The SFK Project: #7 compass and anti-gravity miracleFollow article
What you need:
- 4 Neodym magnets, cylinder 15 mm diameter, 8 mm height (search google for “super magnet” store, SFK #7 needs just 2 magnets but in SFK #8 we will use all 4!)
- 50 cm Copper tube, 22 mm diameter (will also be used for SFK #8)
- low drinking glass
- soup plate filled with water
- small piece of wood
- cardboard tube
- blunt table knife
- sticky tape
- gaffer tape
- black and red waterproof marker
What you will get at the end:
A compass and an anti-gravity miracle
Words of warning:
An adult should always supervise these experiments. Please do not leave your children alone with strong Neodym magnets and knives. When strong magnets bang together, they can severely squeeze skin and fingers. It will be impossible for kids to separate two of those magnets when they cling together. An adult may use a blunt table knife to carefully get the blade between the magnets and the slide them sideways to separate them again:
When you simply lay two of those magnets on the table, the magnetic force will attract them even over many centimetres, and they also jump over barriers to end up clinging together. So use the table knife to stick unused magnets to it. This way, they will stay where they are:
When operating the “electric worm” for more than a few seconds, the battery and the magnets may get extensively hot and could burn the skin. Let not touch your children the battery or the magnets before they have cooled down again.
Do not drop the magnets on a hard surface from a distance. Neodym magnets are hard but brittle and may burst when crashing in the floor.
How to build:
Look for a very plain surface. Do you know which direction north is? That’s where you will never see the sun. At midday, the sun shines from the south. Mark the direction to the north with an arrow.
Now place one of the magnets with its flat sides upright on the plain surface. Give the magnet a little twist, and it will automatically turn with one of the flat sides pointing to the north and the opposite side to the south. This only works on a very plain surface. I've put a shot of a successful go in the video below.
Please mark this side of the magnet with a black “N” and the opposite side with a red “S”.
If it doesn’t work for you, we will use a soup plate full of water as a plain surface. But we need the magnet to swim on the water. So we need a little boat made of a piece of wood. You can use sticky tape or blue tack to fix the magnet to the boat. When you place it on the water, it will slowly turn until one of the flat sides of the magnet shows to the north. Mark this side with an “N”, the opposite side with a red “S”:
Over many hundred years, people used such a compass to find the correct direction when travelling, especially on the sea when you have no trees, mountains or buildings as points of orientation.
We need to find the north and south “poles” of each magnet. Here is a way to make this as easy as possible: Lay one of the magnets on the bottom of a low drinking glass. Let the other magnet cling to it but with the bottom of the glass in between. As we have learned in our SFK project #4 (“Magic Levitation”) north attracts south and repels north. So when your magnet in the glass lays with “N” Pointing to the bottom, The magnet underneath will cling with “S” to the glass, and you can mark the opposite side with an “N”.
The anti-gravity miracle
I’ve attached my cardboard and copper tubes to a wall to demonstrate this little miracle. But you might also simply hold them in your hand. When you drop a pair of magnets from our worm into the cardboard tube, they will fall very quickly through the tube and come out at the bottom end in less than a second. The gravity pulls the magnets to the bottom. But what happens if you drop the pair of magnets into the copper tube? The magnets somehow magically get slowed down. They take about five seconds to travel through the copper tube until they fall out of the end. In the video, I’ve placed an old stopwatch on the table so you can see how long it takes.
You have seen so many funny and exciting things now. It’s time to understand why all this is working the way it does.
How it works:
Please read FSK project #4 to get an explanation of “magnetism”, “magnetic field” and the “poles” of a magnet. I’ve already mentioned there that our earth is a giant magnet with a north and a south pole. And you’ve learned that a magnet’s north pole is always attracted by another magnet’s south pole. So why then, you may ask, does our magnet’s north pole points to the north of our earth? Well, that’s an excellent question. It is all about naming. When people detected that magnets always turn in a certain direction, they called the end of the magnet, which points to the Northpole of our earth, “north”. But that means that the giant earth magnet does indeed point with its south pole toward the Northpole:
Today we better do not use the term “magnetic north pole of the earth” because that could mean the Northpole (also called the Arctic) or if used in a correct way it would mean the earth’s Southpole (also called the Antarctic). We better say the “arctic magnetic pole of the earth” or the “antarctic magnetic pole of the earth”.
In FSK project #3 you have learned that you can reverse the interaction between a magnetic field and electric current: When you move a magnet over a wire, you “induce” an electric current in this wire. A changing magnetic field is always producing current in a conductive material. A bicycle’s dynamo is working this way: The wheel of the dynamo is turning a magnet inside a coil. This produces an electric current which can make you bike’s lamps lighting.
When we dropped the magnet into the copper tube, the moving magnet caused a changing magnetic field. And this, field induced electric current in the copper tube happens because copper is a very conductive material. The induced currents are somehow “chaotic” because they look for their best way in the material. There are small eddies of currents, like the eddy of water when you empty a basin. That’s why such currents are called “eddy currents”.
The interesting thing is that such an eddy current turns the copper into an electromagnet. The direction of the electromagnet is always so that it “works against” the magnetic field which caused the eddy current. It kind of wants to stop that field from changing. How can it stop the field changing? By slowing down the Neodymmagnet. But once the Neodymmagnet does no longer move, the field is constant and does no longer cause eddy currents. But without eddy currents, nothing is braking the Neodymmagnet from falling down. So it starts moving again. The whole system gets in a state where the magnet is still moving but much slower due to the eddy currents.
By the way:
The compass has been used for many hundred years. The Chinese people knew about magnetic stones (“Magnetit”, also called “Lodestone”) since over 2000 years and they have used them as a compass and other things.
You may know that our earth is like a ball rotating around an axis once per day. This axis runs from the geographic Northpole to the geographic Southpole and in 90° to the equator. The axis of the giant earth magnet is not the same. It is slightly tilted away from the axis of rotation like in this picture:
So the arctic magnetic pole is many kilometres away from the geographic Northpole. And it is not at a stable position but hops around by about 50 km per year. This makes navigation with a compass difficult. Good to know that we can nowadays navigate with GPS. The earth magnet also changes its strength over time. And his already switched its orientation (north became south and south became north) several times. The last flip was about 780,000 years ago.
Slowing a movement down by eddy currents is a principle used in “induction brakes” (also called “eddy current brakes”. High-speed trains are using such brakes:
You can also accelerate things with eddy currents. This is used in energy meters like this one:
The electric energy which is measured is flowing through a small coil. This coil is positioned above the surface of an aluminium disk which can rotate. The alternating current (“AC”) of your home electricity produces an alternating magnetic field when flowing through the coil. This alternating magnetic field is like a moving magnetic field: It induces eddy currents in the disk which let the disk rotate. The number of rotations is counted by the energy meter.
Eddy currents are cool, but they can make things hot like every electric current does. Have you heard of induction stoves? Maybe you even have one? They use a very fast-changing magnetic field to induce eddy currents in a pan or a pot. These currents heat the metal pots and the water or food inside:
Where I’ve got the idea from:
This time things are coming from my time at school. See: you can learn cool things at school :-)
What’s coming up:
Next week in SFK #8, we will build an electric worm!