Electromagnetic induction

Basic knowledge is as follows. An electric field acts on stationary charges in space and can move them from their place. The magnetic field acts on moving charges, that is, it does not act on stationary charges and cannot move them from their place. Let's see how this basic knowledge will allow us to explain the physical principle of electromagnetic induction and how the transformer works.

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It is known that a transformer transforms or transforms only an alternating one. But why is this impossible for direct current? Imagine a direct current flows in the primary winding of a transformer. Constant current causes a constant magnetic field and that's it. This means that there is a constant magnetic field around the conductor with current. This permanent magnetic field penetrates the secondary winding of the transformer, but, as we just said, the magnetic field only acts on moving charges. But we have metal in the secondary winding. And the magnetic field cannot move free electrons. Therefore, no electric current is generated.

Now let's see what happens if an alternating current flows in the primary winding of a transformer. Alternating current produces an alternating magnetic field. And here everything happens differently. The alternating magnetic field penetrates the secondary winding, but by itself it still cannot budge the stationary electrons. An electric field must be generated. Which is what happens. But such a new electric field, in comparison with the initial static electric field, will be called a "vortex field". It is this vortex electric field that shifts the electrons, which leads to the appearance of an alternating current. This is the idea that explains the physical principle of the transformer.

The task:

An electrical circuit is given. It consists of a galvanic cell, a variable resistance rheostat, a transformer and measuring instruments - an ammeter and a voltmeter. At the initial moment of time, the rheostat slider is set in the middle and is stationary. Based on the laws of electrodynamics, explain how the readings of the instruments will change as you move the rheostat slider to the left. Disregard the EMF of self-induction.

Before proceeding to explain the solution to a problem of this type, you should pay attention to the fact that students, picking up the tasks of the Olympiad in physics, begin to read the condition. They should not be tempted to immediately answer the question. The first thing to do is to read the question very carefully and correctly. Some tasks contain so-called "hints". In this task, there may be a hint in the form of the phrase "based on the laws of electrodynamics." If you simply explain how the readings of the instruments will change as you move the rheostat slider to the left and right, then you will not receive the maximum number of points for the task.

Recall that there is an alternating current in the primary winding, an alternating magnetic flux occurs in the secondary winding in accordance with Faraday's law, which creates an EMF of induction, and the voltmeter shows the voltage at the ends of the secondary winding. These are basic data, but there are some comments. They do not follow from the condition, but the best option for a participant in the physics Olympiad would be to analyze the readings of a voltmeter. It is necessary to find out whether these readings will be constant when the rheostat engine moves or not? This is a very interesting question.