Concept of Electricity
Introduction
Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by Maxwell's equations. Various common phenomena are related to electricity, including lightning, static electricity, electric heating, electric discharges and many others.
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The presence of an electric charge, which can be either positive or negative, produces an electric field. The movement of electric charges is an electric current and produces a magnetic field.
Wikipedia.
Electricity is briefly defined as the flow of electric charge, but there's so much behind that simple statement. Where do the charges come from? How do we move them? Where do they move to? How does an electric charge cause mechanical motion or make things light up? So many questions! To begin to explain what electricity is we need to zoom way in, beyond the matter and molecules, to the atoms that make up everything we interact with in life.
Electricity Basics
When beginning to explore the world of electricity and electronics, it is vital to start by understanding the basics of voltage, current, and resistance. These are the three basic building blocks required to manipulate and utilize electricity.
When describing voltage, current, and resistance, a common analogy is a water tank. In this analogy, charge is represented by the water amount, voltage is represented by the water pressure, and current is represented by the water flow. So for this analogy, remember:
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Water = Charge
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Pressure = Voltage
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Flow = Current
Voltage
The pressure at the end of the hose can represent voltage. The water in the tank represents charge. The more water in the tank, the higher the charge, the more pressure is measured at the end of the hose.
We can think of this tank as a battery, a place where we store a certain amount of energy and then release it. If we drain our tank a certain amount, the pressure created at the end of the hose goes down. We can think of this as decreasing voltage, like when a flashlight gets dimmer as the batteries run down. There is also a decrease in the amount of water that will flow through the hose. Less pressure means less water is flowing.
Battery: source of dc voltage
Technically
Voltage, electrical potential difference, electric tension or electric pressure is the electric potential difference between two points. Given two points in the space, called A and B, voltage is the difference of electric potentials between those two points. It is denoted as ΔV and measured in units of electric potential: volts, or joules per coulomb. A voltage may represent either a source of energy (electromotive force), or lost, used, or stored energy (potential drop). Voltage can be caused by static electric fields, by electric current through a magnetic field, by time-varying magnetic fields, or some combination of these three.
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​Instruments for measuring voltages (or potential difference) include the voltmeter, the potentiometer, and the oscilloscope. The voltmeter works by measuring the current through a fixed resistor, which, according to Ohm's Law, is proportional to the voltage across the resistor. The potentiometer works by balancing the unknown voltage against a known voltage in a bridge circuit. The cathode-ray oscilloscope works by amplifying the voltage and using it to deflect an electron beam from a straight path, so that the deflection of the beam is proportional to the voltage.
Typical voltages
A common voltage for flashlight batteries is 1.5 volts (DC). A common voltage for automobile batteries is 12 volts (DC). Common voltages supplied by power companies to consumers are 110 to 120 volts (AC) and 220 to 240 volts (AC). The voltage in electric power transmission lines used to distribute electricity from power stations can be several hundred times greater than consumer voltages, typically 110 to 1200 kV (AC). The voltage used in overhead lines to power railway locomotives is between 12 kV and 50 kV (AC).
Electricity transmission lines
Electric current
We can think of the amount of water flowing through the hose from the tank as current. The higher the pressure, the higher the flow, and vice-versa. With water, we would measure the volume of the water flowing through the hose over a certain period of time. With electricity, we measure the amount of charge flowing through the circuit over a period of time.
This increases the pressure (voltage) at the end of the narrower hose, pushing more water through the tank. This is analogous to an increase in voltage that causes an increase in current.
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Water = Charge (measured in Coulombs)
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Pressure = Voltage (measured in Volts)
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Flow = Current (measured in Amperes, or "Amps" for short)
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Hose Width = Resistance
Technically
An electric current is a flow of electric charge. In electric circuits this charge is often carried by moving electrons in a wire. It can also be carried by ions in an electrolyte, or by both ions and electrons such as in a plasma. The SI unit for measuring an electric current is the ampere, Current is measured in Amperes (usually just referred to as "Amps"). An ampere is defined as 6.241*10^18 electrons (1 Coulomb) per second passing through a point in a circuit. Amps are represented in equations by the letter "I". Electric current can be measured using an ammeter. Electric current can flow through some things, electrical conductors, but will not flow through an electrical insulator. Electric currents cause many effects, notably heating, but also induce magnetic fields, which are widely used for motors, inductors and generators.
Types of current
AC and DC
The abbreviations AC and DC are often used to mean simply alternating and direct, as when they modify current or voltage.
Direct current
Direct current (DC) is the unidirectional flow of electric charge. Direct current is produced by sources such as
batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through semiconductors, insulators, or even through a vacuum as in electron or ion beams. The electric charge flows in a constant direction, distinguishing it from alternating current (AC). A term formerly used for direct current was galvanic current.
Alternating current
In alternating current (AC, also ac), the movement of electric charge periodically reverses direction. In direct current (DC, also dc), the flow of electric charge is only in one direction. AC is the form in which electric power is delivered to businesses and residences. The usual waveform of an AC power circuit is a sine wave. In certain applications, different waveforms are used, such as triangular or square waves. Audio and radio signals carried on electrical wires are also examples of alternating current. In these applications, an important goal is often the recovery of information encoded (or modulated) onto the AC signal.
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One of the main reasons that we use alternating AC voltages and currents in our homes and workplace’s is that AC supplies can be easily generated at a convenient voltage, transformed (hence the name transformer) into much higher voltages and then distributed around the country using a national grid of pylons and cables over very long distances.
The reason for transforming the voltage to a much higher level is that higher distribution voltages implies lower currents for the same power and therefore lower I *R losses along the networked grid of cables. These higher AC transmission voltages and currents can then be reduced to a much lower, safer and usable voltage level where it can be used to supply electrical equipment in our homes and workplaces, and all this is possible thanks to the basic Voltage Transformer.
Current measurement
Current can be measured using an ammeter.
At the circuit level, there are various techniques that can be used to measure current:
• Shunt resistors
• Hall effect current sensor transducers
• Transformers (however DC cannot be measured)
• Magnetoresistive field sensors
Digital multimeter for measuring voltage, current and resistance.
Resistance
Consider again our two water tanks, one with a narrow pipe and one with a wide pipe.
It stands to reason that we can't fit as much volume through a narrow pipe than a wider one at the same pressure. This is resistance. The narrow pipe "resists" the flow of water through it even though the water is at the same pressure as the tank with the wider pipe.
Technically
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In electrical terms, this is represented by two circuits with equal voltages and different resistances. The circuit with the higher resistance will allow less charge to flow, meaning the circuit with higher resistance has less current flowing through it.
This brings us back to Georg Ohm. Ohm defines the unit of resistance of "1 Ohm" as the resistance between two points in a conductor where the application of 1 volt will push 1 ampere, or 6.241×10^18 electrons. This value is usually represented in schematics with the greek letter "Ω", which is called omega, and pronounced "ohm".
Resistor is used to reduce current flow in the circuit
Ohm's law
Ohm's law states that the current through a conductor between two points is directly proportional to the potential difference across the two points. Introducing the constant of proportionality, the resistance, one arrives at the usual mathematical equation that describes this relationship: where I is the current through the conductor in units of amperes, V is the potential difference measured across the conductor in units of volts, and R is the resistance of the conductor in units of ohms. More specifically, Ohm's law states that the R in this relation is constant, independent of the current.
Ohm's law triangle
Summary
According to our water tank analogy, the tank represents the battery, the water in the tank represents electric charge, the water flowing through the hose from the tank is current, then the pressure at which the water flows is the voltage and the width of the hose is the resistance.
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The lesser the water in the tank (electric charge), the lesser the pressure of the flowing water (voltage), and the lesser the width of the hose (increased resistance), the lesser the amount of flowing water (current): if the battery is drained to a certain level, the voltage drops. This is noticed when a flashlight gets dimmer as the battery starts to run down.