What is the voltage drop

For a person who is familiar with electrical equipment at the level of a simple user (knows where and how to turn on / off), many terms used by electricians seem to be some kind of nonsense. For example, what exactly is a "voltage drop" or "circuit assembly". Where and what falls? Who disassembled the scheme for the details? In fact, the physical meaning of the processes occurring behind most of these words is quite understandable even with the school knowledge of physics.

To explain what a voltage drop is, it is necessary to remember which voltages are in the electrical circuit (the global classification is meant). There are only two kinds. The first is the voltage of the power supply, which is connected to the circuit under consideration. It can also be called applied to the entire circuit. And the second kind is the voltage drop. It can be considered both for the entire contour, and for any single element.

In practice, it looks like this. For example, if you take a conventional incandescent lamp, screw it into the socket, and connect the wires from it to a home power outlet, then the voltage applied to the circuit (power source - conductors - load) will be 220 Volts. But it is worthwhile using a voltmeter to measure its value on the lamp, as it becomes obvious that it is slightly less than 220. This is because a voltage drop occurred on the electrical resistance that the lamp has.

Perhaps there is no one who does not hear about Ohm's law. In general, its formulation looks like this:

I = U / R,

Where R is the resistance of the circuit or its element, measured in Ohms; U is the voltage in volts; And, finally, I is the current in amperes. As can be seen, all three quantities are directly related. Therefore, knowing any two, you can quite easily calculate the third. Of course, in each specific case it is necessary to take into account the kind of current (variable or constant) and some other specifying characteristics, but the basis is the above formula.

Electric energy is, in fact, movement along the conductor of negatively charged particles (electrons). In our example, the lamp spiral has a high resistance, that is, it slows down the moving electrons. Due to this, a visible luminescence appears, but the total energy of the particle flux decreases. As can be seen from the formula, as the current decreases, the voltage also decreases. That is why the results of measurements at the outlet and on the lamp are different. This difference is the voltage drop. This value is always taken into account to prevent too much reduction in the elements at the end of the circuit.

The voltage drop across the resistor depends on its internal resistance and the current flowing through it. The temperature and current characteristics also have an indirect effect. If an ammeter is included in the circuit under consideration, the drop can be determined by multiplying the current value by the lamp resistance.

But it is far from always possible to calculate the voltage drop with the help of the simplest formula and a measuring device. In the case of parallel-connected resistances, the finding of the quantity becomes more complicated. Alternating current has to additionally take into account the reactive component.

Consider an example with two parallel-connected resistors R1 and R2. The resistance of the wire R3 and the power source R0 is known. Also given the value of EMF - E.

Bring the parallel branches to one number. For this situation, the formula is:

R = (R1 * R2) / (R1 + R2)

Determine the resistance of the whole circuit through the sum R4 = R + R3.

We calculate the current:

I = E / (R4 + r)

It remains to find out the value of the voltage drop on the selected element:

U = I * R5

Here the factor "R5" can be any R - from 1 to 4, depending on which particular element of the circuit needs to be calculated.