Physiology & Medicine / Re: Is the continuous application of anti-perspirant to your armpits dangerous?« on: 08/11/2019 13:52:10 »
Dunno about dangerous, but I made myself allergic to the stuff by putting it on broken skin.
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Hydroelectricity is indeed free of charge as long as you collect the water in your own reservoir at the top of your own mountain and move it through your own pipes. It may be worthwhile comparing your domestic piped water charges with your electricity bill.
UK domestic water costs about £4 per tonne (you pay more for the sewerage to take it away than you do for delivering the clean stuff!). Assume you have a 5 m pressure head from a 1000 liter brake tank in the loft. 1 tonne falling through 5 m will deliver
m g h = 1000 x 9.81 x 5 = 49,050 joules = 13.625 Wh for £4, about 30,000 times the cost of the same amount of domestic electricity.
If you drive your turbine direct from the water main, at say 5 bar pressure, you will reduce the cost to about 3,000 times the cost of mains electricity.
I drove my parent's Ford Torino off the road once due to a stall turning at an intersection. Impossible to steer or stop.I've always been sceptical of accounts like this.
I think you are slightly misquoting me, what I said was "For a pendulum the restoring force is mg (vector)"I thought you were saying "For a pendulum the restoring force is mg (mg is a vector)....."
If it's left open for the user to decide how approximately equal, and how small, it can't not be true.Since sinϴ ≈ ϴ for small ϴ, then F ≈ mgϴ,But that isn't always true.
You are making an approximation. As @chiralSPO said, we often make approximations in order to simplify calculations, and that is valid in both practical and teaching situations.The question was why doesn't gravity affect the spring-mass system, and the answer is because gravity exerts a constant force and not one dependent on displacement. That's true regardless of any approximation in the linearity of the pendulum restoring force, and/or Hooke's law.
For a pendulum Sinϴ ≈ ϴ is only useful for ϴ<10į, although at 20į the error is only around 1%, for ϴ>22į the difference is noticeable.
Similarly for the spring/weight I did say "for a perfect spring/small oscillation" and so using a similar small deflection the variation of g is considered to be zero.
If you use approximations for one you ought to allow them for the other.
For a pendulum the restoring force is mgThe restoring force on a pendulum is mgsinϴ.
In fact a dynamo works by moving a conductor through a magnetic field which does work on the charged particles in the conductor which can then be used to generate energy in a circuit. But there's no need for an electric field in this case and therefore no electric potential.If I move a wire through a magnetic field there will be a voltage difference generated between the ends. If I were to place a speck of charged dust nearby, would it not be attracted to one end of the wire because there is now an electric field surrounding the wire?
I don't see what being a radio engineer has to do with anything.The relevance is that radio engineers all understand the terms EMF and PD in the context of a Thevenin source, as defined above.
An electromagnetic force (EMF) is defined as that which can produce energy to generate an electric currentOk, so how is that different from PD? Using my definitions of EMF and PD, I could construct a Thevenin source with an infinite resistance, such that the EMF can deliver energy but the PD cannot, would that not then satisfy your distinction between the two terms?
It's quite complicated isn't it? I could really do with a walk-through diagram to show what is happening at each stage of the waveform to understand how it works.If you install a (free) copy of LTspice, there's a dimmer in the examples folder which you can play with to your heart's content: http://www.linear.com/designtools/software/
There's a difference in potential between two points in a static electric field but such a field cannot produce a steady current in a resistor whereas an EMF can.This seems to me a bit like arguing that there is a difference, in kind, between the metre used to measure the position of a point in empty space, and the metre used to measure the height of a reservoir that can generate power in a hydro plant. Distance is distance.
A couple of years ago m neighbour knocked on the door and pointed out that the front garden wall was a few inches over his side of the boundary. I forget how much, but the angle it made with the front of the house was about one degree off the normal. I told him that that's where it had been since the house was built, but he was still suspicious. He went quiet when I showed him that the rooms of the houses were even more off square though. I suppose if the mirrors are perfect, the light would keep going round until it found its way back to the flame eventually in any shape room though..........Quote from: vhfpmrBut a corner reflector returns a beam of light parallel to the incident beam, not convergent on the source.I agree - but then it will bounce off the diagonally opposite corner; won't this put it back on another parallel path, towards the candle flame?
I believe that what Pete is saying is that a battery does not necessarily have an electric field about it. An electrochemical (galvanic) battery is not a capacitor, which has an EMF that is determined purely by a macroscopic electric field. Batteries instead have an EMF that is determined by microscopic electric fields (atomic scale), and so when completely isolated from a circuit, a battery is unlikely to have a significant dipole moment (uneven distribution of charge within the battery)
Imagine a simple electrochemical cell, containing a piece zinc metal at one end of a tube, and a piece of copper at the other end. The copper end of the battery contains an electrolyte solution composed of copper chloride dissolved in water (Cu2+ and ClĖ ions), while the zinc side is dissolved sodium chloride (Na+ and ClĖ ions), and a thin membrane separates the two electrolytes, allowing chloride ions to cross, but not copper. When the two electrodes are not connected to anything, no electrochemical reaction occurs, and the tube shouldn't have any significant electric field about it. (you could measure the amplitude and direction of the field anywhere around the cell, and would probably not observe any--if you touch a meter to the two ends of the cell it would detect the EMF, but the potential difference between points in space each a micron away from the electrode is probably pretty close to zero).
Once a circuit is completed, electrons flow from the zinc electrode to the copper electrode, which dumps the electrons into the empty orbitals of the Cu2+ ions, reducing them to copper metal, and chloride ions move to the zinc end of the cell. So negative charge (moving either as electron or chloride) has moved all around the circuit, but the cell itself doesn't have any change in charge distribution on a macroscopic scale, only changes in the composition of matter within.
most of the visible light would bounce of corner reflectors (which form every corner of the room), and end up getting absorbed by the candle flame and turned into invisible heatBut a corner reflector returns a beam of light parallel to the incident beam, not convergent on the source.
Not all boilers have the ability to sense when all the trvs are shut which can lead to what is known as dry cycling where the boiler just keeeps heating water in its own loop usually around a bathroom radiator with no trv or a bypass loop. However, the room thermostat is rarely in a position to stop this happening.