① Polarity
The lack of electrons (positive charge) or excess electrons (negative charge) cause the material to carry an electric charge. When the electron distribution is equal, the material is electrically neutral. Along with the change of the electronic balance, there will be a potential difference, which will cause the movement of the current to do work. For this, we need to provide energy for the material. This process may be as simple as rubbing a piece of plastic to generate a static charge, or it may involve storing chemical energy in the battery to generate positive and negative electrodes.
The process of discharging a rechargeable battery with a potential difference until the potential difference is zero is shown in Figure 1.
②Conductor
When electrons can be easily transferred from one atom to another in a material, this material is a conductor. Generally speaking, all metals are conductors. Silver is the best conductor, followed by copper. Copper is usually used because its cost is lower than silver, but the current long-distance transmission lines are more inclined to use aluminum. Aluminum is indeed slightly inferior to copper in conductivity, but it is lighter and cheaper than copper. The conductor allows current (electrons) to flow from the power source to the load with minimal obstruction. For example, the wire connecting the flashlight to its battery is a conductor.
③Insulator
Electrons cannot move easily, but materials that are more tightly bound to their electron orbits are called insulators. This material will prevent the flow of current, and is usually used where it is necessary to isolate the current or where the current flow may cause danger, such as cable coating. Because the insulator prevents electrons from moving inside its structure, it can store charge. The insulator used to store charge is called a dielectric. Dry air is a kind of dielectric, and any electrostatic shock can be restrained. Once the air becomes humid, static electricity can flow into the earth through the air.
④Semiconductor
Materials that are neither insulators nor conductors, but can exhibit some of the properties of the two are called semiconductors. For example, carbon, silicon and thread. Germanium was widely used in early transistor manufacturing, but it has been largely replaced by silicon. The microchip and solar cell industries consume a lot of silicon.
⑤ Potential difference
Potential refers to the possibility of doing work. When we attach energy to the material to cause a change in its charge state (that is, provide a “potential”), it gives it the ability to try to return to its neutral state and do work. A unit of charge is called Coulomb (C). It is equivalent to the number of charges carried by 6.25×1018 electrons. The potential difference between two charges is called the potential difference, and its unit is called volts (V).
⑥Volt
Volt is a unit of work done by movement between unit charges. In short, moving 1C of charge between two points requires 1J of energy, and the potential difference between these two points is 1V. Voltage is the potential difference between two points. Voltage is sometimes called electromotive force (EMF), the symbol is E, but for the voltage drop on the power source or passive components, the standard symbol of potential difference is U.
⑦Current
The potential difference causes the charge to move between two points, and the moving charge is called current. The number of moving electrons depends on the potential difference between the two points. The greater the potential difference, the greater the current. The current is measured in amperes (A).
Current is the flow of electrons between two points, and 1C (6.25×1018 electrons) electrons flowing through a given point within 1s is a current of 1A. The symbol of current is I (representing “intensity”, which indicates the concentration or intensity of electron flow). All electrons move at the same speed, only the amount is different. Therefore, if the potential difference is doubled, the number of electrons will also double, but their moving speed will not change. Electrons flow from the negative electrode to the positive electrode, that is, the direction of current, as shown in Figure 2.
The flow of electrons from the negative electrode to the positive electrode (electron current) is equivalent to the positive charge flowing from the positive electrode to the negative electrode (traditional current). Traditional current flow is often used to explain the operation of electrical, electronic, equipment, and circuits.
⑧Resistance
A conductor carrying current will always produce a certain amount of resistance in the opposite direction to the current. Since voltage is required to do work to overcome current resistance in the opposite direction, the conductor temperature will also rise. This reaction is called resistance, and it reduces the amount of current that can flow through the conductor. Good conductivity The resistance of the conductor is small, and the resistance of the insulator is high. The unit of resistance is ohm (Ω).
A current of 1A flowing through a 1Ω resistor for 1s will generate 1J of heat. The resistance symbol is R, which can be represented by the Greek letter Q. Conductors such as steel wires have typical electrical resistance, and copper wires have a resistivity of 0.018Ω·mm²/m. Resistance wires, such as heating elements used in toasters, have a resistance of about 24Ω. In the circuit diagram, the resistance is represented by a rectangle, as shown in Figure 3.
⑨Circuit
The path of current l flowing from one electrified point to another is called a circuit. The potential difference U acts on the circuit to cause current to flow. The current flows from the power supply to the load through the circuit, and the current flowing through is called the load current. When any part of the current path is disconnected, an open circuit is formed, and no current flows at this time.
If a fault occurs, current flows in the closed path at both ends of the power source, forming a short circuit. Under short-circuit conditions, the current will be very large. We generally install a fuse in the circuit to prevent this from happening. The circuit is divided into series circuit and parallel circuit, or a combination of the two.
⑩Fuses and circuit breakers
A fuse is a device used to prevent excessive current from causing damage to the conductor in the circuit. It can also reduce the risk of fire caused by overheating of the conductor. It is usually composed of a melt and an insulating shell. When a fault occurs, the fuse can break the circuit. The installation of the fuse may require rewiring or plug-in type.
A circuit breaker is a mechanical device that can open a circuit in a fault state and can be reset when the fault is removed.
A current exceeding the rated current of the fuse or circuit breaker will cause the device to work (break the circuit).
The reconnected fuse is currently considered insufficient to protect the wired system, so a box-type (HRC) fuse or a corresponding rated current (AC or DC) circuit breaker should be used.
| DC voltage | AC voltage |
| Fixed polarity | Reversal of polarity regularly |
| Stable or variable size | The size keeps changing during the cycle |
| Traditional DC voltage cannot be easily increased or decreased, but with the development of DC-DC converters, changes in DC voltage become possible | Can be easily increased or decreased by transformer |
⑪Direct current (DC) and alternating current (AC)
Figure 2 depicts a DC current flowing in one direction. In the DC circuit, because the battery polarity is fixed, current can only flow in one direction. If you want the current to flow in the reverse direction, you need to exchange the connecting wires at both ends of the battery. The battery voltage is relatively fixed, so there is a stable DC voltage in the circuit.
The AC power source regularly reverses its output polarity. The polarity reversal provided by the Australian power grid is 100 times/s, the cycle is 50Hz, and the voltage is 240V. The advantage of alternating current is that it is easy to achieve voltage conversion by using a transformer.
⑫Magnetic effect
When current flows through the conductor, a magnetic field is established around the wire, which exists in a plane perpendicular to the current. Magnetic fields are the basis of many electromagnetic applications, such as speakers, electromagnets, relays, transformers, motors, etc.
The magnetic effect produced by the current in the conductive wire is shown in Figure 4.
Figure 4 shows the magnetic field generated by the flow of current inside the wire. The magnitude of the magnetic field is directly related to the magnitude of the current flowing through the wire. The magnetic field can be strengthened by winding the wire into a coil (primary winding). Through current changes, such as providing alternating current, the size of the magnetic field will also change. Placing another conductor (secondary winding) in a magnetic field will induce current, which is the principle of a transformer. The magnitude of the voltage flowing through the circuit is related to the transformation ratio of the primary winding and the secondary winding.
⑬Electromagnetic
By moving a conductor in a fixed magnetic field, an induced current can be generated inside it. This is the principle of a generator. The output current varies with the intensity of the magnetic field and the number of conductor coils in the magnetic field. The output current can be converted into direct current (generator) by using a converter or alternating current (alternator) can be generated by using a slip ring.
⑭ Ohm’s law, power and energy
There is a direct relationship between the current I, the voltage U and the resistance R. The relationship is expressed by Ohm’s law, namely
U=IR
Can also be written as
I=U/R or R=U/I
According to these formulas, given two known parameters, the third unknown parameter can be calculated.
The unit of power is watts (W). 1W is equal to the work done by moving 1C of electric charge under a voltage of 1V. Because 1C/s is equal to 1A, you can get power equal to the product of voltage and current.
1W=1V×1A or P=lU W is the unit of power and the rate of work done. For example, the energy used to climb a flight of stairs is equal in amount to the energy used to walk to a slope of the same height, but the rate of work is different. kW is often used to measure large-scale power, 1000W=lkW.
Energy is defined as the ability to do work, that is, 1W·h=1W·1h or E=Pt.
A large number of levels of electric power or energy are expressed in kw·h, which can be used by multiplying power by time. The difference between power and energy is an important concept, and the output of a renewable energy system within a certain period of time is expressed in energy.
[Example] A 60W bulb can illuminate for 12 hours. Will consume 720W·h or 0.72kW*h energy, the calculation process is
60W×12h=720W·h=0.72kW·h
⑮License requirements
The policy defines an AC voltage not exceeding 50V and a non-pulsating DC voltage not exceeding 120V as ultra-low voltage (ELV). Australia’s states and territories require electrical machinery/fitter contractor licenses to work in any environment exceeding ELV (remember that restricted electricity licenses are also useful for some transactions). The work related to the ultra-low voltage system must be undertaken by individuals who have been trained or have relevant experience in this field. If a renewable energy system is to be designed and installed in Australia, there are currently some mandatory standards that need to be complied with.
In addition, all regions have local workplace health and safety laws. In addition to the obvious risk of injury, failure to comply with workplace health and safety laws can also result in fines for employers and workers and possible insurance claims.
Be sure to clarify the relevant local requirements.
⑯ electric shock
Any time you work on the circuit, you will face the risk of electric shock. An electric current of one ten millionth of an ampere is sufficient to produce an electric shock. “Shock” is a sudden involuntary muscle contraction caused by electric current passing through the body. When a sufficiently large current passes through the body, it will cause electrocution. Due to the existence of the skin, the human body usually has a relatively large resistance, but when the skin is damaged, a small voltage will generate a deadly current. High voltage will damage the skin resistance and produce a large enough current to be fatal. The body will act as a conductor and cause severe burns inside. A voltage of 500V passing through a body with a resistance of about 25000Ω will generate a current of 20mA, which will be fatal.
Due to muscle contraction, the human body may not be able to let go of a live conductor. This is more obvious in a DC circuit with a constant unidirectional current, where the muscles in the hand will grip the live conductor. The allowable current threshold for men is about 9mA, while that for women is only about 6mA.
Even if you “confirm” that the power is off, always use appropriate insulation test equipment to test the voltage on the wires.
⑰First aid
Anyone working in the electrical and related industries should be familiar with CPR and keep in mind that if you try to rescue a person from a live conductor, you may also receive a fatal electric shock. Therefore, the power should be turned off first, and if it cannot be turned off, an insulator should be used to rescue it.
Always put safety first!