Convection currents ....terrible prac

[bigmack]

The are heaps of them out there but this one uses a Take away container sitting on three beakers . One in the middle has hot water , the ones at each end have cool water . 2/3 fill takeawy container with cool water . Add red food colour to bottom of middle , add Blue food colour to each end and watch .

Yay , so far so good , the colours move to show the convection currents .

Now it gets all funky when you are now asked to place two similar sized pieces of craft foam in the middle to simulate tectonic plates .The idea is that the two pieces will move apart from each other moving to the two cooler ends .

Well they don't . They actually are attracted to each other by electro statics and instead collide together .
I''ve tried Paper , paddle pop stick pieces , various foams , even balsa wood in a variety of sizes . All just move toward each other instead of apart .

I saw a You-tube where it seemed to work , but they were heating a glass dish of oil in the middle with a tealight candle .presumably the oil currents are stronger and can tear the electrostatic attraction apart ...I dunno ....I hate pracs that the teachers use that don't work .

Convection.jpg

[bucketchem]

we dont even bother with this one, we do a convection prac with poppy and mustard seeds, and then a separate tectonic plate model prac

[bigmack]

Tending to think we should scrap it too bucketchem .

[RosalieL]

I wonder if you added a few drops of dishwashing liquid, would that be enough to let them separate?

[bigmack]

I wonder if you added a few drops of dishwashing liquid, would that be enough to let them separate?

Sounded like a good idea Rosalie . Alas

[RosalieL]

I wonder if you added a few drops of dishwashing liquid, would that be enough to let them separate?

Sounded like a good idea Rosalie . Alas

oh maybe metho? Or have you tried using oil yourself? even just enough to cover the surface of the water? It would be a nightmare to clean up though...

[Pm706Narrogin]

We do a simplified version of that but scale it up with larger containers so the students can see the convection currents better. We haven't tried putting something on top to simulate plates. All they're looking to do is give the kids an idea of what mantle convection looks like.

The teachers can still use this as a teachable moment because (for different non-staticky reasons) this is why tectonics research has moved beyond the simple notion of convection currents. The two terms they should be emphasising are ridge push and slab pull. Convection currents do play a role - especially in circulating the heat that drives plate tectonics - but the primary driving forces are ridge push and especially slab pull. Science Assist has a great article on this subject. Colin Price is a former exploration geologist turned Earth & Environmental Science teacher who has clearly been driven bonkers by how we teach lower school on this topic.

So fear not bigmack - this is not a failure. This is technically a super accurate representation of what happens when you don't have slab pull helping move things along. Next time a teacher says "the convection experiment is a dud" just reply "of course it didn't work. Where's your slab pull?" (And then duck from whatever item on the bench they've grabbed to throw at you

[RosalieL]

Thank you so much Pm706Narrogin for sharing that! I have just shared it with my teachers. And thanks Bigmack for asking the questions! I love learning so many random things.

[bigmack]

Woow thanks Pm706 .Earth science is definitely not a subject I am knowledgeable on.
Interesting stuff .That extract makes more sense to me than the other idea.

[Pm706Narrogin]

No worries fellow labbies! Sorry I can't help more with getting the prac to work.

I'm biased but earth science is great cause it requires you to apply a bit of everything from the other topics - a lot of chemistry, a fair bit of physics and even a bit of biology. I'll convince these teachers rocks can be exciting yet!