The Most Dense Metals

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Did you know that the liquid metal mercury has a density of 13.55 grams per cubic centimetre (g/cm³), more dense than lead which has a density of 11.34 g/cm³ ? Gold is even more dense at 19.28 g/cm³. The most dense metal however is osmium, with a density of 22.59 g/cm³ and is the densest naturally occurring element. Most elements don't occur naturally, and have to be refined and extracted from ore which is often a compound of the element of interest and other elements such as oxygen and sulphur. Osmium is too brittle to be used on its own, and alloyed with other metals for applications that require durability such as instrument pivots and electrical contacts. It's also used as an abrasive for polishing optical lenses.

There used to be a bottle of mercury (which was quite weighty) in the BNS in Kilcullen when it was situated in the current library building. Hopefully the mercury was disposed of safely or is now stored properly. There was also a collection of scientific apparatus, glassware, and a balance from a time when science was taught in primary schools. (Before revised curriculums in more recent decades when it was reintroduced, although we did have "Nature Studies" in the 70s). I wonder what happened all that equipment? According to this Irish Times article, science was dropped as a compulsory subject following independence in 1922 (I've run out of my free article quota for this month, so I can't read the full article yet)

Explosions and Condensation of Air

Interesting the shockwave here caused by a defence forces Javelin missile striking an armoured car and condensing the air.
Usually increases in pressure cause moisture to condense out (as in the case of an air compressor which has to be drained regularly). But that's when there's an isothermal compression and time for air to cool. In an explosion, the air compresses rapidly which makes it hot (so it should hold more moisture before condensing). But this competes with the moisture tending to condense out due to the air being compressed. Higher temperature wins out and moisture doesn't condense. It's actually the negative phase of the explosion that causes underpressure and partial vacuum that's responsible for causing air cooling and condensation (the second of the "booms" of the double boom heard during an explosion, when there's a rebound and pressure drops ). This phenomenon can also be observed when gas from an aerosol, e.g. deodorant, is released and can be at near freezing point or even produce frost, again due to the drop in temperature because of gas expansion.

Arcs and Sparks

There's something mesmerising about electric arcs and lightning. I've always found them fascinating. The dielectric strength of air is 3000 volts per mm, meaning it takes 3kV for a spark to jump a distance of one mm between rounded electrodes, less if there are sharp edges. Between the electrodes, a plasma is formed, the fourth state of matter (the other three being solid, liquid and gas). The temperature of an electric arc can be 5000 degrees C or more, hotter than the surface of the Sun, the arc generating intense short wavelength UV radiation, light and heat (and can cause sunburn when arc welding, if PPE isn't used). Arcs also generate radio waves and spark transmitters were used in the early part of the 20th century for transatlantic and ship to shore communication. Information was sent by Morse Code, and although telephones had been in use since the end of the 19th century, it would be several more years before the transmission of voice using radio waves became possible and commonplace.

Why Are the Northern Lights Different Colours?

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Usually green and pink, but sometimes blue. The display is effectively like a giant fluorescent tube (which is a type of discharge lighting, along with sodium and mercury vapour lamps, used for street lighting.) Just like the way a neon lamp gives out pink light as electrons collide with neon molecules in the lamp (an electric current is simply a flow of electrons), high energy charged particles from the Sun hit the atmosphere, causing various gases to fluoresce and emit light of different colours. Green is produced by oxygen in the atmosphere and red and pink are produced by oxygen at higher altitudes when the electrons in oxygen atoms are excited to higher levels. More info here:
https://www.space.com/aurora-colors-explained

Inertial Guidance Systems and Velocity Measurement

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The Solar System and the Earth within it is moving around the Milky Way galaxy at a velocity of 826,000 km/h. The human body can't sense constant speed, only acceleration when we're speeding up or slowing down, so we're not aware of the motion. (In physics we tend to call it velocity rather than speed. Velocity is a vector quantity having both magnitude and direction. The magnitude of the vector is the speed). Instrumentation can't directly detect velocity, except from relative motion to something else. So for instance in a car, speed was traditionally detected mechanically by having a flexible steel cable connected between the mechanical speedometer and transmission, but more likely electronically nowadays using a Hall Effect sensor or similar to count rotations of a component in the transmission such as the front axle. (Like the sensor and magnet on your bicycle wheel to measure speed). Aircraft airspeed can be measured using tubes called pitot tubes connected to the fuselage, which measure drop in air pressure as air flows over the openings. (The same pressure drop principle used in carburetors of petrol engines and paint sprayers). The variation in air pressure then gets processed and ultimately results in a display of airspeed on a gauge on the instrument panel.

What Happens if There's Nothing to Reference Against?

Navigation is possible using the Sun, stars, a compass and a good timepiece. However another way without resorting to astronomical observations or knowing longitude is to use dead reckoning. So if you know the direction you're travelling from a compass and measure the length of time for which you were travelling and the average speed you were travelling at, you can simply multiply speed by time to find the distance and angle from the start point. This can be done repeatedly at intervals, using compass readings and speed to calculate position and plot a trajectory of a vehicle or other object. If speed isn't constant, an integration process can be used to add up all the instantaneous speed x time values. For a spacecraft or missile that doesn't have wheels or air around it to measure velocity from, accelerometers have to be used. These measure acceleration, and from acceleration, velocity and also distance travelled can be calculated. (Mathematically, velocity is the integral of acceleration and distance travelled is the integral of velocity). Direction of travel is also measured in three dimensions by determining the relative angle of a craft (attitude) with respect to references provided by three gyroscopes. The complete system with gyroscopes, accelerometers and associated components and electronics is known as an inertial guidance system.

5 Live Science Podcast — Artificial Pancreas

 

 

 

 

 

 

 

 

 

 

 

 

 

This week's program featured a segment about an "artificial pancreas" for Type 1 diabetes sufferers, which uses a closed loop control system to monitor blood glucose and administer a dose from an insulin pump.

What's a Closed Loop, Negative Feedback Control System?

Many systems use close loop control with negative feedback to control an output of interest. An example is the speed control on small engines, e.g. the one on your petrol lawn mower. If you run into longer grass, this slows down the blade and engine. To reduce the change in rotational speed (the output variable) and keep it at a setpoint (typically 3000 RPM for a mower), a governor is used. A vane, mounted adjacent to the engine's cooling fan that's integral to the flywheel, forms part of the governor mechanism. When the fan slows down, it blows less air onto the vane. The vane is connected via a linkage to a butterfly valve in the carburetor. This valve forms part of the throttle mechanism and as the valve opens up due to movement of the vane, more air/fuel mixture enters the engine. This generates more power which tends to compensate for the slowdown of the blade due to the long grass. Other systems use negative feedback and closed loop control, e.g. your phone charger which has electronics to keep the output at 5 V DC, irrespective of load. Without a voltage regulator, all power sources, including batteries, experience a voltage drop as they're loaded and more current drawn from them. Voltage stabilisation by a regulator is necessary so that the electronics in your phone for instance works in a predictable manner. Other examples of feedback control systems are servos on vehicles and aircraft and temperature and pressure control in industrial processes.

Why Negative Feedback and Not Positive Feedback?

The function of a feedback control system is to keep an output variable at a fixed or variable setpoint, irrespective of load. So in the lawn mower example the speed was the parameter that needed to be kept stable. In negative feedback, the actual speed is fed back and subtracted from the desired speed (the setpoint) to create an error signal. This error, via a controlled variable, drives the system in a direction to bring the output closer to the setpoint. In the lawnmower example, the controlled variable was the fuel. The term negative feedback comes from the subtraction that's involved. Positive feedback on the other hand causes the fed back signal to add to the set point, resulting in instability and the output diverging from a setpoint (e.g. positive feedback in a PA system, causing the squealing from loudspeakers). Many of the systems in our bodies are negative feedback control systems and like any control system if not tuned properly, there can be oscillations and instability. Hence the wobbling from side to side when we're learning to ride a bike!

 

Nova Due in a Binary Star System, 3000 Light Years Away

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It won't be as spectacular as Tycho's Supernova of 1572 or Kepler's Supernova which occurred some decades later and was visible in daylight. However, a seemingly new star will appear in the constellation of Hercules and should be as bright as the North Star. The nova is predicted to happen before September 2024 in a binary system 3000 light years away, made up of a dead white dwarf and ageing red giant. The white dwarf, consisting of "star stuff" so dense that a teaspoon of it would weigh five tonnes, is sucking material off the "nearby" red giant. Every 80 years or so, the the material it accumulates reaches a critical mass and a thermonuclear fusion reaction is sparked off, the star lighting up so that it can be seen from Earth, 18 thousand trillion miles away. In reality, because the star system is 3000 light years away, the event really occurred 3000 years ago, the light taking this length of time to reach us.

This BBC article provides more details.

Move over, solar eclipse: Scientists predict a once-in-a-lifetime nova explosion in the coming months