Electromagnetism
Posted: 23 April 2021
In this article I hope to share some useful resources and help tie these topics closely together. The first one is, of course, the spark website produced by the institute of physics:
In physics, we deal with forces and we need a language to describe the areas in which those forces would have effect. A field then is a region in which each point is affected by a force. Objects fall to the ground because they are affected by the force of earth’s gravitational field, if a magnet is causing an effect on a compass, for example, then the compass would be described as within the magnets magnetic field.
This concept of a field is often a hard one for students to swallow but there are several things we can do to show the field.
The first in these is an activity I like to do even at A-level; by placing plotting a plotting compass near a bar magnet it is possible to draw a chart or picture representing the effect of the magnet on the compass at any given point. We describe these illustrative lines as “field lines”.
Though it’s a common misconception, field lines are not real, though we talk of them as if they were because they are such a useful mental image.
The other classic visualisation, though fantastic, is often the cause of the misconception. By sprinkling iron filings round a magnet you can show the effect of the magnet at each of the points where there is a bit of iron. (top tip, put the magnet in a plastic bag before sprinkling)
One of the great mysteries of Physics is the fact that the effects of the permanent and induced magnets mentioned above can be reproduced with only a wire and a current. In fact there’s a classic investigation where a set-up like this:
We had better start from the beginning –
Oersted made the discovery for which he is famous in 1820. At the time, most scientists thought electricity and magnetism were not related, though there were some indicators to the contrary – it had long been known that a compass, for example, when struck by lightning, could reverse polarity. Oersted had previously noted a similarity between thermal radiation and light, though he did not determine that both are electromagnetic waves. He seems to have believed that electricity and magnetism were forces radiated by all substances, and these forces might somehow interfere with each other. He was not far off!
The discovery that bears his name came about during the setup for a lecture demonstration, on April 21, 1820. Oersted noticed that when he turned on an electric current by connecting the wire to both ends of the battery, a compass needle held nearby deflected away from magnetic north, where it normally pointed. The compass needle moved only slightly, so slightly that the audience didn’t even notice. But it was clear to Oersted that something significant was happening.
A few months later he published his results in a pamphlet, which was circulated privately to physicists and scientific societies. His results were mainly qualitative, but the effect was clear–an electric current generates a magnetic force. The publication caused an immediate sensation, and raised Oersted’s status as a scientist. Others began investigating the newly found connection between electricity and magnetism.
French physicist André Ampère developed a mathematical law to describe the magnetic forces between current carrying wires for whom we name the Amp and about a decade later Michael Faraday demonstrated that a changing magnetic field induces an electric current. Following Faraday’s work, James Clerk Maxwell developed Maxwell’s equations, formally unifying electricity and magnetism and in turn formalised the fundamental nature of the speed of light spawning relativity!
We can calculate the strength of this interaction using one of Fleming’s excellent mnemonics – the left-hand rule for electric motors.When current flows through a conducting wire, and an external magnetic field is applied across that flow, the conducting wire experiences a force perpendicular both to that field and to the direction of the current flow (i.e they are mutually perpendicular). A left hand can be held, as shown in the illustration below, so as to represent three mutually orthogonal axes on the thumb, fore finger and middle finger. Each finger is then assigned to a quantity (mechanical force, magnetic field and electric current). The right and left hand are used for generators and motors respectively.
(a) the current on the wire I,
(b) the length of the conductor in the field L,
(c) the sine of the angle θ that the conductor makes with the field , and
(d) the strength of the field – this is measured by a quantity known as the magnetic flux density B of the field.
Now that we have the difficult bit out of the way (the idea that electricity and magnetism interact) it’s relatively easy to build devices that exploit this effect. The simplest motor is probably the homopolar motor:
Though it’s possible to show it in all manner of fantastic ways:
The rotary electric motors that power so much of our world are the very same physics as above, the only complex part is how to stop the wires of the coils from tangling up as the motor spins – this is accomplished by the commutator, the function of which can be seen below.
Bruce Yeany has a great video on paper speakers here:
As with so many things in physics – there is a mirror to the motor effect. The work of Faraday tells us that “Any change in the magnetic environment of a coil of wire will cause a voltage (emf) to be “induced” in the coil”.
In other words, no matter how the change is produced, a voltage will be generated. The change could be produced by changing the magnetic field strength, moving a magnet toward or away from the coil, moving the coil into or out of the magnetic field, rotating the coil relative to the magnet, etc.
Phet has a great simulation of this:
It’s a small step from realising that the effect is reversible to the transformer, a device that uses this interchange between oscillating magnetic and electric to useful effect, either increasing voltage or decreasing it.
