Now we can understand why metals exhibit resistance electric current, i.e. why, in order to maintain a long current, it is necessary to maintain a potential difference at the ends of the metal conductor all the time. If electrons did not experience any interference in their movement, then, being brought into ordered motion, they would move by inertia, without action electric field, indefinitely. However, in reality, electrons experience collisions with ions. In this case, the electrons, which had a certain speed of ordered motion before the collision, will rebound after the collision in arbitrary, random directions, and the ordered motion of the electrons (electric current) will turn into disordered (thermal) motion: after eliminating the electric field, the current will very soon disappear. In order to obtain a long-lasting current, after each collision it is necessary to drive the electrons again and again in a certain direction, and for this it is necessary that a force acts on the electrons all the time, that is, that there is an electric field inside the metal.
The greater the potential difference maintained at the ends of a metal conductor, the stronger the electric field inside it, the greater the current in the conductor. The calculation, which we do not present, shows that the potential difference and current strength must be strictly proportional to each other (Ohm's law).
Moving under the influence of an electric field, electrons acquire some kinetic energy. During collisions, this energy is partially transferred to the lattice ions, causing them to undergo more intense thermal motion. Thus, in the presence of current, the energy of the ordered movement of electrons (current) is constantly transformed into the energy of chaotic movement of ions and electrons, which represents the internal energy of the body; which means that internal energy metal increases. This explains the release of Joule heat.
To summarize, we can say that the reason for electrical resistance is that electrons, during their movement, experience collisions with metal ions. These collisions produce the same result as the action of some constant force friction, which tends to slow down the movement of electrons.
Conductivity difference different metals due to some differences in the number free electrons per unit volume of metal and under conditions of electron movement, which comes down to the difference in medium length free path, i.e., the path traveled on average by an electron between two collisions with metal ions. However, these differences are not very significant, as a result of which the conductivity of some metals differs, as the table shows. 2 (§ 47), from the conductivity of others by only a few tens of times; at the same time, the conductivity of even the worst of metal conductors is hundreds of thousands of times greater than the conductivity good electrolytes and billions of times higher than the conductivity of semiconductors.
The phenomenon of superconductivity (§ 49) means that conditions have arisen in the metal under which electrons do not experience resistance to their movement. Therefore, to maintain a long current in a superconductor, there is no need for a potential difference. It is enough to set the electrons in motion by some kind of push, and then the current in the superconductor will exist even after the potential difference is eliminated. We have already talked about this experiment in § 49.
Now we can understand why metals resist electric current, that is, why in order to maintain a long-lasting current it is necessary to maintain a potential difference at the ends of the metal conductor all the time. If the electrons did not experience any interference in their movement, then, being brought into ordered motion, they would move by inertia, without the action of an electric field, for an unlimited time. However, in reality, electrons experience collisions with ions. In this case, the electrons, which had a certain speed of ordered motion before the collision, will rebound after the collision in arbitrary, random directions, and the ordered motion of the electrons (electric current) will turn into disordered (thermal) motion: after eliminating the electric field, the current will very soon disappear. In order to obtain a long-lasting current, after each collision it is necessary to drive the electrons again and again in a certain direction, and for this it is necessary that a force acts on the electrons all the time, that is, that there is an electric field inside the metal.
The greater the potential difference maintained at the ends of a metal conductor, the stronger the electric field inside it, the greater the current in the conductor. The calculation, which we do not present, shows that the potential difference and current strength must be strictly proportional to each other (Ohm's law).
Moving under the influence of an electric field, electrons acquire some kinetic energy. During collisions, this energy is partially transferred to the lattice ions, causing them to undergo more intense thermal motion. Thus, in the presence of current, the energy of the ordered movement of electrons (current) is constantly transformed into the energy of chaotic movement of ions and electrons, which represents the internal energy of the body; which means that the internal energy of the metal increases. This explains the release of Joule heat.
To summarize, we can say that the reason for electrical resistance is that electrons, during their movement, experience collisions with metal ions. These collisions produce the same result as the action of some constant frictional force, tending to slow down the movement of electrons.
The difference in the conductivity of different metals is due to some differences in the number of free electrons per unit volume of the metal and in the conditions of electron motion, which comes down to a difference in the mean free path, i.e., the path traveled on average by an electron between two collisions with metal ions. However, these differences are not very significant, as a result of which the conductivity of some metals differs, as the table shows. 2 (§ 47), from the conductivity of others by only a few tens of times; at the same time, the conductivity of even the worst metal conductors is hundreds of thousands of times greater than the conductivity of good electrolytes and billions of times greater than the conductivity of semiconductors.
The phenomenon of superconductivity (§ 49) means that conditions have arisen in the metal under which electrons do not experience resistance to their movement. Therefore, to maintain a long-term current in a superconductor, there is no need for a potential difference. It is enough to set the electrons in motion with some kind of push, and then the current in the superconductor will exist even after the potential difference is eliminated. We have already talked about this experiment in § 49.
Including electrical circuit any current source, different conductors and an ammeter, you can notice that for different conductors the ammeter readings are different, i.e. the current strength in a given circuit is different. So, for example, if instead of iron wire AB (Fig. 70) you include nickel wire CD in a circuit of the same length and cross-section, then the current strength in the circuit will decrease, and if you include copper EF, the current strength will increase significantly.
Rice. 70. Dependence of current strength on the properties of conductors
A voltmeter, alternately connected to the ends of these conductors, shows the same voltage. This means that the current strength in the circuit depends not only on the voltage, but also on the properties of the conductors included in the circuit. The dependence of the current strength on the properties of the conductor is explained by the fact that different conductors have different electrical resistance.
Electrical resistance - physical quantity. It is designated by the letter R.
The unit of resistance is taken to be 1 ohm - the resistance of a conductor in which, at a voltage at the ends of 1 volt, the current strength is 1 ampere.. Briefly it is written like this:
1 Ohm = 1 V / 1 A
Other units of resistance are also used: milliohm (mOhm), kilohm (kOhm), megaohm (MOhm).
1 mOhm = 0.001 Ohm;
1 kOhm = 1000 Ohm;
1MOhm = 1000,000 Ohms.
What is the reason for the resistance? If the electrons in the conductor did not experience any interference in their movement, then they, being brought into ordered motion, would move by inertia for an unlimited time. In reality, electrons interact with ions crystal lattice metal At the same time, the ordered movement of electrons slows down and through cross section conductor passes in 1 s less than their number. Accordingly, the charge transferred by electrons in 1 s decreases, i.e., the current strength decreases. Thus, each conductor, as it were, counteracts the electric current and provides resistance to it.
The cause of resistance is the interaction of moving electrons with ions of the crystal lattice.
Different conductors have different resistance due to differences in the structure of their crystal lattice, due to different lengths and cross-sectional areas.
Questions
- How can you show experimentally that the current strength in a circuit depends on the properties of the conductor?
- What is the unit of conductor resistance? What's her name?
- What units of resistance other than the ohm are used?
- What is the reason for the resistance?
Exercise 28
- Draw a diagram of the circuit shown in Figure 70 and explain the experiment carried out on this drawing.
- Express the values of the following resistances in ohms: 100 mOhm; 0.7 kOhm; 20 MOhm.
- The current in the spiral of an electric lamp is 0.5 A when the voltage at its ends is 1 V. Determine the resistance of the spiral.
