A circuit refers to the path through which current passes, composed of various electrical devices or components connected in a certain way to form a whole. The basic components of a circuit include power supply, load, and intermediate links.
Table of Contents
1. Current
The directional movement of the charge produces a current, which is often represented by I. There are two types of current: direct current and alternating current.
A current whose magnitude and direction do not change with time is called direct current.
The magnitude and direction of the current that changes with time is called alternating current.
The unit of current is ampere (A), and the unit of milliampere (mA) or microampere (uA) is also commonly used. 1A=1000mA, 1mA=1000uA.
Current can be measured with an ammeter. When measuring, connect the ammeter in series in the circuit, and select the range where the pointer of the ammeter is close to full deflection. This prevents excessive current from damaging the ammeter.
2. Voltage
The reason why river water can flow is because of the water level difference; the reason why the electric charge can flow is because of the electric potential difference. Potential difference is also voltage.
Voltage is what makes current. In the circuit, the voltage is often represented by U. The unit of voltage is volt (V), and the unit of millivolt (mV) or microvolt (uV) is also commonly used. 1V=1000mV, 1mV=1000uV.
Voltage can be measured with a voltmeter. When measuring, connect the voltmeter in parallel with the circuit, and select the range where the pointer of the voltmeter is close to full deflection.
If the voltage on the circuit cannot be estimated, use a large range first, and then use a suitable range after rough measurement. This prevents damage to the voltmeter due to excessive voltage.
3. Resistance
The size of the resistance of the conductor to the current is called resistance, and the resistance is often represented by R.
The unit of resistance is ohm (Ω), and the unit of kiloohm (kΩ) or megohm (MΩ) is also commonly used. 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 a multimeter in the ohm range. When measuring, choose the ohm range where the pointer of the meter is nearly deflected by half. If the resistor is in the circuit, lift up one end of the resistor before measuring.
Ohm’s Law: The current I in a 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 you know two of the three quantities of voltage, current, and resistance, you can find the third quantity according to Ohm’s law, namely
I=U/R, R=U/I, U=I×R
In the AC circuit, Ohm’s law also holds, but the resistance R should be changed to the impedance Z, that is, I=U/Z
4. Power supply
Devices that convert other forms of energy into electrical energy are called power sources. Generators can convert mechanical energy into electrical energy, and dry batteries can convert chemical energy into electrical energy.
Generators, dry batteries, etc. are called power sources. A device that converts alternating current into direct current through a transformer and a rectifier is called a rectifier power supply.
An electronic device that provides a signal is called a signal source. The transistor can amplify the signal sent in front, and transmit the amplified signal to the circuit behind.
For the following circuits, the transistor can also be regarded as a signal source. Rectified power supplies and signal sources are sometimes called power supplies.
5. Load
A device that converts electrical energy into other forms of energy is called a load. Electric motors can convert electrical energy into mechanical energy, resistors can convert electrical energy into thermal energy, light bulbs can convert electrical energy into thermal energy and light energy, and speakers can convert electrical energy into sound energy.
Motors, resistors, light bulbs, speakers, etc. are all called loads. The transistor can also be regarded as a load for the previous signal source.
6. Circuit
The path through which current flows is called a circuit. The simplest circuit consists of a power supply, a load and components such as wires, switches, etc. A circuit connected everywhere is called a path.
There is only a path where current flows in a circuit. A break in a circuit is called an open circuit. If the two ends of a certain part of the circuit are directly connected, so that the voltage of this part becomes zero, which is called a short circuit.
7. EMF
Electromotive force is a physical quantity that reflects the ability of a power source to convert other forms of energy into electrical energy. The electromotive force creates a voltage across the power supply.
In the circuit, the 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 the power supply can be measured with a voltmeter. When measuring, do not connect the power supply to the circuit, use a voltmeter to measure the voltage across the power supply, and the obtained voltage value 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 with a 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 through the internal resistance r has an internal voltage drop, and through the external resistance R has an external voltage drop.
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, use a voltmeter to measure the voltage across the power supply, the voltmeter becomes an external circuit, and the measured voltage is less than the electromotive force.
However, due to the large internal resistance of the voltmeter and the small internal resistance of the power supply, 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.
8. Inductance
Inductance is a physical quantity that measures the ability of a coil to generate electromagnetic induction. When a current is applied to a coil, a magnetic field will be generated around the coil, and the coil will have a magnetic flux passing through it.
The greater the power supply to the coil, the stronger the magnetic field and the greater the magnetic flux through the coil.
Experiments have shown that the magnetic flux passing through the coil is proportional to the incoming current, and their ratio is called the self-inductance coefficient, also called the inductance.
If the magnetic flux through the coil is denoted by φ, the current is denoted by I, and the inductance is denoted by L, then
L = φ/I
The unit of inductance is Henry (H), which is also commonly used as a unit of millihenry (mH) or microhenry (uH). 1H=1000mH, 1H=1000000uH.
9. Inductive reactance
Alternating current can also pass through the coil, but the inductance of the coil hinders the alternating current, which is called inductive reactance.
If the inductance is large, it is difficult for the alternating current to pass through the coil, which means that the inductance is large and the inductance has a great hindering effect.
Experiments have shown that the inductive reactance is proportional to the inductance, and also proportional to the frequency. If the inductance is denoted by XL, the inductance is denoted by L, and the frequency is denoted 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, you can use the above formula to calculate the inductive reactance.
