Nature's Porter - Network Forwarding - Basic Concepts of Circuits

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Basic Concepts of Circuits The directional movement of electric charges is called current. In a circuit, current is often represented by I. There are two types of current: direct current and alternating current. The magnitude and direction of current that does not change with time is called direct current. The magnitude and direction of current that changes with time is called alternating current. The unit of current is ampere (A), and milliampere (mA) or microampere (uA) are also commonly used as units. 1A=1000mA, 1mA=1000uA. Current can be measured with an ammeter. When measuring, connect the ammeter in series in the circuit, and select a range where the ammeter pointer is close to full deflection. This can prevent the current from being too large and damaging the ammeter. Voltage The reason why river water can flow is because of the water level difference; the reason why electric charges can flow is because of the potential difference. Potential difference is also voltage. Voltage is the reason for the formation of current. In a circuit, voltage is often represented by U. The unit of voltage is volt (V), and millivolt (mV) or microvolt (uV) are also commonly used as units. 1V=1000mV, 1mV=1000uV. Voltage can be measured with a voltmeter. When measuring, connect the voltmeter in parallel to the circuit and select a range where the voltmeter pointer is close to full deflection. If the voltage on the circuit cannot be estimated, use a large range first, make a rough measurement, and then use an appropriate range. This can prevent the voltmeter from being damaged due to excessive voltage. The part of the resistor circuit that hinders the passage of current and causes energy consumption is called a resistor. Resistance is often represented by R. The unit of resistance is ohm (Ω), and kiloohms (kΩ) or megaohms (MΩ) are also commonly used as units. 1kΩ=1000Ω, 1MΩ=1000000Ω. The resistance of a conductor is determined by the material, cross-sectional area and length of the conductor. Resistance can be measured with the ohm range of a multimeter. When measuring, select an ohm range where the meter pointer is close to half deflection. If the resistor is in the circuit, open one end of the resistor before measuring. Ohm's Law The current I in the conductor is proportional to the voltage U across the conductor and inversely proportional to the resistance R of the conductor, that is, I=U/R. This law is called Ohm's law. If two of the three quantities of voltage, current and resistance are known, the third quantity can be calculated according to Ohm's law, that is, I=U/R, R=U/I, U=I×R. In AC circuits, Ohm's law also holds true, but the resistance R should be changed to impedance Z, that is, I=U/Z. Power supply A device that converts other forms of energy into electrical energy is called a power supply. Generators can convert mechanical energy into electrical energy, and dry cells can convert chemical energy into electrical energy. Generators, dry cells, etc. are called power supplies. A device that converts AC into DC through a transformer and a rectifier is called a rectifier power supply. An electronic device that can provide signals is called a signal source. A transistor can amplify the signal sent from the front and transmit the amplified signal to the subsequent circuit. A transistor can also be regarded as a signal source for the subsequent circuit. Rectifier power supplies and signal sources are sometimes also called power supplies. Load A device that converts electrical energy into other forms of energy is called a load. Motors can convert electrical energy into mechanical energy, resistors can convert electrical energy into heat energy, light bulbs can convert electrical energy into heat energy and light energy, and speakers can convert electrical energy into sound energy. Motors, resistors, light bulbs, speakers, etc. are all called loads. For the previous signal source, the transistor can also be regarded as a load. The path through which the circuit current flows is called a circuit. The simplest circuit consists of a power supply, a load, and components such as wires and switches. A circuit that is connected everywhere is called a path. Only when there is a path can current flow through the circuit. A circuit that is disconnected at a certain point is called a short circuit or an open circuit. When the two ends of a part of a circuit are directly connected, making the voltage of this part zero, it is called a short circuit. Electromotive force Electromotive force is a physical quantity that reflects the ability of a power supply to convert other forms of energy into electrical energy. Electromotive force generates voltage at both ends of a power supply. In a circuit, electromotive force is often represented by δ. The unit of electromotive force is the same as that of voltage, which is also volts. The electromotive force of a power supply can be measured with a voltmeter. When measuring, the power supply should not be connected to the circuit. Use a voltmeter to measure the voltage across the power supply. The voltage value obtained can be regarded as equal to the electromotive force of the power supply. If the power supply is connected to the circuit, the voltage across the power supply measured by the voltmeter will be less than the electromotive force of the power supply. This is because the power supply has internal resistance. In a closed circuit, the current has an internal voltage drop through the internal resistance r and an external voltage drop through the external resistance R. The electromotive force δ of the power supply is equal to the sum of the internal voltage Ur and the external voltage UR, that is, δ=Ur+UR. Strictly speaking, even if the power supply is not connected to the circuit, the voltage across the power supply is measured with a voltmeter, and the voltmeter becomes an external circuit, and the measured voltage is also less than the electromotive force. However, since the internal resistance of the voltmeter is large and the internal resistance of the power supply is small, the internal voltage can be ignored. Therefore, the voltage across the power supply measured by the voltmeter can be regarded as equal to the electromotive force of the power supply. When the dry battery is worn out, the voltage across the battery is measured with a voltmeter. Sometimes it is still relatively high, but after connecting to the circuit, the load (radio, tape recorder, etc.) cannot work normally. This happens because the internal resistance of the battery has increased, even larger than the resistance of the load, but still smaller than the internal resistance of the voltmeter. When using a voltmeter to measure the voltage across the battery, the internal voltage divided by the internal resistance of the battery is not large, so the voltage measured by the voltmeter is still relatively high. However, after the battery is connected to the circuit, the internal voltage divided by the internal resistance of the battery increases, and the voltage divided by the load resistance decreases, so the load cannot work normally. In order to determine whether the old battery can be used, the voltage across the battery should be measured when there is a load. For some voltage-stabilized power supplies with poor performance, the voltage across the power supply measured with and without load is quite different, which is also caused by the large internal resistance of the power supply. The time required for periodic alternating current to complete a complete change is called the cycle, which is usually represented by T. The unit of cycle is second (s),Milliseconds (ms) or microseconds (us) are also often used as units. 1s=1000ms, 1s=1000000us. Frequency The number of times that AC completes periodic changes within 1s is called frequency, which is often represented by f. The unit of frequency is hertz (Hz), and kilohertz (kHz) or megahertz (MHz) are also often used as units. 1kHz=1000Hz, 1MHz=1000000Hz. The frequency f of AC is the reciprocal of the period T, that is, f =1/T. Capacitance Capacitance is a physical quantity that measures the ability of a conductor to store charge. When a certain voltage is applied to two insulated conductors, they will store a certain amount of electricity. One conductor stores positive charge, and the other conductor stores an equal amount of negative charge. The greater the applied voltage, the more electricity is stored. The stored electricity is proportional to the applied voltage, and their ratio is called capacitance. If voltage is represented by U, electric quantity is represented by Q, and capacitance is represented by C, then C＝Q／U. The unit of capacitance is Farad (F), and microfarad (uF) or picofarad (pF) is also commonly used as the unit. 1F=106uF, 1F=1012pF. Capacitance can be measured with a capacitance tester, or roughly estimated with the ohmmeter of a multimeter. When the red and black test leads of the ohmmeter touch the two legs of the capacitor respectively, the battery in the ohmmeter will charge the capacitor, and the pointer will deflect. After charging, the pointer returns to zero. When the red and black test leads are swapped, the capacitor will charge in the reverse direction after discharge. The larger the capacitance, the greater the pointer deflection. By comparing the deflection of the measured capacitance and the known capacitance, the value of the measured capacitance can be roughly estimated. In general electronic circuits, except for the tuning circuit and other capacitors that require relatively accurate capacitance, the most commonly used DC isolation, bypass capacitors, filter capacitors, etc. do not require capacitors with accurate capacitance. Therefore, it is practical to roughly estimate the capacitance value with the ohmmeter. However, the ohm range of an ordinary multimeter can only estimate capacitance with a larger value. For capacitance with a smaller value, the ohm range of a transistor multimeter with a large median resistance must be used to estimate it. Capacitances less than tens of picofarads can only be measured with a capacitance tester. Capacitive reactance AC can pass through a capacitor, but the capacitor still has an obstruction effect on AC. The obstruction effect of a capacitor on AC is called capacitive reactance. When the capacitance is large, AC can easily pass through the capacitor, which means that the capacitance is large and the obstruction effect of the capacitor is small; when the frequency of AC is high, AC can also easily pass through the capacitor, which means that the frequency is high and the obstruction effect of the capacitor is small. Experiments have shown that capacitive reactance is inversely proportional to capacitance and frequency. If capacitive reactance is represented by XC, capacitance is represented by C, and frequency is represented by f, then XC = 1/(2πfC) and the unit of capacitive reactance is ohm. Knowing the frequency f of AC and the capacitance C, the capacitive reactance can be calculated using the above formula. Inductance Inductance is a physical quantity that measures the ability of a coil to generate electromagnetic induction. When a current is passed through a coil, a magnetic field will be generated around the coil, and magnetic flux will pass through the coil. The greater the power supplied to the coil, the stronger the magnetic field and the greater the magnetic flux passing through the coil. Experiments have shown that the magnetic flux passing through the coil is proportional to the current supplied, and their ratio is called the self-inductance coefficient, also called inductance. If the magnetic flux passing through the coil is represented by φ, the current is represented by I, and the inductance is represented by L, then L = φ/I. The unit of inductance is Henry (H), and millihenry (mH) or microhenry (uH) is also commonly used as a unit. 1H = 1000mH, 1H = 1000000uH. Inductive reactanceAlternating current can also pass through the coil, but the inductance of the coil has an obstruction effect on the alternating current, and this obstruction is called inductive reactance. When the inductance is large, it is difficult for alternating current to pass through the coil, which means that the inductance is large and the obstruction effect of the inductance is large; when the frequency of the alternating current is high, it is difficult for the alternating current to pass through the coil, which means that the frequency is high and the obstruction effect of the inductance is also large. Experiments have shown that inductive reactance is proportional to inductance and frequency. If inductive reactance is represented by XL, inductance is represented by L, and frequency is represented by f, then XL＝ 2πfL. The unit of inductive reactance is ohm. Knowing the frequency f of the alternating current and the inductance L of the coil, the inductive reactance can be calculated using the above formula. Impedance In a circuit with resistance, inductance and capacitance, the obstruction to alternating current is called impedance. Impedance is often represented by Z. Impedance consists of resistance, inductive reactance and capacitive reactance, but it is not a simple addition of the three. If the three are connected in series, and the frequency f of the alternating current, resistance R, inductance L and capacitance C are known, then the impedance of the series circuit is in ohm. For a specific circuit, the impedance is not constant, but changes with the frequency. In a series circuit of resistance, inductance and capacitance, the impedance of the circuit is generally greater than the resistance. That is, the impedance is reduced to the minimum value. In a parallel circuit of inductance and capacitance, the impedance increases to the maximum value when resonating, which is the opposite of the series circuit. Phase Phase is a physical quantity that reflects the state of alternating current at any time. The magnitude and direction of alternating current change with time. For example, the formula for sinusoidal alternating current is i=Isin2πft. i is the instantaneous value of the alternating current, I is the maximum value of the alternating current, f is the frequency of the alternating current, and t is time. As time goes by, the alternating current can change from zero to the maximum value, from the maximum value to zero, and from zero to the negative maximum value, and from the negative maximum value to zero, as shown in Figure 3A. In trigonometric functions, 2πft is equivalent to an angle, which reflects the state of the alternating current at any time, whether it is increasing or decreasing, positive or negative, etc. Therefore, 2πft is called phase, or phase. Figure 3 If i is not equal to zero when t is equal to zero, the formula should be changed to i=Isin(2πft+ψ), as shown in Figure 3B. Then 2πft+ψ is called phase, and ψ is called initial phase, or initial phase. Phase difference The difference in phase between two alternating currents of the same frequency is called phase difference, or phase difference. The two alternating currents with the same frequency can be two alternating currents, two alternating voltages, two alternating electromotive forces, or any two of these three quantities. For example, the phase difference between the alternating voltage applied to the circuit and the alternating current passing through the circuit is studied. If the circuit is a pure resistor, the phase difference between the alternating voltage and the alternating current is equal to zero. That is to say, when the alternating voltage is equal to zero, the alternating current is also equal to zero, and when the alternating voltage reaches its maximum value, the alternating current also reaches its maximum value. This situation is called in-phase, or in-phase. If the circuit contains inductance and capacitance, the phase difference between the alternating voltage and the alternating current is generally not equal to zero, that is, they are generally out of phase, or the voltage leads the current, or the current leads the voltage. The phase difference between the alternating voltage applied to the base of the transistor amplifier and the alternating voltage output from the collector is exactly equal to 180°. This situation is called anti-phase, or anti-phase.As shown in Figure 3B. Then 2πft+ψ is called phase, and ψ is called initial phase, or initial phase. Phase difference The difference between the phases of two alternating currents with the same frequency is called phase difference, or phase difference. These two alternating currents with the same frequency can be two alternating currents, two alternating voltages, two alternating electromotive forces, or any two of these three quantities. For example, study the phase difference between the alternating voltage applied to the circuit and the alternating current passing through the circuit. If the circuit is a pure resistor, then the phase difference between the alternating voltage and the alternating current is equal to zero. That is to say, when the alternating voltage is equal to zero, the alternating current is also equal to zero, and when the alternating voltage reaches the maximum value, the alternating current also reaches the maximum value. This situation is called in phase, or in phase. If the circuit contains inductance and capacitance, the phase difference between the alternating voltage and the alternating current is generally not equal to zero, that is, they are generally out of phase, or the voltage leads the current, or the current leads the voltage. The phase difference between the alternating voltage applied to the base of the transistor amplifier and the alternating voltage output from the collector is exactly equal to 180°. This situation is called anti-phase, or anti-phase.As shown in Figure 3B. Then 2πft+ψ is called phase, and ψ is called initial phase, or initial phase. Phase difference The difference between the phases of two alternating currents with the same frequency is called phase difference, or phase difference. These two alternating currents with the same frequency can be two alternating currents, two alternating voltages, two alternating electromotive forces, or any two of these three quantities. For example, study the phase difference between the alternating voltage applied to the circuit and the alternating current passing through the circuit. If the circuit is a pure resistor, then the phase difference between the alternating voltage and the alternating current is equal to zero. That is to say, when the alternating voltage is equal to zero, the alternating current is also equal to zero, and when the alternating voltage reaches the maximum value, the alternating current also reaches the maximum value. This situation is called in phase, or in phase. If the circuit contains inductance and capacitance, the phase difference between the alternating voltage and the alternating current is generally not equal to zero, that is, they are generally out of phase, or the voltage leads the current, or the current leads the voltage. The phase difference between the alternating voltage applied to the base of the transistor amplifier and the alternating voltage output from the collector is exactly equal to 180°. This situation is called anti-phase, or anti-phase.