2 Minute Physics: Electromagnetism

Many of the concepts that are present in special relativity are reserved for objects that are moving at extremely high speeds (i.e. near the speed of light). With that in mind, it is hard to believe that it’s possible to see the effects of special relativity in everyday life. Interestingly, electromagnets, which are used in speakers and automatic doors for example, operate on concepts developed in special relativity.

Electromagnets exist through length contraction. Length contraction is when something such as a particle is moving so quickly that it appears shorter than it actually is. Essentially, an electrically stable wire copper wire will have no effect on any positively charged metal that is sitting stationary near the wire. However, this is not the case when that positively charged metal starts moving with respect to the wire. In this case, there will be repulsion between the metal and the wire.

To understand why this works, imagine being in space with no planets or stars around you. The only other object that exists is an infinitely long train, horizontal to you so that you can see the train’s whole length. You are able to move with respect to the only other object in the universe, the train. However, in your perspective you remain stationary and the train is moving. It is impossible for you to distinguish what is moving without another object to compare. Now imagine that two pipes filled with colored water are looped around the train, one red and another black, and both you and the train are stationary.  In this perspective the black water is flowing to your right. You are set in motion again in the same direction and at the same speed as black water is flowing in the pipe. In this perspective, again, it seems like the train is moving and you are stationary. However, it also seems as if the black water is no longer flowing but instead the red water is. You also notice that the red water is flowing in the opposite direction that the black water had been flowing. Take this perspective and apply it to a copper wire (the train) and the positively charged metal (You). The water is replaced with protons (the red water) and electrons ( the black water). As the piece of metal  moves with respect to the wire, it appears as if the protons are flowing through the wire. The protons experience length contraction as they flow through the wire. The contraction allows for a higher proton density and causes an electrical imbalance making the wire positively charged. In turn, the imbalance repels the positively charged metal , which creates an electromagnet.  

Written by Shane Warga, Edited by Alexis K.

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