Electricity and Magnetism
Electricity and Magnetism
Human civilisation has dealt with magnetism for millennia, earliest evidence dating as far back as 1000 BC where the ancient Chinese civilisation discovered a naturally occurring magnetic ore, magnetite. This ore, commonly known as lodestone was used by the ancient Chinese as a geomagnetic compass, however, no one really knew what magnetism was at the time. For centuries on, we have had basic knowledge regarding electricity based upon static electricity found from rubbing amber and fur. However, up until the early 1800s, our understanding of electricity and magnetism was severely limited – considering both of them as entirely separate phenomena. Before we delve deeper into the topic, what is electricity and magnetism? Electricity is, simply, the flow of electrons on a conductive medium.
Electricity is produced when an electric charge moves along some sort of medium such as the conventional current. Magnetism is a force related to an electron’s orbital angular motion around the nucleus and its spin magnetic moment. From this we can observe that both phenomena have one thing in common – they both involve electrons. The link between electricity and magnetism was not confirmed until Romagnosi, who in 1802 noticed that connecting a wire across a voltaic pile deflected a nearby compass needle. Yet this discovery was not widely known until Oersted’s accidental findings in 1820. During a lecture at his house on April 21 1820, he noticed a compass needle deflected from magnetic north when an electric current from a battery was switched on and off, keeping the abnormality to himself which he later experimented on.
However, Oersted could not explain the link between electricity and magnetism and published his findings with no explanation. So what is the link between electricity and magnetic fields – or a phenomenon which we now call electromagnetism? Through Oersted’s discovery in 1820, Andre-Marie Ampere, a French physicist and mathematician began his own experimentation. Ampere had discovered that a current running through two parallel wires will either attract of repel each other based on the direction of each current (whether they ran in same or opposite directions). His findings led to the discovery of Ampere’s Law which is given by F/L=k (I_1 I_2)/d
Where F = force between the wires, L = length of wires, k = constant (2 * 10-7), I = currents and d = distance between the wires. His works lead him to devise a physical understanding of the electromagnetic relationship, proposing a theory of an existence of an electrodynamics molecule, the forerunner idea of the electron. Further researches and development of electromagnetism was conducted by Michael Faraday to which he discovered electromagnetic induction in 1831. Faraday predicted that when coils are attached on opposite sides of a metal ring, when a current is passed through one of the coils, the current will travel through the ring and onto the opposite coil. This phenomenon is known as electromagnetic induction.
Electromagnetism has many uses; among them are principles which are the basis of a variety of today’s technologies. Such principles include the motor principle, solenoids and of course mass spectrometry. Mass spectrometry, in particular represents one of humanity’s most advanced and useful inventions. It is essentially the science of displaying the mass spectrum of molecules comprising a sample. It is widely used in the fields of science, in particular chemistry, to determine the mass of isotopes or the chemical composition of a certain sample (space meteorites and a variety of rocks and minerals). This technology relies on the mass-to-charge ratio of particles which involves the ionisation of the sample.
Mass-to-charge ratio is a physical quantity and two particles with the same mass-to-charge ratio move in the same path in the vacuum when subjected to the same electromagnetic field. This principle plays a significant role in mass spectrometry for it is the basis of detecting and analysing materials. A mass spectrometer works by first vaporising the particles and then ionising them by knocking off some electrons in an electron trap to produce positive ions. The ions are then accelerated so that they have the same kinetic energy. These accelerated ions are then passed through a vacuum tube (remember mass-to-charge ratios), subjected with an electromagnetic field.
Lighter ions will be deflected more and the more positively charged they are (less electrons / more electrons knocked off) the more they are deflected. One can alter the strength of the field to accommodate for the mass-to-charge ratio range needed to be detected. Accelerated ions which are still within the path of the tube are then captured by a detector which analyses the amount of ions carried through the ion beam. The vacuum tube where ions were accelerated through is called a mass analyser. The relationship between the accelerated ion beams and the electromagnetic field being subjected is best summarised through Newton’s 2nd Law and Lorentz’ Force Law: F=ma
F=q(E+V ×B) Where F = Force, m = mass of ions, a = acceleration, q = ionic charge, E = electric field and V x B = the vector cross product of the ion velocity and the applied magnetic field. When a charged particle is accelerated perpendicular to a magnetic field, it will curve, hence the deflection of ion beams in the mass analyser. This deflection can be best described as a centripetal force and the force applied on the charge can also be given by F = qv x B. Since the force is perpendicular, the magnitude of the forces is simply vB. F=(mv^2)/r=qvB
F=mv/qBr Where r = the radius of the path; B = magnetic field, m = mass of the charged ion, v = velocity of the ion and q = charge of the ion. We can now see the mathematical principles behind the velocity selector of a mass spectrometer which is the foundation of the technology. Ever since the discovery of electromagnetism, society has always made use of the principle and applied it to the frontiers of science. The first account of electromagnetism being practically applied is at 1895 when Guillermo Marconi, using Heinrich Hertz’ discovery of electromagnetic waves, used it to send messages over long distances by means of radio signals.
Prior to the discovery of electromagnetic waves, this concept of electromagnetism sparked interest in mechanical physics enthusiasts and led to the discovery of electric motors (dynamo) using the concept of electromagnetism to spin a disk which can be used to generate electricity. The first of these electric motors was invented by Michael Faraday in 1831. These early concepts of electromagnetic devices help shape the world we live in today. There are several fundamental principles of electromagnetism which serve as a basis of technological devices; creating a steady motion, varying field, turning magnetic field on and off and deflection. By creating a steady motion through several magnets movement is induced and such applications include the electric motor and maglev (magnetic levitation) trains. By simply running a current through several coils and magnets, we can produce kinetic energy and develop public transportation technology. Other examples include varying field; by altering the magnitude and direction of a current, one can reverse the polarity of the magnetic field.
This principle is used in loudspeakers (production of sound through movement of parts which produces vibrations) and a variety of memory disks. Magnetic locks, junkyard electromagnets and doorbells all use the principle of turning a magnetic field on and off. By switching the current on and off, one can manipulate the attractive forces of a magnetic field. And one of the final principles is of course deflection. As discussed, mass spectrometers use the deflection of charged particles as the basis of its operations. As we can see, electromagnetism has various applications in the real world today and will always continue to develop.
For example, the concept of maglev trains will likely be adapted to personal transportation (cars). A magnetic field produced can suspend a car and provide frictionless travel (ignoring air resistance) and increase the speeds of travel exponentially. Electric cars are not quite popular due to their short travelling distance and power shortages, however in the future it will be possible to install electromagnetic roads so that you may charge cars while driving. From this we can deduce that the development of electromagnetism is crucial to the advancement of devices which we use today and in the future.
University/College: University of California
Type of paper: Thesis/Dissertation Chapter
Date: 29 September 2016
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