Demonstration of Lenz's Law

If you drop the little metal slug down the copper tube, it falls under the pull of gravity, and drops out at the bottom, just as you would expect.

But take the second, apparently identical, slug; drop it down the tube, and hey presto! the slug travels down the tube extremely slowly. You can even see it's slow passage via the window in the side of the tube.

It is a great magic trick, but the truth is just as amazing. The second slug is highly magnetic. Now Michael Faraday, the British physicist, discovered as long ago as 1831 that when you pushed a bar magnet into the opening within a copper coil, an electric current was produced within the coil. This is known as electromagnetic induction.

The Russian physicist Heinrich Lenz observed that the direction of the induced current in a conductor is always such as to oppose the motion which produced it.

So as the magnetic slug moves down the copper tube, a series of transient "eddy currents" are induced in the copper tube, and these currents will create a magnetic field that opposes the downward motion of the slug.

[hlodog]

I honestly feel like this channel deserves to be a top subscribed channel, but at the same time maybe it takes a certain kind of person to watch videos like this. Lovely videos right down to their core.

[grandillusions]

No - we are rubbish at marketing! We made a CD many years ago, featuring a selection of toys from Tim's collection, but these days it is just quicker to go straight to YouTube. And this way we can post several videos every week. And it does not even cost you anything! Although you can always visit our online Toy Shop if you are minded - a few of the toys we show are on sale there ;-)

[amaggard]

lol! "It goes down at a nice speed. It's called... Gravity."

[michelinerobinson]

I'll help with some of the marketing on twitter right now! I really enjoy these videos of him explaining the toys in a rather genteel manner and love that the toys either have a scientific bent or are just pure magic. I am subscribed as often get inspired.

[grandillusions]

The Leprechaun puzzle comes in different sizes. The one at £9 is quite a big version, and is difficult to ship. I sent one to the US the other day. I put it in a big box (which cost GBP1.50) and the air mail postage to the US was over GBP5.00. As that was the only thing the customer bought, we lost money on the deal. But at least I know it will get there in good shape. People selling bits and pieces from their spare bedroom, and not charging for their time will always be cheaper. cont...

[systempatcher]

This is due to the change in magnetic flux as the magnet moves through copper. Lenz's Law states that this creates an opposing magnetic field in the direction that is opposing the change in magnetic flux, hence slowing the fall.

The equation is taken directly from Faraday's Law:
ε= -N(d(ΦB)/dt)
Where N is the number of coils and ΦB is the magnetic flux. The negative sign is what Lenz added to this equation to demonstrate this phenomena.

[grandillusions]

We spend tens of thousands of pounds on shipping every year. Really, there is no such thing as free shipping - it has to be paid for one way or another. We do not have the scale of Amazon, we cannot compete with their prices. But we offer other things - the free videos you have been watching the last few years, real people at the end of the phone, and a free monthly newsletter with lots of editorial stuff, not just things you can but. In the end tho, the customer chooses. I know that.

[ParaglidingManiac]

This channel and your online store are just amazing! Hope you're doing well!

[Dreeev]

I would advice you to put the link to the shop higher up in the description. Might get you more visitors.

[grandillusions]

OK. Remember though that Amazon are big and so get very low prices with suppliers, and - apparently - pay no taxes here in the UK. All helps keep prices low. Also, goods mass produced in China are cheap, partly because they ARE mass produced, partly because wages in China are very low. We choose to focus on items that are hand made in Europe and USA by small producers, often just one person. And some items we make ourselves. Most of our stuff is not on Amazon. And that's the point really.

[devilpunsher]

I love your videos, you make science extremely fun and interesting for me. :) Keep them coming!

[Melthornal]

I wish I had thousands of dollars to buy all the neat physics toys you have been showing.

[grandillusions]

You have a good memory! Now re-shot at better quality.

[matthewlien]

I stumbled across your videos and have enjoyed them very much!

[Steinninn]

"It's called... gravity" I love that. 0:18

[uncle]

Thank you for demonstrating this. It helped me a lot.

[primodernious]

the slowing effect is caused by the magnet moving in a separate frame of reference. the magnet induce a stationary magnetic field in the copper and the magnet resist because it want to move in the same relative frame of reference as the induced field which is at rest. gravity overcome the force of the stationay induced field and forcefully relocate the posistion of the stationary induced field as it drops. the electric field induces the magnetic field after the magnet altered the alignment of particles inside the copper tube 90 degree out of phase with itself.

[LevisLabOfficial]

There could be a back up battery and a switch to activate an electromagnet that way it could go up

[wangus]

And the brakes in tons of high speed rollercoasters. You can even see them steaming after the car goes by - heating up from all that magnetic "friction" (yes, not the right term but you know what I mean)

[superfour]

Does the weight of the tube increase by the weight of the magnet while it is dropping? I suppose it must ...