SI UNITS
A system of units of measurement, also known as a system of units or system of
measurement, is a collection of units of measurement and rules relating them to each other.
Systems of measurement have historically been important, regulated and defined for the
purposes of science and commerce.
The SI comprises a coherent system of units of measurement starting with seven base units,
which are the second (symbol s, the unit of time), metre (m, length), kilogram (kg, mass), ampere
(A, electric current), kelvin (K, thermodynamic temperature), mole (mol, amount of substance),
and candela (cd, luminous intensity)
The SI standard also includes units derived from the base units. The derived units are defined as
products of powers of the base units. For example, one derived unit is the newton, which can be
expressed in terms of base units as 1 kg m/s2. Other derived units include the hertz,
the pascal (unit of pressure or stress), the ohm, the farad, the joule, the coulomb, the tesla, the
lumen, the becquerel, the siemens, the volt and the watt.
SI base units
Symbol Name Quantity
s second time
m metre length
kg kilogram mass
A ampere electric current
K kelvin thermodynamic temperature
mol mole amount of substance
cd candela luminous intensity
Unit prefixes of the International System of Units (SI)
In order to keep figures short and presentable, SI prefixes are often attached to units such as m
(meter) or g (gram).
For example, a substance weighing 0.000 000 001 g may be described more simply as 1 ng
(nanogram) or as 1000 pg (picograms).
Common prefixes of the International System of Units (SI) are:
p pico = 1 000 000 000 000th = 10-12
n nano = 1 000 000 000th = 10-9
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µ micro = 1 000 000th = 10-6
m milli = 1 000th = 10-3
no prefix = 1
k kilo = 1 000 = 103
M Mega = 1 000 000 = 106
G Giga = 1 000 000 000 = 109
T Tera = 1 000 000 000 000 = 1012
A passive component, also known as a passive device, is an electronic component which can
only receive energy, which it can either dissipate, absorb, or store in an electric field or a
magnetic field. Passive components do not need any form of independent electrical power to
operate. Passive devices are incapable of controlling current by means of another electrical
signal. Passive elements include resistances, capacitors, and coils (also called inductors).
What are Passive Components Used For?
Passive components can either be used individually or connected together within a circuit either
in a series or in a parallel combination to control complex circuits or signals, produce a phase
shift to the signal or to provide some form of feedback but they cannot multiply a signal by more
than one because they have no power gain. Passive components are bi-directional, meaning that
they can be connected in either direction within a circuit, unless they have a particular polarity
marking (such as electrolytic capacitors). The polarity of the voltage across them is determined
by conventional current flow from the positive to the negative terminal. In fact, passive devices
consume power within an electrical or electronic circuit as they act like attenuators unlike active
elements that generate or provide power to a circuit.
Passive devices can be divided into two types: Resistors are dissipative, in that they dissipate the
energy they receive in heat, rather than store it.
Resistors
A resistor is a primary type of physical component that is used in electronic circuits. It has two
(interchangeable) leads. The material placed internally between the two leads of a resistor
opposes (restricts) the flow of current. The amount of that opposition is called its resistance,
which is measured in ohms (Ω). Resistors are used to control the various currents in areas of a
circuit and to manage voltage levels at different points therein by producing voltage drops. When
a voltage is applied across a resistor, current flows through it. Ohm's law for resistors is E = IR,
where E is the voltage across the resistor, R is the resistance of the resistor, and I is the current
flowing through the resistor. That current is proportional to the applied voltage, and inversely
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proportional to the resistance. Thus, as resistance goes up, the current through the element comes
down, so that at high resistances the current is very small.
Ohm's law makes it possible to calculate any one of three circuit values (current, voltage, or
resistance) from the other two.
Capacitors
A capacitor is another primary type of physical component used in electronic circuits. It has two
leads and is used to store and release electric charge. A capacitor's ability to store charge is
referred to as its capacitance, measured in farads (F).
A typical capacitor takes the form of two conductive plates separated by an insulator (dielectric).
This type of circuit element cannot pass direct current (DC) because electrons cannot flow
through the dielectric. However, a capacitor does pass alternating current (AC) because an
alternating voltage causes the capacitor to repeatedly charge and discharge, storing and releasing
energy. Indeed, one of the major uses of capacitors is to pass alternating current while blocking
direct current, a function called 'AC coupling'.
When a direct current flows into a capacitor, a positive charge rapidly builds up on the positive
plate and a corresponding negative charge fills the negative plate (see Figure 1). The buildup
continues until the capacitor is fully charged—i.e., when the plates have accumulated as much
charge (Q) as they can hold. This amount is determined by the capacitance value (C) and the
voltage applied across the component: (Q = CV). At that point, current stops flowing (see Figure
2).
The capacitor is charging / Figure 2: The capacitor is charged (and stable)
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When an alternating current flows through the circuit, the result is quite different, though.
Because the AC current is continuously changing, the capacitor is repeatedly charging and
discharging (see Figure 3). Despite the fact that the dielectric in the capacitor does not pass any
electrons, a current—which in this case is called a displacement current—effectively moves
through the capacitor. The capacitor's opposition to alternating current is called its capacitive
reactance, which, like resistance, is measured in ohms (Ω).
Figure 3: Repeatedly charging and discharging
Capacitor symbols
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Coils
A coil, also called an inductor, is yet another primary type of physical component that is used in
electronic circuits. It has two leads and is typically implemented as one or more windings (loops)
of conductive wire. That wire is often but not necessarily formed around a core of iron or steel or
some other magnetic material. Current through the coil induces a magnetic field that serves as a
store of energy. Inductance is measured in henries (H).
More specifically, a current flowing through a wire generates a magnetic field, the direction of
which is to the right relative to the flow of the current, as described by the 'right-hand rule' (see
Figure 4). If the wire is coiled, the fluxes are in alignment. According to Lenz's law, changes in
the coil's magnetic field generate a counter-electromotive force (and an induced current) that
opposes those changes. Thus, coils can be used in electronic circuits to restrict the flow of
alternating current while allowing direct current to pass.
Coils symbol
Current and magnetic field
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What is an Electric Circuit?
An Electric Circuit is a closed path for transmitting an electric current through the medium of
electrical and magnetic fields. The flow of electrons across the loop constitutes the electric
current. Electrons enter the circuit through the ‘Source’ which can be a battery or a generator.
The source provides energy to the electrons, by setting up an electrical field which provides the
electromotive force. The electrons leave the circuit through the load, to the earth ground, thus
completing a closed path. The load or output can be any simple home appliance like television,
lamp, refrigerator, or can be a complex load such as that on a hydroelectric power generating
station. A simple electric circuit consists of a source (such as a battery), wires as conducting
medium and a load (such as a light bulb). The battery provides required energy for flow of
electrons, to the light bulb. Basic Circuit Elements As mentioned above in the introduction, a
circuit is an interconnection of elements. These elements are classified into active or passive
elements, based on their capability to generate energy. Active Circuit Elements Active Elements
are those which can generate energy. Examples include batteries, generators, operational
amplifiers and diodes. Note that in an electrical circuit, the source elements are the most
significant active elements. An energy source, whether a voltage or current source, is of 2 types –
Independent and Dependent sources. Example of an Independent source is the battery which
provides a constant voltage to the circuit, irrespective of the current flowing through the
terminals. Example of a dependent source is a transistor, which provides current to the circuit,
depending upon the voltage applied to it. Another example is an Operational Amplifier, which
provides voltage, depending upon the differential input voltage applied to its terminals.
Rules to analyze DC sourced circuits
All DC circuit analysis (the determining of currents, voltages and resistances throughout a
circuit) can be done with the use of three rules. These rules are given below. 1. Ohm's law. This
law states that the current in a circuit is directly proportional to the potential difference across the
circuit and inversely proportional to the resistance in the circuit. Mathematically, this can be
expressed as I = V R (1) Ohm's law can be applied to an entire circuit or to individual parts of the
circuit. 2. Kirchoff’s node rule. This rule states that the algebraic sum of all currents at a node
(junction point) is zero. Currents coming into a node are considered negative and currents
leaving a node are considered positive.
Figure 2.
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For the situation in figure 2, we have -I1 + I2 + I3 = 0 or I1 = I2 + I3 This is a statement of the
law of conservation of charge. Since no charge may be stored at a node and since charge cannot
be created or destroyed at the node, the total current entering a node must equal the total current
out of the node. 3. Kirchoff's loop rule. This rule states that the algebraic sum of all the changes
in potential (voltages) around a loop must equal zero. A potential difference is considered
negative if the potential is getting smaller in the direction of the current flow. For the situation in
figure 3, we have +V1 - V2 - V3 = 0 or V1 = V2 + V3
This is a statement of the law of conservation of energy. Since potential differences correspond to
energy changes and since energy cannot be created or destroyed in ordinary electrical
interactions, the energy dissipated by the current as it passes through the circuit (V2 + V3) must
equal the energy given to it by the power supply (V1 ). Question: To illustrate the application of
these rule, try to solve the following problem to find the total current and voltage drop and
individual across each component. (Simplified version of the circuit has been given in the figure
b.
Current and voltage drop across the DC circuit elements Voltage drop means the reduction in
voltage or voltage loss across a particular element. Due to the presence of the impedance or
passive elements, there will be some loss in voltage as the current moves through the circuit.
That is, the energy supplied from the voltage source will get reduced as the current flows through
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the circuit. The simple idea to calculate the voltage drop across the passive elements is calculated
using the ohm’s law (make sure the circuit only consist of resistors in the circuit). Voltage drops
across inductor for the case of inductor, the ohm’s law takes the form:
When a battery is connected to a series resistor and inductor, the inductor resists the change in
current and the current therefore builds up slowly. Acting in accordance with Faraday's law and
Lenz's law, the amount of impedance to the build-up of current is proportional to the rate of
change of the current. That is, the faster you try to make it change, the more it resists. The
current builds up toward the value it would have with the resistor alone because once the current
is no longer changing, the inductor offers no impedance. The rate of this build up is characterized
by the time constant L/R. Establishing a current in an inductor stores energy in the magnetic field
formed by the coils of the inductor.
Voltage across capacitor for the case of capacitor, the situation is different from both the resistors
and inductors. Below we are showing the response of the capacitor for a constant voltage source.
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When a battery is connected to a series resistor and capacitor, the initial current is high as the
battery transports charge from one plate of the capacitor to the other. The charging current
asymptotically approaches zero as the capacitor becomes charged up to the battery voltage.
Charging the capacitor stores energy in the electric field between the capacitor plates. The rate of
charging is typically described in terms of a time constant RC.
Single-phase and three-phase alternating current sources The AC power (alternating current) is a
kind of electricity in which the flow of current is frequently changing directions. At the
beginning of the 1900-year, AC power supply is used for businesses as well as homes. The
system of the power supply is categorized into two types’ namely single-phase power supply, as
well as three phase power supply. For most industrial and businesses settings, three-phase supply
is used to run the high loads, whereas homes are generally supplied by a single phase, because
home appliances require less power. This article discusses the difference between single phase
and three phase power supplies. What is Phase in Electricity? Generally, the phase in electricity
is the current or the voltage among an existing wire as well as a neutral cable. Phase means the
distribution of load, if a single wire is used, an additional load will occur on it & if three wires
are used then loads will be separated between them. This can be called as less power for 1-phase
and more power for 3-phase. If it is a 1-phase system, it includes two wires as well as if it is a 3-
phase system, then it includes 3 wires (or) four wires. Single Phase Supply In the field of
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electrical, single-phase supply is the delivery of AC power by a system in which all the supply
voltages change in simultaneously. This type of power supply sharing is used when the loads
(home appliances) ate generally heat and lighting with some huge electric motors. When a
single-phase supply is connected to an AC motor doesn’t generate a rotating magnetic field,
single phase motors require extra circuits for working, but such electric motors are rare over in
rating of 10 kW. In every cycle, a single-phase system voltage achieves a peak value two times;
the direct power is not stable.
A load with single-phase can be power-driven from a three-phase sharing transformer in two
techniques. One is with the connection between two phases or with connection among one phase
and neutral. These two will give dissimilar voltages from a given power supply. This type of
phase supply provides up to 230V. The applications of this supply mainly use for running the
small home appliances like air conditioners, fans, heater, etc. Single Phase Supply Benefits The
benefits of choosing a single-phase supply include the following.
The design is less complex
Design cost is less
Most efficient AC power supply for up to 1000 watts
Single Phase AC Power Supply is most competent for up to 1000 watts.
Wide-range of application uses Single Phase Supply Applications The applications of single-
phase supply include the following.
This power supply is applicable for homes as well as businesses.
Used to supply plenty of power for homes, as well as nonindustrial businesses.
This power supply is sufficient to run the motors up to about 5 horsepower (hp).
Three Phase Supply The three-phase power supply includes four wires such as one neutral as
well as three conductors. The three conductors are away from phase & space 120º distant from
each other. Three phase power supplies are utilized as a single-phase AC power supply. For the
small load, 1-phase AC power supply, as well as neutral, can be chosen from the 3-phase AC
power supply system. This supply is constant and not at all totally falls to zero. The power of this
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system can be illustrated in two configurations namely star connection (or) delta connection. The
connection of star configuration is used in long-distance communication as it includes a neutral
cable to the error current.
Three Phase Supply Benefits The benefits of choosing a three-phase supply include the
following.
Copper Utilization Reduction
Lessening of Security Risks for Employees
Labor Treatment Costs
The efficiency of the Conductor is Greater
Facility to Run High Power Loads
Three Phase Supply Applications
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The applications of the three-phase supply include the following.
These types of supplies are used in power grids, mobile towers, data centres, aircraft,
shipboard, unmanned systems, as well as other electronic loads larger than 1000 watts.
It is applicable to industrial, manufacturing, and large businesses.
These are used in power-hungry and high-density data centers. Key Differences between Single
Phase and Three Phase Supplies The key differences between single phase and three phases
include the following.
The definition of the single-phase power supply is, the power supplies through a single
conductor
The definition of the three-phase power supply is, the power flows through three conductors.
The single-phase power supply has one distinct wave cycle whereas; three phases have three
distinct wave cycles.
Single phase requires the single wire to connect the circuit whereas; 3-phase needs 3- wires.
The voltage of the single phase is 230V, whereas three phase voltage is 415V.
The phase name of the single phase is split phase, whereas three phase has no other name.
The capacity of power transfer in the single phase is minimum, whereas three phase has the
maximum.
The connection of single phase is simple whereas in 3-phase is complicated.
The power failure happens in a single phase, but not occurs in three phases.
The loss in single phase is maximum whereas in three phase is minimum.
The single-phase efficiency is less whereas in three phase is high.
The single-phase is inexpensive whereas the 3-phase is expensive.
The single-phase AC power supply is utilized for home appliances and three phase power
supply is used in huge industries to run heavy loads.
From the above information finally, we can conclude that with the right care in the design part
of a power supply, the designer can make suitable advice for the highest efficiency & cost
savings of your project. Choosing a single phase (or) three-phase system mainly depends on the
power requirements of a particular application.
Basic AC Circuits Unlike DC current, AC voltage or current changes its direction periodically as
it increases from zero to maximum, and decreases back to zero, then negatively continue to
maximum, and then again back to zero. The frequency of this cycle is about 50 cycles per sec in
India. For high-power applications, AC is more predominant and efficient source than DC. The
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power is not a simple product of voltage and current as in DC, but it depends on the circuit
components. Let’s see the AC circuit behavior with the basic components
AC Circuit with a Resistor
In this type of circuit, the voltage dropping across the resistor is exactly in phase with the current
as shown in the figure. This means that when instantaneous value voltage is zero, the current
value at that instant is also zero. And also, when the voltage is positive during the positive half
wave of the input signal, the current is also positive, so the power is positive even when they are
in negative half wave of the input. This means that the AC power in a resistor is always
dissipating as heat while taking it from the source, irrespective of whether the current is positive
or negative. AC Circuit with Inductors Inductors oppose the change in the current through them
not like the resistors that oppose the flow of current. This means when the current is increased,
the induced voltage tries to oppose this change of the current by dropping the voltage. The
voltage dropped across an inductor is proportional to the rate of change in the current.
Therefore, when the current is at its maximum peak (no rate of change in shape), the
instantaneous voltage at that instant is zero, and reverse happens when the current peaks at zero
(maximum change of its slope), as shown in the figure. So there is no net power dissipation in
the inductor AC circuit. Thus, the instantaneous power of the inductor, in this circuit, is entirely
different from the DC circuit, where it is in same phase. But, in this circuit, it is 90 degrees apart
so the power is negative, at times, as shown in the figure. Negative power means the power
releases back to the circuit as it absorbs it in the rest of the cycle. This opposition of current
change is called as reactance, and it depends on the frequency of the operating circuit. AC Circuit
with Capacitors A Capacitor opposes a change in the voltage, which is dissimilar to an inductor
that opposes a change in the current. By supplying or drawing current, this type of opposition
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takes place, and this current is proportional to the rate of change of the voltage across the
capacitor.
Here, the current through the capacitor is the result of the change in the voltage in the circuit.
Therefore, the instantaneous current is zero when the voltage is at its peak value (no change of
voltage slope), and it is maximum when the voltage is at zero, so the power also alternates in
positive and negative cycles. This means it does not dissipate the energy but just absorbs and
releases the power. AC circuit behavior can also be analyzed by combining the above circuits
like RL, RC and RLC circuits in series as well as in parallel combinations. And also the
equations and formulas of the above circuits are exempted in this article to reduce the
complexity, but the overall idea is to give a basic concept about the electrical circuits.
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