Why Most Metals Are Silver (but Copper And Gold Aren’t)

Why most metals are silver (but copper and gold aren’t)

If we want to understand what gives a metal its color we first need to understand a little bit about the definition of a metal. Metals are materials that experience metallic bonding - wherein the atoms are so close that there is a veritable “sea of electrons” in the substance. (This is also what makes metals conductors, but that’s another story). Basically each atom donates an electron or two that is free to flow throughout the material, unattached to any particular nucleus.

This proximity leads to an overlap in the allowed energy levels of electrons (shown in the lower left hand image above); basically the higher empty electronic levels are so close to the uppermost filled levels (also called the Fermi level) that they form an essentially continuous band of allowed energies.

Now, backtracking a little bit, color in a substance is caused when a material doesn’t absorb a particular wavelength of light. Because of the empty energy levels mentioned above, metals generally can absorb all wavelengths of light in the visible spectrum. This implies that a metal should look black, except that the excited electron can immediately fall back to the state that it came from, emitting exactly the same energy, causing a flat piece of metal to appear reflective. Thus, the reason why most metals are silver. (Also, the flatter a metal, the more reflective, thanks to diffuse vs. specular reflection).

For a few select metals, like copper and gold, the absorption and emission of photons are noticeably dependent on wavelength across the visible part of the spectrum. The graph in the lower right image above shows the reflectance of aluminum, silver, and gold, including wavelengths in the infrared and ultraviolet. Aluminum is pretty reflective overall, and silver is highly reflective in the visible region (about 400 to 700 nm), but gold clearly absorbs wavelengths about 500 nm or below. Thus, it most strongly reflects yellow, giving it its characteristic appearance.

Sources: (first image), 2 (second image), 3 (third image), 4

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t-sci-eng

6 years ago

This month on FYP!

It has been a blissful month in physics with the nobel prize for the work on gravitational waves and LIGO swiftly striking back with the detection of a neutron star merger.

At FYP! we have started to dwell a little bit into the essence of condensed matter physics while exploring some really cool science and engineering along the way.

Here’s what went down on FYP! this month:

Nobel prize in physics article on Gravitational waves

Gravitational waves, Light and Merging neutron stars

Paramagnets and Combinatorics

Diamagnetic levitation - Ig Nobel prize(2000)

Levitating frogs and superconductivity

Revisiting rolling shutter

Cooking with a computer (the importance of a heat sink on a computer)

Cartoon laws of physics

Beautiful proofs (#3) - Euler’s sum

Have a great day!

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t-sci-eng

7 years ago

But That’s Not All It Can Do. Microsoft And NASA Teamed Up To “bring” You, Yes You, To Mars.

But that’s not all it can do. Microsoft and NASA teamed up to “bring” you, yes you, to Mars.

Follow @the-future-now

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t-sci-eng

7 years ago

Superstretchable, supercompressible supercapacitors

Flexible, wearable electronics require equally flexible, wearable power sources. In the journal Angewandte Chemie, Chinese scientists have introduced an extraordinarily stretchable and compressible polyelectrolyte which, in combination with carbon nanotube composite paper electrodes, forms a supercapacitor that can be stretched to 1000 percent in length and compressed to 50 percent in thickness with even gaining, not losing capacity.

Supercapacitors bridge the gap between batteries, which are merely energy-storing devices, and normal capacitors, which release and take up electric energy very quickly but cannot store so much energy. With their ability to charge and release large amounts of electric power in a very short time, supercapacitors are preferably used in regenerative braking, as power buffers in wind turbines, and, increasingly, in consumer electronics such as laptop computers and digital cameras. To make supercapacitors fit for future electrics demands like, for example, wearables and paper electronics, Chunyi Zhi from the City University of Hong Kong and his colleagues are searching for ways to endow them with mechanical flexibility. It can be achieved with a new electrolyte material: they developed a polyelectrolyte that can be stretched more than 10 times its length and compressed to half its thickness retaining full functionality, without breakage, cracking, or other damage to its material.

More Than You Ever Wanted to Know About Mechanical Engineering, Part 33: Stress Concentrations With Fluctuating Stresses

There’s one last complication to consider with fluctuating stresses. When we looked at the case of fully reversed stresses (that is, σ_m = 0, σ_a ≠ 0) we found a fatigue stress concentration factor based on the stress concentration factor for a static situation.

With a fluctuating stress, the situation is a little different. Since the mean stress is non-zero, the part is always under some kind of load. We can consider the effects of this constant mean stress separately from the effects of the momentary alternating stress and assign them a separate fatigue stress concentration factor, which we’ll call K_fm.

Let’s think about what’s actually physically happening to a part being subjected to a fluctuating stress. Let’s say we’re dealing with a plate with a slot in it subjected to fluctuating tension.

More Than You Ever Wanted To Know About Mechanical Engineering, Part 33: Stress Concentrations With Fluctuating

There’s obviously a large stress concentration at the slot that we’ll have to take into account.

There’s three different scenarios which can occur here. The first is that the maximum stress the plate sees (the largest value of combined mean and alternating stress, taking stress concentrations into account) never approaches the yield strength of the material. The plate just stretches and contracts elastically. This isn’t really any different from our previous situation with fatigue stress concentration factors - we can use the K_f factor we got earlier here.

But suppose the yield strength is exceeded. What happens then? If the maximum stress is greater than the yield strength, then the plate must deform plastically at that point of maximum stress - the slot must widen. If the slot is wider, then the stress concentration is relieved - there’s more room for movement before the geometry stops you. If other words, the fatigue stress concentration factor is lessened.

If it’s just your maximum stress that exceeds the yield strength but your minimum stress is still below it, this localized yielding will be one-sided - you’ll get a slot that’s widened on one side, but you’ll still have some overall mean stress. If this is the case, you base your stress concentration factor on the relationship of the mean and alternating stresses to the yield strength.

If both your minimum and maximum stresses exceed the yield strength of the material, you get a situation where you’ve widened the slot as far as you can without actually breaking the part on both sides and you’re experiencing a stress of magnitude equal to the yield strength at either extreme of the fluctuation. Since you now have a fluctuation with equal and opposite extremes, your mean stress is zero - the mean fatigue stress concentration factor is zero. The scenario is now one of fully reversed loading and the mean stress drops right out of it.

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t-sci-eng

7 years ago

Physical Science...In Space!

Each month, we highlight a different research topic on the International Space Station. In May, our focus is physical science.

The space station is a laboratory unlike any on Earth; on-board, we can control gravity as a variable and even remove it entirely from the equation. Removing gravity reveals fundamental aspects of physics hidden by force-dependent phenomena such as buoyancy-driven convection and sedimentation.

Gravity often masks or distorts subtle forces such as surface tension and diffusion; on space station, these forces have been harnessed for a wide variety of physical science applications (combustion, fluids, colloids, surface wetting, boiling, convection, materials processing, etc).

Other examples of observations in space include boiling in which bubbles do not rise, colloidal systems containing crystalline structures unlike any seen on Earth and spherical flames burning around fuel droplets. Also observed was a uniform dispersion of tin particles in a liquid melt, instead of rising to the top as would happen in Earth’s gravity.

So what? By understanding the fundamentals of combustion and surface tension, we may make more efficient combustion engines; better portable medical diagnostics; stronger, lighter alloys; medicines with longer shelf-life, and buildings that are more resistant to earthquakes.

Findings from physical science research on station may improve the understanding of material properties. This information could potentially revolutionize development of new and improved products for use in everything from automobiles to airplanes to spacecraft.

For more information on space station research, follow @ISS_Research on Twitter!

Make sure to follow us on Tumblr for your regular dose of space: http://nasa.tumblr.com

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