December 14, 2019, 05:26:24 AM
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Topic: Looking for good teaching contexts regarding the conductivity of NaCl-solutions  (Read 546 times)

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Offline Tlowlow

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Hey,
first time poster here. As the title states, I am looking for contexts to teach the conductivity of ionic solutions. I thought of smartphones that get damaged when dropped into the sea or that "new" Na/NiCl2 accumulator, but the first one can't really be solved in class and the second one is too complex. My students are in the 8th grade and I was wondering if you know a good example, where conductivity is used to achieve its means.

Thanks a lot!
Cheers and greetings from Germany.

Offline AWK

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AWK

Offline Tlowlow

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This is good advice - I know that Mythbuster episode as well  ;D.

Fortunately, my students won't have to use such a device. They use conductometers or I got some batteries with LED diodes soldered on. 

Offline AWK

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My students build Daniell's galvanic cells, then use a voltmeter to check their voltage before and after the salt bridge connection (usually 1.08 V). The diode on one galvanic cell shines very weakly. Then students  connect two cells in series - the voltage increases twice and the diode shines clearly.
AWK

Offline Tlowlow

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Oh that is a good activity for serial circuits and conductivity.

I think my problem is my inexperience. I'm supposed to have a context for what I teach that gives the students a problem to solve... something like a bath murder mystery maybe. Under normal circumstances I would probably present the conductivity similarly to that YT link you provided me with and let the students guess what the reasons for the shown phenomenon is and go on from there. However, there will be people watching this lesson and I need to give the students a "real life" context.

Offline Enthalpy

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Some boat speed indicators have a magnet to induce a voltage in seawater due to speed, and electrodes to observe the voltage. The resistivity determines the parasitic resistance of the sensor.

Seawater resistance is the very limit to MHD propulsion for boats. You may compute how much voltage serves to overcome the electromotive force and produce a mechanical power, and how much is lost to resistive losses in water.

Could we obtain electricity from a dam by flowing the speeding water between two magnets and extracting electricity from two electrodes? Or from the Gulf Stream and the geomagnetic field?

Offline Enthalpy

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Take a pocket radio receiver for the FM band (88-108MHz). Fold the antenna in. Pick a radio station barely received. Set the volume loud.

Enclose the receiver in a plastic bag or several, make it water tight (Gefrierbeutel mit Verschluss?) (Good knot? Rubber bands?) (Tupperware?). Fasten the bag at mid-height of a bucket, which needs spacers and probably weights.

Fill the bucket with water. Tap water should suffice, but if not, take deionised water for car batteries. Let observe that the radio station is still heard (through water and so on, not obvious).

Dissolve de-icing salt in the water (or table salt for faster dissolution). Not a teaspoon: 250g/L, so take a true bag of salt. Let observe that the receiver doesn't catch the radio station any more.

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If tap water has 1000Ω×cm=10Ω×m resistivity (this varies horribly!), then 0.2m×0.2m×0.2m have 50Ω. Free space impedance is 377Ω, and since λ/4~0.75m, the loss will be a bit stronger than 377/50≈18dB (the permittivity does a lot too). Most stations are still received. Deionised water would have no significant conductive loss.

25% NaCl in water conduct 22S/m or 45mΩ×m so 0.2m×0.2m×0.2m have 0.22Ω. Losses are now a bit over 64dB, so a station with properly chosen strength disappears.

By the way, this is what makes communications with submarines difficult.

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This needs true preparation before showing it in front of students! Bag watertight, not floating, spacers stable, which radio station, hear the sound through water, water pure enough, time to dissolve the salt... The first trial won't work.

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