mark space ratio which differs slightly from 50%. Use a low frequency, such as 25 Hz, so the motor...
34. ### Trailer Frame Beam Design

I think you also need to consider dynamic loading caused by acceleration, braking, hitting kerbs at speed and cornering.
35. ### I Light intensity required to shine a light through an opaque object

The intensity will vary over the surface of the cube, so will depend on the radial distance. I thought microwaves and light were both EM waves.
36. ### I Light intensity required to shine a light through an opaque object

If it were a microwave source we would use the Effective Isotropically Radiated Power in Watts, and calculate the intensity at the surface from P/(4 pi R^2) and then apply a correction for the absorption in decibels per metre. I am sure the same is true for light, but the system of units is...
37. ### I Outer layer refractive index - total internal refection on waveguide

As you will realise there are a number of modes that can propagate in an optical waveguide. By making the transition in refractive index at the boundary gentle, those modes having a small angle of incidence at the interface will not be reflected and will pass through. The waveguide then tends to...
38. ### Frequency selective surface radome

The graph is plotted inm decibels. If it was plotted in watts, then the two plots would be the inverse of each other.
39. ### Frequency selective surface radome

If the radome is loss free, the transmitted power and the reflected power should always add up to incident power. To compare transmitted power and reflected power you have to deduct the two powers. To do this you cannot use decibels, you must first convert from decibels (relative to incident...
40. ### B Exploring the Intersection of Oscillations/Waves & Particles in EM Fields

How about describing the cathode ray tube?
41. ### A Resonant vs non-resonant EOM

I think the resonant one will apply the required modulation voltage to the EOM with expenditure of less power. Because it uses resonance to do this, it is a narrow band device, which will restrict the modulation bandwidth. This might or might not be important to your usage.
42. ### I Does the aerial [antenna] not need to be positioned perpendicular?

You are quite correct that I was considering a ground level receiver, rather than the aeroplane.
43. ### I Does the aerial [antenna] not need to be positioned perpendicular?

The case of a vertical antenna over the ground is very interesting. There is some good information in a paper by Bullington of Bell Systems*. Over perfect ground, or sea water, the ground reflection is aiding at low angles, whereas for a low horizontal antenna it is cancelling. Over imperfect...
44. ### B Simple yet effective way to reliably control the DC Motor speed?

A stroboscope requires a fine adjustment of motor speed. A good method is to use a potentiometer wired across the supply. Schools sometimes have a big wire wound rheostat, maybe 10 Ohms, which can be used. This tends to hold the speed steady and allows adjustment down to zero...
45. ### I Does the aerial [antenna] not need to be positioned perpendicular?

It fascinates me to watch the radio control enthusiasts pointing the antenna at the aircraft! This is the direction for minimum signal - I expect the ground reflection saves them.
46. ### How to wind a 5K audio tube amplifier transformer?

I have also read the section in the Radiotron Designers' Handbook, which suggests a lower primary reactance than I previously said. The speaker in its enclosure will probably have a peak of impedance at a bass resonance, traditionally about 50 Hz. If used with pentodes, which have high plate...
47. ### I Neil deGrasse Tyson on Nikola Tesla and electromagnetic energy

If the power line frequency is very high, however, such that the spacing between wires is a significant part of a wavelength, then the bird will be exposed to electric fields acting along the wire.
48. ### How to wind a 5K audio tube amplifier transformer?

You might be interested in the amplifier described at this url; it is lower power than yours but the design considerations for the transformer are discussed. I believe these amplifiers both operate in class AB1. http://www.r-type.org/articles/art-117.htm
49. ### B What is the principle of a magnet?

You can also build a long magnet using a row of shorter magnets, quite instructional to do this as the reverse process.
50. ### How to wind a 5K audio tube amplifier transformer?

If the transformer requires a 5k primary, then the inductive reactance at the lowest frequency needs to be several times greater than 5k. Tube amplifiers are limited by the B supply voltage and current swing (emission) and so it sounds as if 60W is the peak power. I am not sure there is a need...