From U of Torun

Low-Tech experimental hands-on KIT

Nicolaus Copernicus University in Torun,Poland

8. Oersted’s experiment – vertical version (or Ampere’s experiment)

We place four small magnetic needles around a vertical straight wire (or better a few wires in parallel), which is part of an electrical circuit. When the switch is in the open position, we see that the all compasses needles are in the north direction. (If they are too close they tend to couple one with another and to form a kind or ring even without the current in the wire).

When we close the switch and the wire carrying an electric current, the compass needles turns with respect to circles around the wire. In this experiment we show that electric current could be the source of a magnetic force, and that the lines form circles.

The Ampere’s part of the experiment is that the intensity of the field falls down as the inverse of the distance from the wire, and is proportional to the current (1)

This follows from a general formulation of Ampere’s law applied for a circular symmetry. As the direction of the current matches as well, if current in two wires are opposite, the field is null.

11. Falling piece of magnet/metal in tube of copper with/without slit(s)

Lenz Current Tubes (two types) – dropping neodymium magnet down in 0.5 meter long, copper tube, causes a slow fall of the magnet. The moving magnetic field creates a strong opposing magnetic field, due to eddy currents in the cylindric conductor. The fall takes about 20 seconds in our set-up.

Cutting the tube in a special way (©GK), the fall is fast – demonstrating that direction of eddy currents is perpendicular to the direction of the magnet movement. The fall takes only about 12 seconds.

13. Pohl experiment (Lorentz’ force)

In the experiment shown below, a piece of cooper wire has been placed in a magnetic filed produced by three strong magnets. Copper is non-magnetic, so it is feels no force from the magnet.

However, with a current passing through it, there is a force on the wire. The force arises because the current produced its own magnetic filed, which acts on the poles of the magnet.

The force is increased if the current is increased, a stronger magnets are used or if the length of wire in the filed is increased.

When we change the direction of the current in the circuit, the wire movers in opposite directions.

14. Chaotic magnetic pendulum

The platform of this toy is divided into six areas describing six different situations in a game of football: scoring a goal, a penalty kick, a corner, a foul, a throw-in and off-side. There is
a magnet hidden under each area and there is also one in the ball. The bottom pole of the ball and the top poles of the platform are of the same sign. The ball avoids stopping above any of the areas.

The movements of the ball over the magnets is absolutely chaotic. Sometimes you get the impression that the ball is going to stop above one of the areas, and that it is suddenly attracted to another area. Theoretically, it cannot be foreseen where the ball is going to stop. Even a small change of the initial ball position leads to a different result, which is a characteristic feature of the chaotic movement.

A different version of the magnetic pendulum. The movement of it depends on many factors, like the friction (it can be enhanced if the pendulum is submerged in a liquid), the gravity force (which changes the relative direction if the vertical positioning of the pendulum changes), the attracting or repelling force of magnets (different magnets have slightly different strengths and configurations so they never act with exactly the same force).

15. Magnetic construction sticks and balls (many small tasks, experiments)

Magnetic Sticks - demonstrate that magnets have always two poles - sticks placed by the same poles repel each other, but when stainless steel ball is put between, they attract each other, the effect being caused by magnetic domain re-ordering in balls.

The iron ball inserted between poles of different signs (the upper figure) et magnetised in a "normal" way, becoming a two-pole magnet (N-S), with the axis oriented in the direction of external magnets.

The ball between two poles of the same sign "accommodates" (the lower figure) its magnetic poles in a way to be attracted by both external magnets. We find the "missing” poles on a plane perpendicular to the axis of magnets.

You can create amazing constructions out of those colourful sticks. But at least every two magnets you have to place a metal ball between them. If you don't, the whole thing will fall apart.

Now think carefully: magnet either repel each other or attract each other. If one end of the stick attracts another stick, the other end of the first stick should repel the second stick. And it would be so if we didn't place the metal ball between the poles.

16. Set of toy magnets (illustrating multipoles)

Under the transparent foil of magnetic sketcher, there are microscopic iron filings floating in a special solution. Moved by the magnetic force of a magnet, the filings attach to the foil. The magnet in the shape of a strip on the other side of the board is used for erasing your doodles.

The screen of the board is divided into hexagonal cells to avoid drawing all the solution in one place. The paraffin oil is the solvent here, and the magnetic material consists of iron file dust or, better, iron oxide. The iron oxide, the magnetic material of the first Chinese compasses (which was discovered in Europe by sailors from the Italian town of Amalfi, South of Naples) is called magnetite.

The magnetic sketcher works on the same principle thanks to which the iron nails or powders orient themselves in the magnetic field. They get magnetised by the external field and follow the magnetic field lines. Such an aligned configuration has a lower energy than separate grains would have.

We placed four magnets on the magnetic sketcher and we can observe three magnets with two poles and one with four poles.

17. Flux detector (needed for multipoles and other mini-experiments)

The special paper laminated in this amazing card shows the pole locations on any magnet. Two magnets may look the same physically but have radically different magnetization patterns. With this card you can see how many poles a magnet has, and where they are on the magnet. The card is ideal for conveying the concept of magnetic fields and poles.

With this card you can see how many poles a magnet has, and where they are on the magnet. The card is ideal for conveying the concept of magnetic fields and poles.

This is a different kind of magnet.

Using magnetic filed viewer we can observe four poles of the nice blue magnet.

18. Atomic force microscope model (reference to a practical application)

In the experiment shown below, we present a model of atomic force microscope. A thin metal belt (used in anti-robbery shop systems) is moving slow and attracting to the blue "paper" three times.

There are placed three magnets under the paper and they attract real the metal belt. In the same way works atomic force microscope.

19. Tile of 4 magnetic rings around a stick - levitating magnets.

The toy consists of four flat disc-like magnets with a whole in the centre. The magnets are threaded on a plastic pole. The magnets are very unsociable - whenever you try to push them close to each other they oppose and they even push back your hand. But if you turn them they will stick to each other and it will be very difficult to separate them.

Notice that the middle magnet levitates closer to the bottom one, and not somewhere in the middle. This is because it is pushed down by the weight of the top magnet. The middle magnet supports the top one, but is at the same time pushed down. Each action has its reaction, as was stated by Newton in his Third Law.

Description by Andrzej Karbowski, with the use of material from CD "Physics and Toys",

Description by Anna Kamińska, under Grzegorz Karwasz supervision.

Translations by Gloria Zen, and Studio Conrad, Krzysztof Kowalski, Olsztyn.

Photos by Krzysztof Sluzewski.