the world in a different light
For my students to discuss. Please leave your answer and explanation in the comments below. Options are:
A. $$V_0$$ (peak voltage)
B. $$\frac{V_o}{\sqrt{2}}$$
C. Zero
D. None of the above
This is an interesting question on electricity: in order to survive a lightning strike, which of the following costumes offer the best protection? A coat of armour, your birthday suit, a wetsuit or a superman costume? Watch this MinuteEarth video on Faraday’s cage to find out!
This demonstration is called Newton’s nightmare because it involves the slow dropping of a magnet that seems to be inconsitent with gravitational acceleration.
Using the “CFILE” structure, we can explain how the magnet moves much slower in a metal pipe than when it is undergoing free fall (as in the PVC pipe, which serves as a control).
Now, the metal that we use cannot be ferromagnetic, or the magnet will not even drop at all. It will simply be attracted to the pipe and stick to it.
However, if another metal such as copper or aluminum is used, as the magnet moves through the pipe, different sections of the pipe will experience either a change (either decreasing or increasing) in magnetic flux. Sections of the pipe that the magnet has just gone through suffers a decreasing flux while those that the magnet are approaching gains magnetic flux.
By Faraday’s law, which states that an induced emf is proportional to the rate of change of magnetic flux linkage, emf and hence, current is induced within the pipes. These induced currents are called eddy currents.
By Lenz’s law, the induced currents tend to flow in a way so as to oppose the change causing it. The current in the sections of the pipe that the magnet is leaving will trying to attract the magnet while those that the magnet is approaching will try to repel the magnet.
The effect is that the magnet experiences a retarding magnetic force that acts against gravitational force, hence decreasing its downward acceleration.
I didn’t want to spend money on buying a piece of pyrolytic graphite and large neodymium magnets so I made do with what I have to make the following video. While diamagnetism is not in the A-level physics syllabus, it’s good for students to know that there are other classifications of magnetic materials.
What we study in our syllabus is ferromagnetism, which is exhibited by materials such as iron, cobalt and nickel. Some pencil leads are paramagnetic (weakly attracted to magnets) while others such as the one in the video are diamagnetic (repelled by magnets).
I bought my neodymium magnets from DX.com and the shipping to Singapore takes about 3 weeks, so you might want to factor that time in if you want to get some for your lessons. These magnets are great for other demonstrations such as homopolar motors and Newton’s nightmare.