Ohm's law for a closed circuit

Anyone who chose repair and maintenance of electrical installations with his specialty is well aware of the teachers' statement: "Ohm's law for a closed circuit must be known. Even waking up in the middle of the night, it's important to be able to formulate it. Because it is the basis of all electrical engineering. " Indeed, the regularity 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, Om revealed a direct relationship between the current strength brought 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 EMF) and the resistance of the conductor itself. The dependence was theoretically justified, as a result of which the Ohm law for a closed circuit appeared. An important feature: the relevance of the revealed fundamental law is valid only in the absence of external perturbing force. In other words, if, for example, the conductor is in an alternating magnetic field, then direct application of the formulation is impossible.

Ohm's law for a closed circuit was revealed in the study of the simplest scheme: a power source (having an EMF), from two of its leads to a resistor are conductors in which the directed motion of charge-carrying elementary particles occurs. 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 is the sum of the resistor resistors, the conductor (r), and the power supply (r0). I.e:

I = E / (R + r + r0).

If the internal resistance of the source r0 is greater than the sum of R + r, then the current does not depend on the characteristics of the connected load. In other words, the source of the EMF in this case is a source of current. If the value of r0 is less than R + r, the current is inversely proportional to the total external resistance, and the power source generates a voltage.

When performing accurate calculations, even the loss of voltage at the junction points is taken into account. The electromotive force is determined by measuring the potential difference at the source terminals with the load disconnected (the circuit is open).

Ohm's laws for a chain section are applied as often as for a closed loop. The difference is that the calculation does not take into account the EMF, but only the potential difference. Such a site is called homogeneous. In this case, there is a special case, which allows calculating the characteristics of the electrical circuit on each of its elements. We write it as a formula:

I = U / R;

Where U is the voltage or potential difference, in volts. It is measured by a voltmeter by parallel connection of probes to the terminals of an element (resistance). The resulting value of U is always less than the emf.

Actually, it is this formula is the most famous. Knowing any two components, you can find the third from the formula. Calculation of contours and elements is carried out by means of the law under consideration for the chain section.

Ohm's law for a magnetic circuit is in many respects similar to its interpretation for an electrical circuit. Instead of a conductor, a closed magnetic circuit is used, the source is the winding of the coil with the current passing through the turns. Accordingly, the emerging magnetic flux is closed along the magnetic circuit. The magnetic flux (Ф), circulating along the contour, directly depends on the value of the MDS (magnetomotive force) and the resistance of the material of the passage of the magnetic flux:

Ф = F / Rm;

Where Φ is the magnetic flux, in the webs; F - MDS, in amperes (sometimes gilberts); Rm is the resistance causing the attenuation.