The magnetic river

Many years ago, I was lucky enough to work in “Launch Pad” in the science museum. There were lots of wonderful activities and the following blog posts are about the activities. Launch Pad has been replaced by the equally wonderful “Wonderlab”

https://science-projects.org/wp-content/uploads/2016/10/ch2.pdf

https://www.sciencemuseum.org.uk/home

https://www.sciencemuseum.org.uk/see-and-do/wonderlab-equinor-gallery

The magnetic river

https://www.youtube.com/watch?v=OI_HFnNTfyU

https://www.youtube.com/watch?v=0_QBl6-_jJU

https://www.youtube.com/watch?v=Yvfz51moMaQ

https://en.wikipedia.org/wiki/Linear_motor

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When a piece of metal was placed on the magnetic river it was hurled in straight lines across the glass plate. The motor underneath the glass produced a flow (like a river) of magnetism which could transport metal objects at high speeds.

The magnetic river was interesting because it used a motor which moved in straight lines (motors usually spin round) and transported aluminium, which is not a magnetic material.

The exhibit involved linear motors, eddy current and a process called induction.

How does it work?

1) Linear motors

Parts of the motor were visible under the glass plate and looked nothing like the motor parts found in washing machines, lawn mowers and electric fans which usually spin round. Linear motors produce straight line motion.

One way to produce this effect would be to have lots of ordinary permanent magnets moving around and around on a conveyor belt. This would produce a linear flow of magnetism as seen on the exhibit.

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Instead of ordinary magnets the linear motor used electromagnets which produced a stronger pulling force. When an electric current was passed through the coils that make up the electromagnets they behaved as magnets. Turning off the current turned off the magnetism.

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In the magnetic river a changing electric current was fed through the coils in turn producing a moving magnetic field above the glass plate, the magnetic river.

2) Eddy currents and induction

The puzzling thing about the magnetic river was that the metal effected by the electromagnetic coils was aluminium, because aluminium, unlike iron, is not a magnetic material. If you put a magnet near a piece of aluminium nothing happens as aluminium is not attracted to ordinary magnets. There must have been something else operating in this apparatus.

Aluminium behaved like a magnet because eddy currents were induced in the metal.

Eddy currents are formed if something is done to cause a change in the magnetic flux (lines of magnetic force) in a piece of metal.

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You can see these lines of force with the help of iron filings.

The flux change can be brought about by either moving the metal in a constant magnetic field or subjecting the metal to a changing magnetic field.

The main scientist responsible for discovering this behaviour is Michael Faraday.

https://en.wikipedia.org/wiki/Michael_Faraday

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Michael Faraday FRS (22 September 1791 – 25 August 1867) was an English scientist who contributed to the study of electromagnetism and electrochemistry. His main discoveries include the principles underlying electromagnetic induction, diamagnetism and electrolysis

https://en.wikipedia.org/wiki/Michael_Faraday#Electricity_and_magnetism

When Faraday wrapped two insulated coils of wire around an iron ring, he found that upon passing a current through one coil, a momentary current was induced in the other coil. This phenomenon is now known as mutual induction.

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A diagram of Faraday’s iron ring-coil apparatus. Eviatar Bach

Faraday’s experiment to try to induce a current from a magnetic field, with a battery on the left, an iron ring in the centre, and a galvanometer (a type of ammeter) on the right. This diagram is based on one found in page 263 of Physics: Principles with Applications, fifth edition, author Douglas C. Giancoli, illustrators Patrice Van Acker and Tamara Newnam Cavallo. The explanation in the book reads as follows, with the left coil being X and the right coil Y: “In his attempt to produce an electric current from a magnetic field, Faraday used an apparatus like that shown in Fig. 21–1. A coil of wire, X, was connected to a battery. The current that flowed through X produced a magnetic field that was intensified by the iron core. Faraday hoped that by using a strong enough battery, a steady current in X would produce a great enough magnetic field to produce a current in a second coil Y. This second circuit, Y, contained a galvanometer to detect any current but contained no battery. He met no success with steady currents. But the long-sought effect was finally observed when Faraday saw the galvanometer in circuit Y deflect strongly at the moment, he closed the switch in circuit X. And the galvanometer deflected strongly in the opposite direction when he opened the switch.[…] Faraday concluded that although a steady magnetic field produces no current, a [changing one can]. Such a current is called an induced current.”

https://en.wikipedia.org/wiki/Faraday%27s_law_of_induction

https://en.wikipedia.org/wiki/Electromagnetic_induction

In subsequent experiments, Faraday found that if he moved a magnet through a loop of wire an electric current flowed in that wire. The current also flowed if the loop was moved over a stationary magnet. His demonstrations established that a changing magnetic field produces an electric field; this relation was modelled mathematically by James Clerk Maxwell as Faraday’s law, which subsequently became one of the four Maxwell equations, and which have in turn evolved into the generalisation known today as field theory. Faraday would later use the principles he had discovered to construct the electric dynamo, the ancestor of modern power generators and the electric motor.

https://en.wikipedia.org/wiki/James_Clerk_Maxwell

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James Clerk Maxwell FRSE FRS (13 June 1831 – 5 November 1879) was a Scottish scientist in the field of mathematical physics. His most notable achievement was to formulate the classical theory of electromagnetic radiation, bringing together for the first time electricity, magnetism, and light as different manifestations of the same phenomenon. Maxwell’s equations for electromagnetism have been called the “second great unification in physics” after the first one realised by Isaac Newton.

Near the end of his career, Faraday proposed that electromagnetic forces extended into the empty space around the conductor. This idea was rejected by his fellow scientists, and Faraday did not live to see the eventual acceptance of his proposition by the scientific community. Faraday’s concept of lines of flux emanating from charged bodies and magnets provided a way to visualize electric and magnetic fields; that conceptual model was crucial for the successful development of the electromechanical devices that dominated engineering and industry for the remainder of the 19th century.

In the magnetic river the aluminium was subjected to a varying magnetic field as the coils were magnetised in turn.

Eddy currents can be induced in any electrical conductor, and as aluminium is a conductor they are produced as soon as the current was turned on.

Eddy currents in the aluminium plate produced its own magnetic field and this reacted with the field from the electromagnetic coils. It happens that these two fields repelled each other and because the aluminium was free to move it got hurled to one side.

If the glass could have been removed the aluminium plate would have hovered in mid air whilst it was shifted to one side.

Using linear motors

Some railways use linear motors to move their trains.

https://en.wikipedia.org/wiki/Maglev

Maglev (derived from magnetic levitation) is a system of train transportation that uses two sets of magnets: one set to repel and push the train up off the track, and another set to move the elevated train ahead, taking advantage of the lack of friction. Along certain “medium-range” routes (usually 320 to 640 km), maglev can compete favourably with high-speed rail and airplanes.

https://en.wikipedia.org/wiki/Incheon_Airport_Maglev

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The Incheon Airport Maglev is a maglev line in South Korea opened on February 3, 2016. It is the world’s second commercially operating unmanned urban maglev line after Japan’s Linimo. It specifically utilises electromagnetic suspension (EMS) and linear induction motor (LIM) propulsion

https://en.wikipedia.org/wiki/Linimo

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Linimo approaching Banpaku Kaijo Station, located at the main gate of Expo 2005, westbound towards Fujigaoka Station in Nagoya on the line’s opening day.

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