Physics is full of concepts that are difficult to imagine. A striking example of this — the theme of electricity. Almost all phenomena occurring there and the terms are difficult to see or to imagine.
What is electrical resistance? Where does it come from? Why is there tension? And why DC has power? Questions an infinite number. It is necessary to understand everything in order. And it would be good to start with resistance.
What happens in a conductor when it carries current?
There are times when the material that has conductive capability, is between the two poles of the electric field: positive and negative. And then it is an electric current. It is shown that free electrons start directional movement. Because they have a negative charge, their movement is in one direction – to plus. Interestingly, the direction of the electric current, it is customary to specify a different – from plus to minus.During the movement of the electrons hitting the atoms of a substance and transmit them of their energy. This explains what is included in the network, the conductor heats up. And the electrons slow down its movement. However, the electric field accelerates them again, so they again rush to plus. This process occurs indefinitely until around the conductor there is an electric field. It turns out that the electrons experience resistance to electric current. That is, the more obstacles they meet, the higher the value of this quantity.
What is electrical resistance?
It is possible to define on the basis of two positions. The first is the formula for Ohm's law. And it sounds like this: electrical resistance is a physical quantity, which is defined as the ratio of the voltage in the conductor to the current flowing in it. Mathematical notation is given below.
The second is based on the properties of the body. The electrical resistance of a conductor is a physical quantity that indicates a property of the body to convert electricity energy into heat. Both of these statements are true. Only in schools often focus on memorization first. The study indicated the value with the letter R. the Unit that measures electrical resistance, the Ohm.
For some formulas it can be found?
The most famous derives from Ohm's law for subcircuit. It combines electric current, voltage, resistance. Looks like this:
This is the formula 1.
The second takes into account the fact that the resistance depends on the parameters of the conductor:
This formula has the number 2. It introduced the following notation:
Electrical resistivity is a physical quantity, which is equal to the resistance of the material of length 1 m and cross-sectional area 1 m2 .
The table shows the system unit of specific resistance. In real situations this does not happen, that the cross section measured in square meters. Almost always it is square millimeters. Therefore, electrical resistivity is more convenient to take Ω * mm 2 / m, and the area to frame in 2 mm .
What and how depends on the resistance?
First, from the substance from which made the guide. The greater the importance of the electrical resistivity, the worse it will conduct current.
Second, from a length of wire. And here is a direct correlation. With increasing length of the resistance increases.
Third, the thickness. The thicker the wire, the less resistance.
And finally, fourth, the temperature of the conductor. And here things are not so simple. If we are talking about metals, their electrical resistance increases with heat. With the exception of some special alloys is their resistance remains almost unchanged when heated. These include: Constantan, nickeline and manganin. When liquids are heated, their resistance decreases.
What are the resistors?
It is an element that is included in an electrical circuit. He has a very specific resistance. This is what is used in the circuits. It is customary to divide the resistor into two types: fixed and variable. Their name is due to the fact if you can change their resistance. First — constant — do not allow any way to change the nominal value of the resistance. It remains the same. The second variable is given the opportunity to make adjustment by changing the resistance depending on the needs of the specific scheme. In electronics produce another kind — turn trimmer. Their resistance changes only in the moment when you want to configure device, and then remains constant.
As the diagrams it looks like the resistor?
A rectangle with two exits from the narrow sides. It is a constant resistor. If a third party thereto pririsoval arrow, it has AC. In addition to the schemes is signed and the electrical resistance of the resistor. Right inside this rectangle. Usually just numbers or name, if they are very large.
What is insulation and why it should be measured?
Its purpose – ensure electrical safety. The insulation resistance is the main characteristic. It is not allowed to flow through the human body dangerous current.
- working – its purpose is to ensure the normal operation of the equipment, so it not always has a sufficient level of protection of human
- additional followup mind and protects people
- dual combines two of the first insulation
- reinforced, which is an improved form of working, it is as reliable as extra.
All devices that have a domestic purpose, must be equipped with double or reinforced insulation. Moreover, it must possess these characteristics to withstand all mechanical, electrical and thermal loads.
Over time the insulation gets old and its parameters worsen. This explains the fact that it requires regular preventive examination. Its purpose is to eliminate the defects, and the measurement of active resistance. It uses a special device — megger.
Examples of problems with solutions
Condition 1: it is required to determine the electrical resistance of iron wire, which has a length equal to 200 m, and the cross-sectional area of 5 mm2.
Solution. You need to use the second formula. It is unknown only the resistivity. But it can be viewed in the table. It is equal to 0,098 Ohm * mm / m 2. Now we just need to substitute values into formula and calculate:
R = 0,098 * 200 / 5 = 3,92 Ohm.
Answer: resistance is approximately 4 Ohms.
Condition 2: calculate the electrical resistance of a conductor made of aluminum, if its length is 2 km, and the cross-sectional area is 2.5 mm².
Solution. Similar to the first task, the specific resistance is 0.028 Ω * mm / m 2. To get the correct answer, you will need to transfer the miles to meters: 2 km = 2000 m. Now you can read:
R = 0,028 * 2000 / 2,5 = 22.4 Ω.
The answer. R = 22.4 Ω.
Condition 3: any length required wire, if its resistance should be 30 Ohms? Known the area of its cross-section is 0.2 mm2, and the material is nickeline.
Solution. From the same resistance formula to obtain an expression for the length of the wire:
l = (R * S) / ρ. You know everything except the resistivity, which needs to be taken from the table: 0.45 Ω * mm 2 / m. After substitution and calculation, it turns out that l = 13,33 m.
Answer: the approximate length is 13 m.
Condition 4: to determine the material of the resistor, if its length is 40 m, impedance 16 Ohms, the cross section is 0.5 mm2.
Solution. Similarly, the third task is expressed the formula for resistivity:
ρ = (R * S) / l. Substitution of values and calculations give the following result: ρ = 0.2 Ohm * mm 2 / m. This value of resistivity is typical for the lead.