Anyone who has chosen the repair and maintenance of electrical installations as their specialty is well aware of the teachers' statement: “Ohm's law for a closed circuit needs to be known. Even waking up in the middle of the night, it is important to be able to formulate it. Because this is the basis of all electrical engineering. ” Indeed, the pattern discovered by the outstanding German physicist Georg Simon Om influenced the subsequent development of the science of electricity.
In 1826, while conducting experiments to study the passage of electric current through a conductor, Ohm revealed a direct relationship between the amperage supplied to the circuit by the voltage of the power source (although in this case it is more correct to talk about the electromotive force of the emf) and the resistance of the conductor itself. Dependence was theoretically justified, as a result of which Ohm's law for a closed circuit appeared. An important feature: the relevance of the revealed fundamental law is valid only in the absence of an external disturbing force. In other words, if, for example, the conductor is in an alternating magnetic field, then the direct application of the formulation is impossible.
Ohm's law for a closed circuit was revealed in the study of the simplest circuit: a power source (having an EMF), conductors go from its two conclusions to the resistor, in which there is directed movement of elementary particles carrying a charge. Hence, the current is the ratio of the electromotive force to the total resistance of the circuit:
I = E / R,
where E is the electromotive force of the power source, measured in volts; I - current value, in amperes; R is the electrical resistance of the resistor, in ohms. Note that Ohm's law for a closed circuit takes into account all the components of R. When calculating a complete closed circuit, R means the sum of the resistances of the resistor, conductor (r), and power supply (r0). I.e:
I = E / (R + r + r0).
If the internal resistance of the source r0 is greater than the sum R + r, then the current strength does not depend on the characteristics of the connected load. In other words, the EMF source in this case is the current source. If the value of r0 is less than R + r, then the current is inversely proportional to the total external resistance, and the power source generates a voltage.
When performing accurate calculations, even the voltage loss at the joints is taken into account. Electromotive force is determined by measuring the potential difference at the terminals of the source with the load off (circuit open).
Ohm's laws apply as often to a circuit section as they do to closed loop. The difference is that the calculations do not take into account the EMF, but only the potential difference. This section is called homogeneous. In this case, there is a special case that allows you to calculate the characteristics of the electrical circuit on each of its elements. We write it in the form of a formula:
I = U / R;
where U is the voltage or potential difference, in volts. It is measured with a voltmeter by connecting the probes in parallel to the terminals of any element (resistance). The obtained value of U is always less than the EMF.
Actually, it is this formula that is the most famous. Knowing any two components, one can find the third from the formula. The calculation of circuits and elements is performed by means of the considered law for a section of a circuit.
Ohm's law for a magnetic circuit is in many ways similar to its interpretation for an electrical circuit. Instead of a conductor, a closed magnetic circuit is used, the source is a coil winding with current passing through the turns. Accordingly, the resulting magnetic flux closes along the magnetic circuit. The magnetic flux () circulating along the contour directly depends on the value of the MDS (magnetomotive force) and the resistance of the material passing the magnetic flux
Φ = F / Rm;
where f is the magnetic flux in weber; F - MDS, in amperes (sometimes gilbert); Rm is the resistance causing attenuation.