Dr Yan shows you what to expect
|Difficulty: straightforward||Easy once you get hold of the copper and magnet|
|Time/effort: very quick||Just a minute or two|
|Hazard level: low||Do note the safety advice|
SAFETY: Neodymium magnets are surprisingly powerful when they snap towards metal or another magnet. Mind your fingers and don't let an animal or young child play with them.
You do not want a magnet inside your body, so never suck on a pipe that has a magnet in it.
The magnets are quite brittle. Don't let them fall on to hard surfaces (like concrete) from a height – they may shatter into sharp pieces.
Straight copper pipe, at least 30cm long. 15 or 22mm diameter central heating pipe works well. (A hardware store may lend you a piece for a minute just to try this out)
A circular neodymium magnet, sometimes called a rare earth magnet. It needs to be a bit smaller than the inside of the copper pipe, say 10–12mm for a 15mm pipe. (Hardware and electronics stores sell them)
Optional: Straight, plastic pipe of similar dimensions to the copper pipe to use as a comparison
If you have a plastic pipe, hold it vertically over a slightly soft surface such as carpet, a wooden floor or something like a washing up bowl.
Drop the magnet down the tube with its flat sides facing up and down.
Hold the copper pipe the same way and drop the magnet just the same.
The magnet will fall straight down the plastic pipe and emerge out of the other end almost immediately.
In the copper pipe, the magnet slowly hovers downwards. Depending on the diameter of your pipe, the magnet will appear to defy gravity and could take 30 seconds to travel 1m downwards.
This demonstration relies on the magnet moving close to the copper. The size of the magnet compared to the inside diameter of the copper pipe is important. If you don't see any hovering, you may have to find magnets that fit better.
Stacking more than one magnet together in a column will make the effect more obvious.
If you drop a stack of small magnets down a relatively large pipe, they will fall quickly but as they go you should see them tumbling.
The magnet moving near the copper causes a force that acts as a brake, slowing its natural motion down the pipe due to gravity. Where does the force come from?
Electric currents and magnetic fields are closely related. If you have one, you often have (or can make) the other.
The braking force in the demonstration is due to an eddy current. That's an electrical current that forms in the copper, in response to the moving magnetic field of the falling neodymium magnet.
That eddy current produces a magnetic field of its own. The two magnetic fields overlap and slow the fall of the magnet.
The science behind this braking force was described in the 19th century by physicist Heinrich Lenz who devised Lenz's law. Find more through the Go further web links top right.
Fields, eddy currents, forces... if this activity has got you wondering, Dr Yan is Bang Goes the Theory's online expert.
Atoms, the tiny, invisible particles that make up every substance around us, contain even smaller particles called electrons. The way that those electrons arrange themselves in a substance influences how it behaves near a magnet.
At a very simple level, electrons tend to pair up. If they can't do that, lone electrons are left hanging around. Unpaired electrons are important in creating strong magnetic effects.
Magnetism to many people means the way fridge magnets stick to the side of a fridge that's made of steel. That kind of magnetic attraction is quite a strong effect. It is called ferromagnetism.
Some substances that don't appear to be magnetic do in fact show a different, much weaker effect. Copper is one such material. It is repelled (pushed away) by powerful magnets, due to its diamagnetism.
Water is also diamagnetic so things with lots of water in them can be pushed around using your neodymium magnet. Try seeing what sorts of fruit have enough water in them that you can push them around magnetically. There's one suggested method in the web links top right on this page.
In a laboratory, the repulsion of water by really strong magnets (more powerful than the neodymium ones here) has even been used to make small animals levitate. Please don't annoy any pets or wild animals with whatever magnets you have. You won't be able to move them and could harm them.
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