Ask a Mathematician / Ask a Physicist

Physicist: To control power in a house or an outlet you’d generally use a fuse.  But fuses are slow, they need time to heat up.  A surge (or the faster “spike”) happen too fast, so reacting to a surge is no good.  Dealing with a surge properly has to be built into the nature of the machine.  There are two (general) ways to do this: “choking” and “shunting”.

Shunting is what a sink does when it over-flows.  You never worry about water getting to the ceiling above your sink, because the moment the water gets as high as the edge the sink starts overflowing and stops filling up.  The ceiling is protected by the innate nature of the sink (which doesn’t need to react, it just “does”).

If the water in the sink gets too high it simply falls over the edge. Similarly, if the voltage in the load line gets too high, the resistance of the varistor (variable resistor) drops from very, very high to nearly zero. This allows a connection to ground, and electricity can then flow out of the circut, instead of through it.

The zero-finesse surge protector is just a varistor placed between the power line and the ground line.  The ground line (in this metaphor) is the floor around the sink, where all the overflowing water gets dumped.

While there is some fancy quantum mechanics tied up in varistors (I’m talkin’ valence and conduction bands here), what really gets my physics juices jumping is waves and frequencies.

The other type of surge protector is essentially a “band pass filter” centered around 60 hz (the frequency of the current in the electrical grid).  You can think of this as the “radio” of the surge protector tuning into the “station” of the wall outlet (and tuning out everything else).

Now, I hope this doesn’t come as a shock to anyone, but most surges (e.g., lightning strikes) are fast.  It turns out that because of the Uncertainty Principle, things that happen really fast are necessarily spread out over a lot of frequencies (small time uncertainty means large frequency uncertainty).

The frequency spectrum for a wall socket (top) and a lightning bolt (bottom). Keep in mind that this is not a graph in time, but in frequency. If you played a single note on a piano you'd get something like the top graph, and if you banged on all the keys at once you'd get something like the bottom graph.

So while a power surge may have a lot of energy overall, the amount of energy right around 60 Hz, where it can get into the circuit, should be fairly small.  You can build chokes to limit the frequencies that get past the surge protector by using carefully tuned LRC circuits, but generally (since most of the energy is in frequencies much higher than 60 Hz) you can just build a “low pass filter”.

Which is just fancy talk for “an inductor”.

Which is just fancy talk for “a coil of wire“.

The original question was: Would it be possible to create a very dense cloud cover inside a laboratory under controlled conditions and generate “artificial lightning”?  the Power output would be Amazing!!  it would really help solve our energy crisis.

The Chaitén volcano in Chile. Holy shit.

Physicist: Lightning is generated in the same way that static electricity is generated when you drag your feet on a carpet.  A storm cloud or an ash cloud is just a whole mess of feet and carpet.  As ash explodes out of a volcano it rubs together.  Almost all of that kinetic energy becomes more heat, but a very, very small fraction becomes electrical energy.

It is entirely possible to create static electricity, and even lightning using this method.  Van de Graaf generators, for example, use rubbing to generate voltages in excess of a 1,000,000V.  However, it’s a very inefficient method for generating power.  Dynamo generators (the standard generator) is surprisingly efficient.

Essentially, it would take a lot of energy to throw all that dust into the air and get it moving and, because you can’t get more energy out than you put in, it wouldn’t be worth it.  In fact the electrical power you would get out would be tiny compared to the power it took to make it work in the first place.  That being said, it would look pretty cool, so why not?

Artificial lightning (in miniature) is regularly created in places like NEETRAC at Georgia Tech.

An artificial lightning bolt.

This lightning is generated using capacitor banks (not rubbing stuff together, the way natural lightning is created) and is feeble by comparison to the real thing.  Also, it isn’t used to generate power.  It’s used to test the ability of new equipment and machines to survive lightning strikes.

Physicist: The narrative that usually leads to this question is something like: “It was the Wright brother’s brilliant wing shape, among other design innovations, that first made manned flight possible”.  So if the wing shape is so important, why does it still work if it’s flipped upside-down?

A wing (or airfoil or whatever) creates lift by taking advantage of a combination of the Bernoulli force and “angle of attack”.  By increasing angle of attack (tilting the nose up) the oncoming wind hits the bottom of the wing more, and pushes the plane up.  However, this force also increases drag substantially.  The Bernoulli force shows up when the air over the top of the wing is faster than the bottom, but it requires a bit of cleverness to get it to work.  Cleverness like the Kutta-Joukowski condition.

The Kutta-Joukowski condition: If air didn't flow faster over the top of the wing, then the air from the bottom would have to whip around the trailing edge with a very high acceleration. Too high, in fact. K-J assures that this singularity at the trailing edge doesn't show up.

When the Wright brothers built “the Flyer” (they were smart, not particularly creative) the engines available at the time were not powerful enough to lift themselves using only an angle of attack approach, so using a slick airfoil shape to take advantage of Bernoulli forces was essential to get off the ground.  Using the engines we have today (jets and whatnot) you could fly a brick, so long as the nose is pointed up.

So to actually answer the question; back in the day planes couldn’t fly upside-down.  But since then engines have become powerful enough to keep them in the air, despite the fact that by flying upside-down they’re being pushed toward the ground.  All they have to do is increase their angle of attack by pointing their nose up (or down, if you ask the pilot).

Physicist: You could definitely make a device that heats water through mixing.  In fact, that’s exactly how scientists (Joule) figured out how to equate heat energy and kinetic energy in the first place.

Joule's device, which turns the energy of a dropping weight into heat.

When you introduce turbulence to a system the energy flows from large scale eddies into smaller and smaller scale eddies.  At some point the eddies are about the size of molecules (this takes about one minute).  At this point you’re no longer talking about the flow of a fluid, and are instead talking about the random motion of molecules (heat).

Fun fact!: In two dimensions turbulence actually starts at small scales and moves up into larger scales!  You can see this exhibited in weather systems larger than about 15 km across in the atmosphere (at these scales the atmosphere is effectively flat).

A good way to induce large-scale eddy currents.

From this point of view the difference between a mixer and a heater is that a mixer induces large eddy currents, and a heater induces the smallest possible eddies.  Ultrasonic heaters fall neatly in between.

Efficiency is defined as , where is efficiency, is the energy put in, and is the energy lost to heat.  You’ll notice that no lost heat means 100% () efficiency, and if all the energy in is lost to heat then the efficiency is 0%.  So the nice thing about trying to create heat intentionally, is that you’ll always be 100% efficient (if you lose some heat to heat, would you notice?).  Or close enough at least.  What you have to worry about is accidentally heating up the wrong thing.  Mixers generate large eddies which can move the cup, makes noise, and what-have-you.  In other words, some of the energy is wasted heating up stuff near the cup (if you can hear it, then some of the energy is being wasted in your ear).

An electric stove top pushes energy into water at a rate of about 1kW.  A normal blender (mixer) draws power at about 400W, and loses almost all of it to noise and vibration.

So, to actually answer the question, you can heat coffee through mixing, but you’ll get plenty of splashing, it’d be slow, and it’d lose a fair amount of energy through noise and vibration.  You’d be better off with a normal mixing device that has a heating element built in, or heating on a stove first.