Category Archives: Physics

Today is Fara Day

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It’s been my job to wind coils in the lab, and at some point I acquired a strong rare-earth magnet, so I decided to play around with Faraday’s law: A changing magnetic fields will induce a potential difference in a loop of wire, and with many turns present in a coil each turn can contribute, so this can be be a significant effect. I set up an oscilloscope to measure the voltage in the coil and took some movies, which allowed me to grab good still shots and do this without needing more than two hands or a prehensile appendage.

Faraday’s law tells us that
\(V = -Nfrac{dphi}{dt}\)

So lots of windings (N) helps, and the driving effect is the rate at which the flux changes.

Here’s what the signal looks like when I pushed the magnet in and the pulled it out after a short delay (and repeated as necessary); you can see the pulse as the coil sees an increasing field (and therefore flux) as it gets closer to (and enters) the coil, and then the signal drops back to zero when the magnet stops moving.

But that’s not the only way for the coil to see a changing magnetic field. You can flip the magnet, and this actually induces a larger voltage in the coil

You can see that this gives a signal about 4x as large as the simple motion; flipping the magnet with my fingers happened quickly, and changing the direction by 180º gives you the maximum change in the flux that you can achieve, and you get a change in sign as well — the flux starts out at a maximum inside the coil, and the rotation causes it to decrease, increase again, and is a constant when the rotation stops. Decreasing and increasing flux give opposite signs for the induced voltage.

 

So what happens if you combine the two motions? Here’s the signal from dropping a spinning magnet through the coil

I think the signal here can be broken up into two parts: the drop and spin is before the center division, and the behavior from ~50 milliseconds before that division is from the magnet bouncing off of the carpeted floor and coming to rest. The drop & spin, show the envelope of the motion we see in the first plot with the oscillating behavior of the spinning. It looks like there are 5 rotations during the drop, which was from ~1 meter (though the coil itself is shorter), which would take ~450 ms. In the first part of the curve the magnet is still outside the coil, so the flux is getting bigger, which means the signal size increases. I’m not exactly sure why the signal amplitude levels off in the second half of the drop; it’s entirely possible the magnet’s rotation axis was changing direction as it fell, and any component along the axis of the cylinder won’t contribute to the signal we see here. It would be nice to have an independent way of measuring the spin effects to match them up with the electronic signal; this might be a reasonable physics-101 lab measurement to do.

Ice is Nice

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How Much Ice Do You Need For Your Drinks?

The Navy has an ice command, and I assume its members always bring ice to parties simply because the partygoers assume they will, and would be angry if they didn’t. Self-fulfillment. Aka a feedback loop.

Anyway, Rhett does the calculation of how much ice it takes to cool a drink down if you own an ideal cooler, and find that a 12 oz (355ml) can of water is cooled to 0ºC with just 100g of ice. Being a responsible physicist, he does a reasonability check:

[I]f I have a 6 pack of drinks, I would need 600 grams of ice, a 12 pack would need 1.2 kg of ice. Yes, that seems small. Remember this is the ideal case.

I like to look at this another way. This tidbit stems from a NOVA show on getting things cold (Absolute Zero, which aired just before I started blogging — which explains why I can’t find a blog post on it) they explained how people used to ship ice to the tropics and made a big profit on it, and this was made possible because the amount of energy to melt a certain mass of ice was equal to the energy to subsequently heat it up to ~80ºC. We can check this by dividing the latent heat of fusion (334 J/g) by the specific heat (4.18 J/g-K) and we get that answer. Which means that freezing/melting ice involves a LOT of energy compared to changing its temperature by a few degrees. If 80ºC requires an equal mass, cooling by a quarter of that should require a quarter of the ice, plus a little for cooling the aluminum. Which gives you the 100g/can ideal case. (I guess the true “ideal case” would then be 2.4 kg of ice)

Seven Ferrite Rings For the Laser-Lords in Their Dark Labs of Cinderblock

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OK, more than seven. Ferrite rings (aka cores) are used as inductive loads in circuits, among other things. Any alternating current in a wire creates a magnetic field. By looping the wire through the ring a few times, you will create a changing magnetic field inside, which will produce eddy currents. These currents create fields which oppose the induced changes (this is Lenz’s law). From the circuit’s point of view (I hope it will excuse me anthropomorphizing it; I know circuits dislike that) this is an inductive load, so the alternating current sees a higher impedance than the DC, and this knocks down any high-frequency noise you might have and reduces what you write onto the components being powered.

It's Not a Concert in Japan

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The Radioactive Orchestra

Pick your isotopes from the Chart of the Nuclides. The program plays the energy level decay cascade as a series of notes that sound (to me) like a xylophone/marimba. You can change volume, pitch and tempo of each, as well as the waveform played — square and triangle waves sound more “techno” (I picked the isobaric combo of Ce, Xe and Cs, all -135). There’s also a visual of nuclei emitting the gammas. Interesting.

We Are Saved! Again!

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New Alloy Can Convert Heat Directly Into Electricity

[T]he new alloy — Ni45Co5Mn40Sn10 — undergoes a reversible phase transformation, in which one type of solid turns into another type of solid when the temperature changes, according to a news release from the University of Minnesota. Specifically, the alloy goes from being non-magnetic to highly magnetized. The temperature only needs to be raised a small amount for this to happen.

But if it’s a phase transformation, it should be happening at a specific temperature. Once the material has heated up and you get your magnetic field, what then? The article and press release on which it was based don’t go into that.

During a small-scale demonstration in a University of Minnesota lab, the new material created by the researchers begins as a non-magnetic material, then suddenly becomes strongly magnetic when the temperature is raised a small amount. When this happens, the material absorbs heat and spontaneously produces electricity in a surrounding coil.

Faraday’s law tells us this should happen upon the creation of the magnetic field, but once it has happened — nada. You would need to cycle between states to maintain a changing field needed to continually produce electricity, so after heating you would need to then cool the substance.

The paper confirms this requirement

The design of the coil is to give a maximal component of E parallel to the wire, thereby driving a current. A potential difference across the coil of opposite polarity is obtained on the reverse phase transformation upon cooling.

So here “directly” doesn’t mean “directly” in the sense that you slap it on a car engine and produce electricity. You need a temperature gradient, just like always, and in this case, you need to bracket a specific temperature. Promising, but these headlines always seem to outnumber new products by a fair margin.

Physics Works and I am Still Alive. See You Wednesday.

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You need to a flashplayer enabled browser to view this YouTube video

There was a story I heard when I was a student, (perhaps/likely apocryphal) way before youtube made it possible to record and share the examples so easily, of a professor doing this demonstration, wrecking-ball-to-nose, but letting a student give it a try, and the student gave the ball a small push. Broken face.

(See you next Wednesday would have been a gag that probably none of the students would have understood)

Paint It, White

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Cut waste, create jobs, save money — what’s not to like?

There is a link to several ideas to put people back to work, but the focus is on painting rooftops white to save utility costs, and idea that was popularized by Steven Chu, the Secretary of Energy.

There’s a claim raised in the comments:

AC costs less than heating

Obviously the total amount you spend on heating and cooling will depend on the climate where you live, but what if we quantify this? A degree of heating vs a degree of cooling? Then it becomes easy to make a comparison to see how compelling rooftop painting might be — if you have comparable degree-days of each, then whichever is cheaper tells you which way to go.

The basic argument is a simple application of thermodynamics. All nonreversible thermodynamic processes (which include your heating and air conditioning) reject heat. In the case of heating, though, the goal is to produce the heat, which means you can build heaters which are basically 100% effective. Cooling, however, has to reject heat, and becomes less efficient as it becomes hotter outside. Cooling efficiency is necessarily less than 100%, which means cooling an arbitrary mass by a degree costs you more in energy than heating it does.

Then there is an economic analysis — are you using the same energy source for cooling and heating (say, gas heating vs electricity for cooling) and are those energy sources costing you the same, and how much heating and cooling do you do? If you live in a dry area you can use a swamp cooler, which is probably relatively cheap. But make no mistake — the energy to cool by a degree is greater than the energy to heat by a degree.