ELECTRICITY AND MAGNETISM: WHAT IS THE CONNECTION BETWEEN THEM?
As you might have already guessed, the purpose of this lesson is to master the theoretical information regarding electricity and magnetism, and to trace the connection between these two concepts. Because it is precisely thanks to magnetic (electromagnetic) phenomena that we can generate electricity, without which human life is inconceivable today. At the end of the lesson, a simple but rather interesting practical assignment awaits you.
The direct connection between electricity and magnetism was discovered in 1819 by the Danish physics professor Hans Christian Ørsted. While conducting experiments, the scientist found that whenever he turned on the current, a magnetic compass needle located near the current-carrying conductor tended to turn perpendicular to the conductor, and when he turned it off, the magnetic needle returned to its original position. The scientist concluded: a magnetic field is generated around a current-carrying conductor, which affects the magnetic needle.
You can verify this yourself if you conduct a similar experiment. This will require: a battery of galvanic cells, for example, a 4.5V flat battery, a miniature incandescent lamp intended for an electric pocket flashlight, a copper wire 0.2 - 0.3 mm thick in enamel, cotton, or silk insulation, and a compass. Using pieces of wire, having stripped the insulation from their ends, connect the incandescent lamp to the battery. The lamp lights up because an electrical circuit has been formed. The battery in this case is the power source for this circuit. Bring one of the connecting conductors closer to the compass, see the figure, and you will see how its magnetic needle immediately positions itself across the conductor. It will indicate the direction of the circular magnetic field lines generated by the current.
The strongest magnetic field of the current will be near the conductor itself. As you move away from the conductor, the magnetic field scatters and weakens.
And what if we change the direction of the current in the conductor by swapping its connection to the battery poles? The direction of the magnetic field lines will also change — the magnetic needle will turn in the other direction. This means that the direction of the magnetic field lines excited by the current depends on the direction of the current in the conductor.
What is the role of the incandescent lamp in these experiments? It serves as an indicator of the presence of current in the circuit. Furthermore, it limits the current in the circuit. If only a conductor is connected to the battery, the magnetic field of the current will become stronger, but the battery will quickly discharge.
If a direct current of a constant value flows in the conductor, its magnetic field will also remain unchanged. But if the current decreases, its magnetic field will also become weaker. If the current increases, its magnetic field will strengthen; if the current disappears, the magnetic field will vanish. In short, the electric current and its magnetic field are inextricably linked and mutually dependent.
The magnetic field of a current is easy to strengthen if the current-carrying conductor is coiled into a coil. The magnetic field lines of such a coil can be concentrated if a nail or an iron rod is placed inside it. Such a coil with a core will become an electromagnet capable of attracting relatively heavy iron objects. This property of current is used in many electrical devices.
And what if a magnetic needle is brought close to a wire carrying an alternating current? It will remain motionless, even if the wire is coiled into a coil. Does this mean there is no magnetic field around a conductor with alternating current? There is a magnetic field, but it is also alternating. The magnetic needle will not deflect simply due to its inertia — it will not have time to follow the rapid changes in the magnetic field.
The first electromagnet, the basic features of which have been preserved in many modern electrical devices, such as electromagnetic relays and headphone emitters, was invented by the English scientist William Sturgeon in 1821. Two decades after this event, the French physicist André-Marie Ampère made a new, exceptionally important discovery for that time. He established experimentally that two parallel conductors carrying current are capable of performing mechanical work: if the current in both conductors flows in the same direction, they attract each other, and if in opposite directions, they repel.
Can you guess why this happens? In the first case, when the direction of the current in both conductors is the same, their magnetic fields, also having the same direction, seem to merge into a single field, pulling the conductors with them. In the second case, the magnetic fields around the conductors, now having opposite directions, repel and thereby push the conductors apart.
In the first half of the last century, an invaluable contribution to science was made by the self-taught English physicist Michael Faraday. While studying the connection between electric current and magnetism, he discovered the phenomenon of electromagnetic induction. Its essence is as follows. If a magnet is quickly inserted into a coil of insulated wire, the needle of an electrical measuring instrument connected to the ends of the coil will momentarily deflect from the zero mark on the instrument's scale.
With the same rapid movement of the magnet inside the coil, but in the opposite direction, the instrument's needle will also quickly deflect in the opposite direction and return to its original position. There could only be one conclusion: the magnetic field intersects the wire and excites (induces) the movement of free electrons in it — an electric current. However, you can do it differently: move the coil along a stationary magnet instead of the magnet. The result will be the same. The magnet can be replaced by a coil carrying a direct current. The magnetic field of this coil, caused by the current, when intersecting the turns of a second coil, will also excite an electromotive force in it, creating an electric current in its circuit.
The phenomenon of electromagnetic induction is the basis of the operation of an alternating current generator, which is a coil of wire rotating between the poles of a strong magnet or electromagnet (in the figure, the coil is shown as a single turn of wire).
As it rotates, the coil intersects the magnetic field lines, and an electric current is induced (generated) in it.
In 1837, the Russian academician Boris Jacobi discovered a phenomenon opposite to the operation of a current generator. By passing a current through a coil placed in a magnetic field, the scientist caused the coil to start rotating. This was the world's first electromagnetic motor. Faraday, who discovered the law of electromagnetic induction, experimentally found another very important phenomenon — the ability to transfer alternating current from coil to coil over a distance without any direct electrical connection between them. The essence of this phenomenon is that an alternating or interrupted (pulsating) current flowing in one of the coils is converted into an alternating magnetic field, which intersects the turns of the second coil and thereby excites an alternating EMF in it. Based on this, a remarkable device called a transformer was created, which plays a very important role in electrical and radio engineering.
ALTERNATING CURRENT GENERATES ELECTROMAGNETIC WAVES
The experiments of Michael Faraday and his compatriot and follower James Clerk Maxwell led scientists to the conclusion that an alternating magnetic field, generated by a continuously changing current, creates an electric field in the surrounding space, which in turn excites a magnetic field, the magnetic field — an electric one, and so on. Interconnected, mutually generating magnetic and electric fields form a single alternating electromagnetic field, which continuously, as if separating and moving away from its point of excitation, propagates throughout the surrounding space at the speed of light, equal to 300,000 km/s. The phenomenon of the excitation of electromagnetic fields by alternating current is usually called the radiation of electromagnetic oscillations or the radiation of electromagnetic waves. Encountering conductors in their path, the magnetic components of electromagnetic oscillations excite an alternating electric field in these conductors, creating in them the same alternating current as the current that excited the electromagnetic waves, only incomparably weaker. The technique of radio transmission and radio reception is based on this remarkable phenomenon.
The equality of the propagation speed of electromagnetic waves created by alternating current and the speed of light is not accidental, because light rays, just like thermal rays, are also electromagnetic oscillations by their nature. The idea of the kinship of light and electrical phenomena was expressed by the Russian scientist Mikhail Lomonosov back in the middle of the 18th century. The theory of electromagnetic waves was developed by Clerk Maxwell in the first half of the last century. However, it was only in 1888 that the German scientist Heinrich Hertz managed to experimentally prove the very fact of the existence of electromagnetic waves and find a way to detect them. In his experimental setup, the emitter of electromagnetic waves was a dipole oscillator (vibrator) — two rods with metal balls at the ends, the power voltage source for the vibrator was a Ruhmkorff induction coil (found in every school physics room), and the detector of electromagnetic energy was a resonator, which was an open turn of wire, also with balls at the ends.
The halves of the vibrator were charged to such a high voltage that an electric spark — an artificial miniature lightning bolt — jumped through the air between the inner balls. An electrical discharge occurred. At this moment, lasting fractions of a second, the vibrator radiated a short series of rapidly alternating damped, i.e., decreasing in amplitude, electromagnetic waves. Intersecting the wire of the resonator located nearby, the electromagnetic energy excited electrical oscillations in it, as evidenced by a very weak spark appearing between the balls of the resonator. Another discharge, and a new burst of damped electromagnetic oscillations excited a weak alternating current in the resonator. This is how Heinrich Hertz found a way to excite electromagnetic waves and detect them. But he had no idea of the ways to practically use his discovery.
Important concepts and points to remember from this lesson: what a magnetic field is, how it affects surrounding objects, and the basic conditions necessary for the emergence of a magnetic field. The concepts of electromagnetism and electromagnetic induction, as well as the conditions for the emergence of electromagnetic oscillations under the influence of alternating current — electromagnetic waves.
PRACTICAL WORK
In this practical work, you should trace experimentally the effect of an electric current on a coil with a metal core, which, as we have already found out, is called an inductor. Also, in this experiment, you can clearly see how the magnetic field induced in the metal core of the coil affects surrounding objects (in this case, small metal objects), which can be: paper clips, thumbtacks, etc. The diagram of our experimental setup is not complicated and was given above; here I will duplicate it for convenience.
Start your experiments!
Moving on to the next lesson!
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