June 10, 2023

The Beauty of a Copper Pipe and Magnet

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To put the title of this article into context I think a description of the use of the demonstration I am talking about is in order.

Picture a year 6 transition open evening, the school is busy with parents and prospective students, as a Science department you want to show the joy involved in your subjects but at the same time demonstrate the academic nature. You want the prospective students and their parents to get an idea of the expectations within Science alongside how knowledge rich and creative the Science curriculum is. 

The school Science ambassadors are enlisted to demonstrate some of the concepts covered on the curriculum. You have some mechanics experiments setup, light gates and motion sensors at the ready. A laser is being used to transmit audio to a receiver with some of the latest popular music. Marshmallows are undergoing the full effects of a vacuum and as always the teacher says, ‘No you can’t eat these….they are being used for a Science experiment and have been in the prep room for about 2 years.’

There by a lab table watching the hustle and bustle is that Physics teacher. All they have in front of them is a copper pipe, roughly between half a metre and a metre ±0.001m. An eager and confident Y6 student arrives at the table and challenges them to entertain. The teacher picks up a magnet and holds the pipe vertical, they drop the magnet freely falling next to the pipe, nothing to see here. The teacher takes the magnet back to top of the pipe anticipation building, the pressure to perform, the magnet drops into the pipe, waiting, waiting and more waiting. The prospective year 7 looks confused and baffled why no magnet, suddenly they hear it drop onto the table. The great part is the discussion that follows, why did that happen? How could this be? Are some of the questions of disbelief and confusion the need for knowledge, an explanation? What follows next should be as good if not better that the demonstration. 

Our society and the technological developments we have seen over history hinge on an understanding of an equation. In the words of the Spice Girls, ‘When two become one’. An English experimental Scientist called Michael Faraday and his relentless work on electromagnetic effects produced results that led to practicality of using electricity in technology. The next Scientist involved is Emil Lenz, a Russian working on electromagnetism separately. Step in, figuratively, the ‘Spice girls’ or Franz Ernst Neumann, Physicist and Mathematician. Franz derived the mathematical relationship of Faraday’s law of electromagnetic induction, combining the knowledge of Lenz and Faraday.

Faraday’s law discovered experimentally tells us electromotive force (EMF) can be induced when there is relative motion between a magnet and a conductor. A coil has the greatest EMF induced when the normal of the coil is at right angles to the magnetic field, figure 1.

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Figure 1

Lenz’s law, figure 2, tells us that the induced current is in such a direction to oppose the change in magnetic flux causing it. Magnetic flux is given by the product of the magnetic flux density (field strength) and the area of the coil. In figure 2 we can see a conductor being moved at constant velocity in the direction of the green arrow. The EMF on the electrons causes a flow in the direction of the red arrows and conventional current flow is in the opposite direction to this. We now have a current carrying conductor in a magnetic field which experiences a motor force. The direction of this force is in the opposite direction to the constant velocity, can be deduced using Fleming’s Left hand rule, if you are so inclined. This is a statement of conservation of energy, work must be done to maintain the constant velocity and hence the induced EMF.

Figure 2

Franz Ernst Neumann derived the mathematical relationship of Faraday’s law, this incorporates Lenz’s law, in Equation 1, responsible for the negative sign.

There are two strands to this article, the first to discuss in simple terms the Physical phenomena that our technology and society is built on. Without this knowledge we would not have the simple things we take for granted at home, more so now than ever before.

The second is a discussion of practical demonstrations, the discussion and story that can follow and how inspiring they can be to budding Scientists. In an age of computer simulations and virtual lab software, all of which can be excellent, do not dispose of the copper pipe and the magnet. The demo could be simple, lacking in moving parts, technology etc, but the discussion of the theory, history of experimentation and knowledge are priceless.

Do that demo that you feel could be cut from the curriculum as time is limited, they have more impact than the credit they sometimes get.

Featured photo by Ilze Lucero on Unsplash

Head of Science, CSciTEACH and CPhys. Love the history of Science and Physics.

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