Conditions for the formation of current (1) The directional movement of charge generates current, whether it is positive charge (cations, holes in semiconductors) or negative charge (anions, electrons). If the conductor is a metal or semiconductor device, the atoms will not change chemically, because the lost electrons will be replenished from other places. However, if electricity is conducted by ions, the ions will ionize into atoms at the electrode and attach to the electrode, resulting in chemical changes.
(2) Positive charges will also move, and the easiest thing to imagine is the movement of cations in conductive solutions. It is convenient to designate the moving direction of positive charge as the current direction. For example, if the negative charge is substituted into the calculation, a negative value is obtained, indicating that the current direction is opposite to the negative charge movement direction.
(3) There is no doubt that the battery provides voltage. Under the power supply voltage, an electric field is generated in the conductor, and the charge moves under the action of the electric field to form a current. But if the current is to last, then the battery must provide electrons, otherwise all the electrons in the wire will run away! But where did the electrons in the wire go? There is no doubt that it ran to the power supply. So the electrons run out of the power supply and return to the power supply. After the circuit is disconnected, the wire is not charged, so the electrons in the wire have not increased or decreased, so there must be no more or less electrons in the battery. Therefore, the battery does not provide electrons and does not consume electrons. The battery only provides voltage.
Under the non-electrostatic force of the power supply, charged particles of the same kind will move directionally, with positive charges moving towards the negative pole of the power supply and negative charges moving towards the positive pole of the power supply. The directional motion of charged particles is current, and it is generally stipulated that the direction of positive charge motion is the positive direction of current. A current whose current direction does not change with time is called direct current, and a current whose current direction changes with time is called alternating current. The difference between DC and AC is only its direction, which has nothing to do with other quantities. Although the current has a direction, it is a scalar. The magnitude of current is called current intensity, which is equal to the amount of charge passing through the circuit every second. The common unit of current is ampere (a) or milliampere (mA), that is,1000 ma =1a. The path through which current flows is the circuit. In the closed circuit, the transmission and conversion of electric energy are realized. The circuit consists of power supply, connecting wires, switching appliances, loads and other auxiliary equipment. A power supply is a device that provides electrical energy. The function of power supply is to convert non-electric energy into electric energy. For example, batteries convert chemical energy into electric energy, generators convert mechanical energy into electric energy, and solar cells convert solar energy into electric energy.
Dry batteries, storage batteries and generators are the most commonly used power sources. Load is a device that consumes electric energy in the circuit, and its function is to convert electric energy into other forms of energy. For example, an electric furnace converts electric energy into heat energy, and an electric motor converts electric energy into mechanical energy. Lighting appliances, household appliances and machine tools are the most common loads. Switchgear is the load control equipment, such as knife switch, circuit breaker, electromagnetic switch, decompression starter, etc. Auxiliary equipment includes various relays, fuses and measuring instruments. Auxiliary equipment is used to control, distribute, protect and measure circuits. Connecting wires connect power supply, load and other equipment into a closed loop. The function of connecting wires is to transmit electric energy or electrical signals.
Brief introduction of ammeter ammeter is an instrument for measuring current. The main types are rotating coil ammeter, rotating iron piece ammeter, thermocouple ammeter and hot wire ammeter. There is a permanent magnet inside the ammeter, which generates a magnetic field between the two poles. In the magnetic field, there is a coil with a hairspring at both ends. Each spring is connected to a terminal of an ammeter, and the spring and the coil are connected through a rotating shaft. There is a pointer at the front end of the rotating shaft relative to the ammeter. When a current passes through, the current passes through the magnetic field along the spring and the rotating shaft, and the current cuts the magnetic induction line, so the coil deflects under the action of the magnetic field force, driving the rotating shaft and the pointer to deflect.
An ammeter is made according to the action of magnetic field force in a magnetic field on a charged conductor. There is a permanent magnet inside the ammeter, which generates a magnetic field between the two poles. In the magnetic field, there is a coil with a hairspring at both ends. Each spring is connected to a terminal of an ammeter, and the spring and the coil are connected through a rotating shaft. There is a pointer at the front end of the rotating shaft relative to the ammeter. Pointer deflection. Because the magnitude of magnetic field force increases with the increase of current, the magnitude of current can be observed by the deflection degree of pointer. This is called magnetoelectric ammeter, which is the kind we usually use in the laboratory.
Generally, microamperes or milliamperes can be directly measured. In order to measure more current, ammeter should have shunt resistance (also called shunt). Mainly adopts the measuring mechanism of magnetoelectric instrument. When the resistance value of the shunt makes the full-scale current pass, the ammeter is completely deflected, that is, the ammeter indicates the maximum value. For several amperes of current, a special shunt can be set in the ammeter. For currents above a few amperes, external shunt is used. The resistance of high current shunt is very small. In order to avoid the error caused by the lead resistance and contact resistance attached to the shunt, the shunt should be made into a four-terminal form, that is, there are two current terminals and two voltage terminals. For example, when a shunt and a millivoltmeter are connected to measure a large current of 200A, if the standardized range of the millivoltmeter is 45mV (or 75mV), then the resistance value of the shunt is 0.045/200=0.000225? (or 0.075/200=0.000375? )。 If a ring (or cascade) shunt is used, a multi-range ammeter can be made.
The main factors causing electric shock casualties due to the harm of current to human body are generally as follows:
1. The current passing through the human body. According to the analysis of electric shock accidents, it is concluded that when the power frequency current is 0.5 ~ 1 mA, people's fingers and wrists will feel numb or painful; When the current increases to 8 ~ 10 mA, the acupuncture sensation and pain increase, and spasm occurs, and the charged body is grasped, but it can finally get rid of the charged body. When the contact current reaches 20 ~ 30 mA, people will soon be paralyzed, unable to get rid of the charged body, and their blood pressure will rise, making it difficult to breathe. When the current is 50mA, it will paralyze people's breathing, and the heart will start to vibrate, which can be fatal in a few seconds. The greater the current passing through the human body, the stronger the physiological response of the human body, the more serious the pathological state and the shorter the death time.
2. The length of power-on time. The longer the current passes through the human body, the more serious the consequences will be. This is because the longer the time, the lower the resistance and the greater the current. At the same time, the human heart has a time difference of 0. 1s every time it contracts and expands. During this period, the human body is most sensitive to the action of current. Therefore, the longer the electric shock time, the more times it coincides with this gap period, and the greater the danger.
3. The way the current passes through the human body. When the current passes through the important internal organs of the human body, the consequences are serious. For example, through the head, it will destroy the brain nerve and cause death. Through the spinal cord, it will destroy the central nervous system and paralyze people. Passing through the lungs can make it difficult to breathe. Passing through the heart will cause the heart to vibrate or stop beating and die. Among these injuries, the heart injury is the most serious. According to accident statistics, the most dangerous route through human body is from hand to foot, followed by hand to foot, and the least dangerous route is from foot to foot, which may lead to secondary accidents.
4. Type of current. Current can be divided into direct current and alternating current. Alternating current can be divided into power frequency electricity and high frequency electricity. These currents are harmful to human body, but the degree of harm is different. The ability of human body to withstand direct current and high frequency electricity is stronger than that of power frequency electricity. Therefore, power frequency electricity is the most harmful to human body.
5. The health status of the person who gets an electric shock. The consequences of electric shock are related to the health status of the electric shock victim. According to statistics, muscular people and adults have stronger ability to get rid of current than children, and men have stronger ability to get rid of current than women. For patients with heart disease, lung disease, endocrine disorders and mental illness, electric shock is the most dangerous. They have the highest death rate from electric shock. In addition, people who are psychologically prepared for electric shock have less electric shock damage.
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