Basics of Electricity. Understanding the Essential Concepts

1. What is electricity?

Electricity has become an integral part of our daily lives so much so that there is no need to reiterate this fact. However, when we say electricity, it can range from weak electricity generated inside a human body to a few volts of electricity in a battery, a 100V power supply for household use, high-voltage electricity in transmission lines ranging from a few thousands to a few hundreds of thousands of volts, and destructive electricity of lightning that exceeds a few hundreds of millions of volts.

To understand the fundamentals of electricity, it is necessary to have a correct understanding of the concepts such as, voltage, electric current, electric power and electrical resistance. It is easy to understand these concepts if we think of electricity in comparison with the flow of water. We can understand the concepts faster by comparing electricity, which is not visible to the human eye, with water, which is familiar to us and for which we have an experiential understanding.

Supplementary Explanation

* Though there is a precise definition forthe unit ofvolt V, it is enough if you can instinctively feel its magnitude from, say, a dry cellwhich is of 1.5 volts, and a general household power supply whichis 100 volts.The higher the pressure of electricity, which is same asvoltage, tries to forcibly go through a passage that is difficult for electricity to pass through.

* Pascal, the unit of physical pressure such as hydraulic pressure, is defined as “Pressure of 1 Newton per meter square”. 1 pascal Pa is a very small unit of pressure. For example, 1 atmospheric pressure is 101, 300Pa, and hydraulic pressure in the water supply system is about 150, 000Pa.

However, as this leads to excessively large numerical values, normally, pascal is used with prefixes such as hecto (100 times) or mega (1,000,000 times). With this, 1 atmospheric pressure becomes equal to 1013 hectopascal, a well-known term in weather forecast. In the industrial sector, a more commonly used term is 0.1013 megapascal. An atmospheric pressure of 1 Pa or less is said to be a “High-vacuum” range.

* 1 ampere of current represents “1 coulomb of electrical charge flowing past a given point per second”. 1 coulomb of electrical charge is enormous depending on how you perceive it. The number of electrons is 600, 000, 000, 000, 000, 000, 000 (6 X 1020) and if we keep 2 electrical charges of 1 coulomb each 1m apart, then the electric force working between them is approximately 9 X 109 newton (Force capable of lifting approximately 900,000 tons).

Let’s take a suitable junction box with2 electric wires. When electric current flows out of 1 wire, the same amount of current always flows into the other wire at the same time, and the currents are balanced out. This is an important law of electricity (Kirchhoff’s first law). Let’s assume that there are only leaving currents, then a huge force of attraction, as mentioned previously, will operate between those currents (electrical charges) that has gone out and the current will be drawn back. Therefore, such a situation does not occur in reality.

Furthermore, Kirchhoff’s first law is not restricted to 2 wires and states that irrespective of the number of wires, the algebraic sum of the currents is always zero. (Assuming that leaving currents are + and entering currents are -) * Watt W, which is the unit of energy such as electrical energy, represents the energy consumption rate of 1 joule per second. 1W = 1J/s. You can instinctively feel it from, say, an electric bulb which is 20W to 100W, a microwave oven which is approximately 1000W. The working of a human body is extremely efficient, consuming only about 100W. As compared to warm-blooded animals (human beings, cats, birds, etc.), the energy consumption per body weight in cold-blooded animals (fish, lizard, etc.) is almost one digit lesser. In a passenger car,consumption can reach up to 200,000W. (However, if we take an hourly average, it is about 1/tenth.) 

2. Meaning of flow of electricity

Electricity generally flows through electric wires, but if this is compared to water, then the electric wire is equivalent to a hose.However, it is like a hose that is fully filled with water. The electric wire is not a pathway with nothing in it, but something like a hose that is filled with electricity.The objects it is filled with are “Electrons” that move freely, but it is also not a problem to call this “Electricity”.

Let’s think about the flow of water. When water is pushed in from one side of a long hose filled with water, it will instantly come out from the opposite side.Even if water is slowly pushed in, that flow will instantly become the same as within the entire hose. Even if the hose is hundreds of meters long, the water will instantly gush out from the other end of the hose.

Even though the movement of water within the hose itself is slow, it should be understood that the speed of the flow is transmitted at an intense speed.

It is the same in the case of electricity too. If “Electrons” are pushed into one end of the electric wire that is filled with electrons, no matter how long it is, and the movement is transferred throughout the entire wire with all the electrons moving simultaneously and the same number of electrons go out from the other end.At this time, the speed of the electrons moving within the wire itself is not very fast, but the speed with which electricity flows within the wire is instantaneous (precisely almost the speed of light → approximately 300,000 km/sec).

In other words, when electricity is flowing through an object, it does not mean that electricity flowed from outside the object and passed through it. It must be considered that the electricity (electrons) in the object itself has caused a large migration on the whole, due to electrons pushed in from the outside.Therefore, electricity will not flow in objects that do not have free moving electrons originally.Conversely, even if an object has electrons that move freely, electric current can flow even if electricity does not flow in from the outside.Recently, the much-used electromagnetic cooker gives a changing magnetic field to the all-metal bottom of the pot to move its own electrons. This causes current to flow and cooking takes place by the heat generated due to it.

In addition to that, the principle that electric current is generated by a changing magnetic field in many electric devices such as transformers and motors is also being used.

A magnetic field is a special temporal state around a magnet.The magnetic field is generated by the flow of electric current. The interaction with the magnetic field induces an electric current and this is a very important phenomenon for using electricity.

3. Voltage

Voltage is the electric pressure which is equivalent to the water pressure of water. The unit is volt. In water flow, water pressure is necessary according to its condition. If water pressure is zero, water cannot flow. It is the same even in the case of electricity and current does not flow unless voltage is applied (Excluding the phenomenon of superconductivity).

When water flows through the hose, if pressure is too high, the hose bursts and water will flow out. In the case of electricity, when the voltage increases and exceeds the withstanding voltage limit of the electric wire, the current leaks from the wire resulting in a very dangerous condition.This is called dielectric breakdown, and it is often accompanied by ignition and explosions. For that reason, limitations on voltage that can be used are defined for each wire.

Electrical wires are usually coated with an insulating coating around copper wire (Part in which electricity flows) so that electricity does not leak out. The most widely used coating are polyvinyl chloride and rubber. For applications requiring heat resistance, silicone rubber coating is widely used (Approximately200°C). However, since silicon has weak mechanical strength, electrical wires whose outside is further covered with glass fiber cloth are often used. Electric wires using fluororesin (Teflon) have good heat resistance, mechanical strength, and chemical resistance and a thin insulation layer can be used, butit is more dangerous than silicone rubberif the limit temperature is exceeded. Silicon rubber will become hard and brittle when it exceeds the heat resistance limit, but since Teflon softens, it tends to lead to short circuit accidents and leakage. Of course, there is no problem if it is properly used within the heat resistance limit (Up to approximately 250°C).

Air is a good insulator. Copper wires without insulation coating are used for high voltage transmission lines. By keeping it in the air, it is insulated so that electricity will not escape. However, there is a limit to the withstanding voltage, withstanding voltage of air is about 1000 V per 1 mm. Therefore, for example, when approaching approximately 1 cm near a metal exposed portion to which a voltage of 10,000 volts is applied, even if it is not in contact, electricity jumps out and may cause electrocution. Similarly, for transmission lines of 200,000 volts, it is dangerous up to approximately20 cm.

Using air to insulate the transmission line is mainly a matter of cost. Various plastics, rubber, ceramics etc., have much better insulation performance than air.

4. Current

Current is the amount of electricity flowing equivalent to the water flow rate. Unit is AmpereA or Coulomb per secondC/s (This is not normally used)

When water flows through the hose, a hose with thickness corresponding to the desired flow rate is required. When flow of a large amount of water is required, a hose of a certain thickness has to be used. Similarly, in the case of electricity, a wire with a thickness (capacity) according to the value of the current flow is required. When more than the specified current flows, heat generation in the wire is large which increases the temperature. At that time, the coating of the electric wires (Layer of rubber covering the copper wire to prevent the leakage of electricity) may cause fire or electric leakage.

Electric shock occurs due to the flow of electricity in the body.Even if you touch the path of electricity, you will not get an electric shock unless current flows through the body.For the current to flow in the body, application of a voltage (High and low electric potentials come in contact with the human body at the same time) is required. However, even if voltage is applied to the human body, when the voltage is low, electric current is not felt as the current value flowing through the body is weak. There is influence of individual difference and humidity and it is felt when the voltage is about 20 V. At 100 V, the electric shock is severe and dangerous. In general, higher the voltage, greater is the risk of electrocution. It is said that thousands of volts are more dangerous. Just approaching tens of thousands to hundreds of thousands of volts makes us feel abnormal and we will be more careful, and accidents are not likely to happen.

The current capacity of the electric wire depends on the current value that can flow determined from the relationship between the temperature rise caused by passing an electric current and heat resistance of the insulating coating. Therefore, it is not only determined by the thickness of the conductor (Part of the copper wire where electricity flows), but it depends on the heat resistance of the coating as well as the ambient temperature. For example, just bundling a large number of electric wires result in less heat dissipation and current that can flow decreases. However, selecting wires is difficult without a guideline and upper limit value of the current that can flow with respect to the sectional area of the conductor is determined. (This is our company standard)

0.3 Sq --- (Cross sectional area of the conductor (0.3mm2) up to about 3A

0.5 Sq --- (Cross sectional area of the conductor (0.5mm2) up to about 5A

0.75 Sq --- (Cross sectional area of the conductor (0.75mm2) up to about 8A

1.25 Sq --- (Cross sectional area of the conductor (1.25mm2) up to about 15A

2.0 Sq --- (Cross sectional area of the conductor (2.0mm2) up to about 20A

The current value that makes the surface temperature of the conductor the same, is proportional to its diameter compared to the cross-sectional area of the conductor. The above standard has a slight inconsistency. However, Electric cables are difficult to handle if they become too thick, and when the current is large, the wire is selected without any allowance.

5. Power

Power is the ability of electricity to do work and release heat. Comparing with water, it is the magnitude of the ability of water to turn a water wheel.The power to turn the water wheel will be greater as the volume of water and water pressure increases. It is the same in the case of electricity, the power will be higher as the voltage and electric current increases.

Hydraulic power= Hydraulic Pressure x Water flow rate

Power= Voltage x Electric current

From the above equation, it is understood that energy will not be generated even if there is only water pressure, but the water flow rate is zero.It is the same even in the case of electricity. Even if only voltage is applied, energy will not be generated unless electric current flows. In addition, no matter how much current is flowing, if the voltage is close to zero, then the generated energy will also be close to zero.

The same units are used even if the form of energy is thermal energy, kinetic energy or any other forms of energy.They are Joule → J or Watt → W which is J/s. Until recently, Calorie was used as a unit of thermal energy or Horsepower were used for rate of kinetic energy, but now Joule or Watt is used universally.

Watt W is very well known as the unit of power.1-Watt W means the state where energy is continuously supplied at 1 Joule/second. For 100 Watts, energy is continuously supplied at a rate of 100 Joules/ second, which will be 360,000 Joules/ hour. To raise the temperature of 1 gram of water by 1°C, the amount of heat needed is approximately 4.2 Joules. The old unit of 1 Calorie is the energy that raises the temperature of 1 gram of water by 1°C, it means that 1 Calorie is approximately 4.2 Joules.

Power = Voltage x Electric current, if 15 Amperes of current flows with a voltage of 100 Volts, then 1500 Watts of power will be consumed.1500 W is also written as 1.5 kW, which us called Kilowatt, since k (kilo) means 1000 times.

Explanation about the SI prefixes: If the following symbols are placed before the symbols of the units, the number will be multiplied by the numerical values below.

There are also other units such as Deci [d] → one-tenth, Deca [da or D] → ten times, but they are not used much.

6. Transportation of electricity

When using electricity, it becomes necessary to transport electricity from the site where it is generated to the place of consumption. Electric power stationsare mostly located away from the place of consumption. To transport large amounts of electric power, it is necessary to increase the product of voltage and current, since Electric power = Voltage x Electric current.

However, if we increase the current, thick wires become necessary, and transmitting electricity over a few tens to a few hundreds of kilometers in thick wires becomes extremely costly. Therefore, if the same amount of electricity is to be transported, then by increasing the voltage as much as possible, current can be reduced in the same proportion. Thus, high voltage (several hundreds of thousands of volts) is adopted for power transmission over long distances. Also, if voltage is increased, then thinner wires can be used, which, however, increases the cost of insulation (tall and huge steel transmission towers, huge insulators etc.). Therefore, the optimum transmission voltage is determined in due course. Also, when using household electrical appliances, there is a limitation on wires that can be used and the capacity of plugs. Considering from the viewpoint of safety, it is desirable to have a voltage as low as possible, however, this will necessitate the use of thicker wires (cords) to provide the required current, leading to less flexibility and increased difficulty in handling. Practically, about 10Amp is the limit for the wires or types of plugs that can be used in general household electrical appliances.

In Japan, 100V has been adopted as voltage for general households, appliances with a maximum electricity consumption of about 1000 wattscorresponding to the current of 10 ampere can be used. However, an electric range used for cooking could not produce enough heatwith this much electricity, and electric ranges could not compete with gas ranges. This also limits the power of air conditioners.

Many countries use voltage of around 220V, Japan is one of the few countries in the world where the general household voltage is only 100 V. Recently, household appliances using 200V are on the increase in Japan as well. With 200 V, equipment consuming up to about 2kW can be used and an electric range that can match gas ranges can be manufactured.

7. Electric Resistance

Even in the case of water and electricity, there is some resistance to the flow path. If there is no resistance, the hoses are connected in a ring shape and if the water is made to flow initially using some method, water will flowcontinuously through the ring-shaped hose. However, since there is resistance, water will stop flowing after some time. It is the same even in the case of electricity. (However, in the case of a superconductor as a special example, the current continues to flow continuously to the ring-shaped thing.)

If the hose where water is flowing is squeezed by hand at the middle, the resistance of that part increases. Then a difference in water pressure can be established on both sides of the part that has been squeezed. In the squeezed part, water flows intensely with rubbing, generating heat. (In the case of water flow, the heat generated is carried away by water and the heat is not felt.)

In case of electricity, when inserting a resistance (such as a resistor) that resists the flow of electricity in the middle of the electric wire through which current flows, Voltage (potential difference) is generated at both ends and heat is generated. The unit of electric resistance is ohm →Ω. In the case of heat generation by electricity, heat remains in that part and since the amount of heat generated is generally larger than water flow, heat generation can be clearly confirmed. Utilizing this heat, high temperature that cannot be obtained by normal method can be easily obtained. In a bulb, the filament is heated to about 3000°C by this heat generation and it shines brightly.

For example, when a current of 4 ampere flows through a 3-ohm resistor, a voltage is generated across the both ends, its value is resistance value x current value. (3 x 4 = 12 V)

At this time, the resistor generates heat, and the amount of generated heat is resistance value x (current value)₂Amount of heat generated by the resistor = 3 x 42 = 48 Watts.

For convenience, electrical expert’s express voltage using E, currentusing I, power using P,and resistance using R, and the relationship is summarized as follows. These are called Ohm's Law.

I = E/R      E = RI      R = E/I

P = EI      P=RI2      P = E2/R R = E2/P

For example, when P = E2/R, the electric power consumed by the electric heater is the square of the voltage applied to it (Voltage value multiplied twice) divided by its resistance value.

Examples:

(1) Resistance of heater 200 V, 2000 W
From P = E2/R
R= 200 x 200/2000 = 20 Ω
(1) Power when a power of 60 V is applied to a heater 200 V, 2000 W
The resistance of the heater from R = E2/P
R= 100 x 100/200 = 50 Ω
Power P when E is 60 V fromP = E2/R
P = 60 x 50/50 = 72 W
Power can also be obtained from the followingas it is proportional to the square of the voltage.
P = 200 x (60/100)2 = 72 W

8. Conduction of electricity (details)

The freely movable electrons in the electric wires depend on the special properties of the metals. A normal atom consists of nucleus with positive charge and electrons belonging exclusively to the nucleus, the number of + charge of the nucleus and the number of - charge due to the surrounding electrons cancel each other out, and the whole atom has zero electric charge.

The term metal atom is where some electrons are separated from the nucleus and move freely. However, even if it is said that it can move freely, if the positive charge of the nucleus is not cancelled, it will be pulled back with a large electric power, and as a whole it just moves as free electrons according to the distribution of atoms (nucleus). But, since it is independent, it can be shifted or replaced to the next atom freely by applying some force, it will easily move in large groups as well. Of course, even in this case, another free electron group must flow in immediately after moving to cancel the positive charge of the nucleus.

In ordinary substances, electrons belong to specific nucleus as mentioned earlier and there are no freely moving electrons. For this reason, electricity is not conducted. This is called an electrical insulator, and most substances other than metals have this property.

However, even in insulators when the temperature increases, the electrons can become free by receiving the heat energy and can conduct some electricity. When the insulation resistance of ceramics is measured at room temperature, it is more than 1000 MΩ, at 1000°C it is only 1MΩ.

In the relationship between temperature and electric resistance, metals have the opposite tendency to the above-mentioned insulating substance. That is, as the temperature of the metal increases, movement of free electrons is hindered by atomic vibration, and the resistance value tends to increase. At 0°C and 1000°C, the resistance value of some metals increases after 10 years. However, though the resistance value increases, digit of the resistance value differs significantly from that of the insulator. (It differs at the level of thousands of trillions)

Semiconductors are substances having intermediate properties between metals and ordinary insulators. Basically, electricity is not conducted as there are no freely moving electrons.Since the binding of electrons by the nucleus is weak, electricity can be easily passed with some kind of external stimulus. Utilizing this unstable property, semiconductor products with various functions are manufactured.

Liquids easily conduct electricity by a different mechanism other than free electrons. This is a method of conducting electricity by moving atoms with less or more electrons called ions through a liquid. Vapor or gaseous body can also conduct electricity by moving charged atoms in the same principle. Especially since water often passes electricity, it is likely to cause insulation failure of electrical equipment. Even if it is not wet, insulation resistance may drop by several thousands of millions just by being damp. Since, vapor or gaseous body at super high temperature can provide enough heat energy to the electrons and it exhibits the same properties as that of the metals with free electrons. (Plasma state)

It is mentioned that water passes electricity easily, but pure water does not pass electricity. If there is the so called “Electrolyte” such as salt slightly dissolved in water, it will let the electricity to flow through. Pure water does not exist in the natural state. Since it contains some electrolytes, normal water is able to pass some electricity.