Electric Current
The directional movement of the charge is called a circuit, and the current is often represented by I. Current is divided into DC and AC. The size and direction of the current do not change with time is called direct current. The magnitude and direction of current change with time is called alternating current.
The unit of current is ampere (A), and milliampere (mA) or microamp (uA) is also commonly used as the unit. 1A=1000mA，1mA=1000uA。
The 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 will prevent the ammeter from being damaged by excessive current.
Voltage
The river can flow because of the water level difference; The charge can flow because of the potential difference. The potential difference is the voltage. Voltage is the cause of the current. In a circuit, the voltage is usually expressed in 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。
The voltage can be measured with a voltmeter. When measuring, connect the voltmeter in parallel to the circuit, and select the range where the voltmeter pointer 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 will prevent damage to the voltmeter due to excessive voltage.
Resistor
The part of a circuit that blocks the passage of current and consumes energy is called resistance. The resistance is usually represented by R. The unit of resistance is ohm (Ω), and kiloohm (k Ω) or megaohm (M Ω) is also commonly used as the unit. 1kΩ=1000Ω，1MΩ=1000000Ω。 The resistance of a conductor is determined by its material, cross-sectional area and length.
The resistance can be measured with a multimeter ohm scale. When measuring, choose the ohm scale where the pointer of the meter is close to half deflection. If the resistance is in the circuit, open one end of the resistance and then measure it.
Ohm’s Law The current I in a conductor is proportional to the voltage U at both ends of 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 calculate the third quantity according to Ohm’s law, that is
I=U/R，R＝U／I，U＝I × R
In an AC circuit, Ohm’s law is also valid, but the resistance R should be changed to the impedance Z, that is, I=U/Z
Power Supply
A device that converts other forms of energy into electrical energy is called a power source. Generators can convert mechanical energy into electrical energy, and dry cells can convert chemical energy into electrical energy. Generators, dry batteries, etc. are called power supplies. The device that converts alternating current into direct current through transformer and rectifier is called rectifier power supply.
The electronic equipment that can provide signals is called signal source. The crystal triode can amplify the signal sent from the front and transmit the amplified signal to the circuit behind. The crystal triode can also be regarded as the signal source for the circuit behind. Rectification power supply and signal source are sometimes called power supply.
Load
A device that converts electrical energy into other forms of energy is called a load. The motor can convert electric energy into mechanical energy, the resistance can convert electric energy into heat energy, the electric bulb can convert electric energy into heat energy and light energy, and the loudspeaker can convert electric energy into sound energy. Motors, resistors, light bulbs, loudspeakers, etc. are called loads. The crystal triode can also be regarded as a load for the front signal source.
Circuit
The path through which current flows is called a circuit. The simplest circuit consists of power supply, load, wire, switch and other components. A circuit connected everywhere is called a path. Only the path, there is current in the circuit. A break in a circuit is called an open circuit or an open circuit. The two ends of a certain part of the circuit are directly connected to make the voltage of this part become zero, which is called short circuit.
