Formula for finding voltage: basic concepts, finding through current strength and resistance

When designing circuits for various devices, a radio amateur needs to make accurate calculations using measuring instruments and formulas. In electrical engineering, formulas are used to calculate the quantities of electricity (formulas for voltage, resistance, current, and so on).

General information about electric current

Electric current is the process of motion of charged particles (free electrons), which has a directional vector. The particles move under the action of the electric field strength, which has a vector direction. This field does the work of moving these particles. Current strength, voltage and resistance affect the work of the electric field.

Physical sense

The physical meaning is understood work of current on the sitecorresponding to the amount of charge. A positive charge moves from one point with one potential to another, and the potential at this point is different from the previous one. As a result of this, a potential difference arises, called voltage or EMF (electromotive force).

To fully understand this physical process and clarify the physical meaning of stress, it is necessary to draw an analogy with a pipe. Let's say the pipe is filled with water and a tap is screwed to it to drain the water. This pipe is also equipped with a tap for filling water with a powerful pump.

To demonstrate the analogy, you need to open the tap completely, the water will start pouring out and it can be concluded that the pressure is insignificant. In the second case, the drain valve is not fully open and water is collected using a pump. Pressure builds up in the pipe and the pressure increases. The pump that builds up the pressure is, in this example, the electric field strength.

Electricity, if not controlled and not aware of the harmful effects on the human body, can create many problems ranging from burning appliances and fires, and ending with a threat to life and health person. Safety is very important in any area.

The detrimental effect on a person

Electricity is very dangerous and is the cause of accidents. Radio amateurs are at risk of electric shock quite often. Some radio amateurs use their fingers to test for voltage and neglect safety precautions. Most of them consider voltages from 500 V to be life-threatening, and 110 and 220 are not harmful to health. Shocks from low-power current sources (low-power power transformer, capacitor), in their opinion, are not dangerous.

According to safety precautions when working with electricity, they are mistaken, but there is another side of this issue: the body of each person is individual, has different parameters. It follows from this statement that the lethal characteristics of electricity (voltage and current) are individual for each person. Some can be hit by 36 V, while others are not penetrated by 220 V.

The effect of electricity on the human body depends on several factors: strength and frequency, time and the path of passage through the body, the resistance of the body or part of the body through which it flows current.

Researchers have found that the value of the lethal current that affects the heart is more than 100 mA. Currents from 50 mA to 100 mA cause loss of consciousness when briefly touching a surface that conducts a current. Currents up to 50 mA can cause injuries such as falling from stairs, loosening a live conductor, etc.

Influence on the factor of defeat it also has the resistance of the human body. The resistance for each individual is difficult to determine and its range is from 30 kOhm to 200 kOhm. This value depends on many factors: skin thickness, body and environmental moisture, fatigue, neuro-emotional state, illness and other factors. Resistance decreases sharply with increased air humidity and work in wet areas.

The formula for calculating a life-threatening voltage, assuming that Rh = 2 kΩ and I = 60 mA, looks like this: U = I * R = 0.06 * 2000 = 120 V. In this situation, a dangerous voltage can be considered 120 V and above.

The frequency of the current is another dangerous characteristic that has a damaging effect. As the frequency increases, the hazard decreases in direct proportion. The current also has a thermal effect, therefore, high-frequency currents cannot be considered safe.

Injuries caused by electricity are called electrical injuries. Each of them carries less or more danger. The most dangerous are injuries sustained from an electric arc, which has a high temperature of 5 thousand. up to 12 thousand degrees Celsius. Types of electrical injuries:

1. Electrical burns occur when heat is applied to the tissues of the human body through which current flows.
2. Burned areas on the skin occur when it comes into direct contact with the live part of the conductor. The affected area becomes gray or pale gray.
3. Leather metallization - impregnation of leather with metal particles during short circuit or welding.
4. Mechanical damage - spontaneous muscle spasm, leading to a fall. When falling, fractures, bruises, dislocations of joints, etc. occur.
5. Electrophthalmia - inflammation of the mucous membrane of the eyes when exposed to radiation from an electric arc.

There is another type of injury - electric shock. This type of defeat can be conditionally divided into 5 groups: without loss of consciousness; with loss of consciousness associated with impaired cardiac activity or without it; clinical death and electric shock.

Units

The work of an electric field to move a charge measured in J (Joule), charge in C (coulomb). This is how the voltage or its unit of measurement is indicated: the ratio of these quantities (work on displacement in J to the electric charge in C) and is the potential difference, measured in volts (V) and denoted U. The potential difference is:

1. Variable (amplitude and polarity change over time, depending on the characteristic frequency).
2. Constant (has a constant amplitude value and polarity is a constant value).

And also the units of measurement have prefixes, for example, kV (Kilovolt = 1000V) and MV (megavolt = 1000000V). There are very low values, such as mV (millivolt = 0.001V).

AC and DC circuits

In AC and DC circuits, U has different properties and produces different effects on conductors. For a constant voltage, there are laws for calculating its characteristics, but for a variable, the methods for calculating indicators are noticeably different. Let's take a closer look at all the differences and similarities.

Calculation and analysis of circuits is performed using Ohm's law: the full circuit current is directly proportional to the voltage and inversely proportional to the sum of the circuit and power supply resistances.

A consequence of the law, provided that the internal resistance of the electricity source is neglected: the current strength of a section of the circuit is directly proportional to the EMF and inversely proportional to the resistance of this section.

Writing Ohm's law, from which the formula for voltage, current and resistance follows: I = U / (Rts + Rvn), where I is the current strength, U is the EMF, Rts is the resistance of the circuit, Rvn is the internal resistance of the source nutrition.

Formula of current strength through resistance and voltage: I = U / Rts.

Formula of voltage of electric current: U = I * Rts.

To calculate the power, you need to multiply U by I: P = U * I = U * U / R, where P is the power.

AC single phase voltage

In circuits for alternating current, completely different phenomena and processes occur, other laws are valid for them. There are such main types:

1. Instantaneous (potential difference at a specific time interval: u = u (t)).
2. Amplitude value (maximum value of instantaneous U at the moment of time: u (t) = Um * sin (wt + f), where w is the angular frequency, t is a specific moment in time and f is the angle of the initial phase of the voltage).
3. Average value (for a sinusoid is zero).
4. Root mean square - Uq (U for the entire oscillation period and for a sinusoid has the form: Uq = 0.707 * Um).
5. Average-straightened - Uv (the average value of the module U: Um is approximately equal to 0.9 * Uq).

In 3-phase current circuits, 2 types of voltages are distinguished: linear (phase-phase) and phase (phase-zero). When connected in a "delta" circuit, the phase and linear U are equal. In the case of a "star" connection, the phase connection is 1.732050808 times less than the linear one.

Recommendations for choosing a device

For calculations, it is necessary to measure the values ​​of quantities of electricity. There are special devices that help make accurate calculations. A voltmeter is used to measure the potential difference.

A voltmeter (volt is a unit of EMF measurement, a meter - I measure) is a device for measuring EMF in a circuit, connected in parallel with the area where it is necessary to measure.

For a specific case, it is necessary to use one or another device. For more accurate calculations, devices with a high accuracy class are purchased. Voltmeter classification:

1. Principle of operation: electromechanical (switch) and electronic.
2. Purpose: direct and alternating current, pulse, selective and universal.
3. Design: panel board, portable and stationary.

Analog Electromechanical Voltmeter has large measurement errors in high-resistance circuits, but has proven itself in low-resistance circuits and the possibility of upgrading (increasing the measurement values ​​of U due to an additional resistor).

Rectifier voltmeter has a higher class of accuracy. Consists of the measuring device itself (has a sensitivity to direct current) and a rectifier device. They are not very widespread due to high errors, and are used as signaling devices (approximate value of U).

Digital voltmeters used in combined multimeter instruments. The incoming voltage to the terminals (measuring probes) of the device is converted into a signal using an analog-to-digital converter (ADC). It is displayed on a digital display. This type of device is widely used due to its high accuracy and versatility.

Pulse voltmeter must be used when measuring the amplitudes of pulsed signals and single pulses.

The main application of phase-sensitive voltmeters is to measure the quadrature components of the complex voltage (the presence of the imaginary and real parts) of the primary harmonic. They are usually equipped with 2 indicators to identify the imaginary and real parts. They are widely used in measuring the AFC (amplitude-phase characteristic) for the selection of parts and tuning amplifiers.

To measure the nominal DC voltage, voltmeters of the B2 subgroup (voltmeters for direct voltage), as well as B7 (universal) are used.

To determine alternating voltage, it is necessary to use devices from subgroup B3 or universal type (B7). However, these voltmeters often use special converters from alternating voltage to direct voltage.

B3 and B7 are calculated for RMS voltage only.. In these electrical measuring instruments, it is possible to use detectors (converters): peak, rectifier and quadratic. The best option is a voltmeter on a square-law detector, while the measured value is outputted directly without any conversion. Measuring instruments on peak and rectifier detectors recalculate the values, thereby reducing the measurement accuracy. To measure the periodic non-harmonic voltage, a voltmeter on a square-law detector is selected.

Thus, voltage calculation plays an important role in electrical engineering. Calculations for alternating and constant circuits of electric current differ significantly, as a result of which it is necessary to first determine the type of current, and then make calculations. But it is also necessary to observe safety precautions when working with electricity. After all, its main provisions are based on the bitter experience of mankind.