Introduction to electricity

Содержание

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Symbol: (q) Unit: Coulomb (C) The fundamental electric quantity is charge.

Symbol: (q)
Unit: Coulomb (C)

The fundamental electric quantity is charge.
Atoms are composed

of charge carrying particles: electrons and protons, and neutral particles, neutrons.
The smallest amount of charge that exists is carried by an electron and a proton.

Charge in an electron:
qe = -1.602x10-19 C
Charge in a proton:
qp = 1.602x10-19 C

Charge

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Current Current moves through a circuit element “through variable.” Current is

Current

Current moves through a circuit element “through variable.”
Current is rate of

flow of negatively-charged particles, called electrons, through a predetermined cross-sectional area in a conductor.
Like water flow.

Essentially, flow of electrons in an electric circuit leads to the establishment of current.
I(t) =
q : relatively charged electrons (C)
Amp = C/sec
Often measured in milliamps, mA

Symbol: I
Unit: Ampere

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Current-Water Analogy

Current-Water Analogy

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Voltage Potential difference across two terminals in a circuit “across variable.”

Voltage

Potential difference across two terminals in a circuit “across variable.”
In order

to move charge from point A to point B, work needs to be done.
Like potential energy at a water fall.

A

B

Let A be the lower potential/voltage terminal
Let B be the higher potential/voltage terminal
Then, voltage across A and B is the cost in energy required to move a unit positive charge from A to B.

Symbol: V
Unit: Volt

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Voltage-Water Analogy

Voltage-Water Analogy

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Voltage/Current-Water Analogy

Voltage/Current-Water Analogy

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Series Connection of Cells Each cell provides 1.5 V Two cells

Series Connection of Cells

Each cell provides 1.5 V
Two cells connected one after

another, in series, provide 3 V, while three cells would provide 4.5 V
Polarities matter
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Parallel Connection of Cells If the cells are connected in parallel,

Parallel Connection of Cells

If the cells are connected in parallel, the

voltage stays at 1.5 V, but now a larger current can be drawn.
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Wire-Water Analogy

Wire-Water Analogy

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Resistor Concept —I Flow of electric current through a conductor experiences

Resistor Concept —I

Flow of electric current through a conductor experiences a

certain amount of resistance.
The resistance, expressed in ohms (Ω, named after George ohm), kilo-ohms (kΩ, 1000Ω), or mega-ohms (MΩ, 106Ω) is a measure of how much a resistor resists the flow of electricity.
The magnitude of resistance is dictated by electric properties of the material and material geometry.
This behavior of materials is often used to control/limit electric current flow in circuits.
Henceforth, the conductors that exhibit the property of resisting current flow are called resistors.

Resistor Symbols

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Resistor Concept —II A resistor is a dissipative element. It converts

Resistor Concept —II

A resistor is a dissipative element. It converts electrical

energy into heat energy. It is analogous to the viscous friction element of mechanical system.
When electrons enter at one end of a resistor, some of the electrons collide with atoms within the resistor. These atoms start vibrating and transfer their energy to neighboring air molecules. In this way, a resistor dissipates electrical energy into heat energy.
Resistors can be thought of as analogous to water carrying pipes. Water is supplied to your home in large pipes, however, the pipes get smaller as the water reaches the final user. The pipe size limits the water flow to what you actually need.
Electricity works in a similar manner, except that wires have so little resistance that they would have to be very very thin to limit the flow of electricity. Such thin wire would be hard to handle and break easily.
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Resistors-Water Analogy

Resistors-Water Analogy

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Resistor V-I Characteristic In a typical resistor, a conducting element displays

Resistor V-I Characteristic

In a typical resistor, a conducting element displays linear

voltage-current relationship. (i.e., current through a resistor is directly proportional to the voltage across it).
I ∝V
Using G as a constant of proportionality, we obtain:
I = GV
Equivalently,
V = RI (or V = IR)
where R = 1/G.
R is termed as the resistance of conductor (ohm, Ω)
G is termed as the conductance of conductor (mho, )
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Resistor Applications Resistors are used for: Limiting current in electric circuits.

Resistor Applications

Resistors are used for:
Limiting current in electric circuits.
Lowering voltage levels

in electric circuits (using voltage divider).
As current provider.
As a sensor (e.g., photoresistor detects light condition, thermistor detects temperature condition, strain gauge detects load condition, etc.)
In electronic circuits, resistors are used as pull-up and pull-down elements to avoid floating signal levels.
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Resistors: Power Rating and Composition It is very important to be

Resistors: Power Rating and Composition

It is very important to be aware

of power rating of resistor used in circuits and to make sure that this limit is not violated. A higher power rating resistor can dissipate more energy that a lower power rating resistor.
Resistors can be made of:
Carbon film (decomposition of carbon film on a ceramic core).
Carbon composition (carbon powder and glue-like binder).
Metal oxide (ceramic core coated with metal oxide).
Precision metal film.
High power wire wound.
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Resistor Examples

Resistor Examples

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Resistor Labels Wire-wound resistors have a label indicating resistance and power

Resistor Labels

Wire-wound resistors have a label indicating resistance and power ratings.
A

majority of resistors have color bars to indicate their resistance magnitude.
There are usually 4 to 6 bands of color on a resistor. As shown in the figure below, the right most color bar indicates the resistor reliability, however, some resistor use this bar to indicate the tolerance. The color bar immediately left to the tolerance bar (C), indicates the multipliers (in tens). To the left of the multiplier bar are the digits, starting from the last digit to the first digit.
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Resistor Color Codes x.01 - Silver X1000000000 9 White X100000000 8

Resistor Color Codes

x.01

-

Silver

X1000000000

9

White

X100000000

8

Grey

X10000000

7

Purple

X1000000

6

Blue

X100000

5

Green

X10000

4

Yellow

X1000

3

Orange

X100

2

Red

X10

1

Brown

X1

0

Black

x.1

-

Gold

Multiplier

Digit

Band color

±5%

Gold

±10%

Silver

±2%

Red

±1%

Brown

Tolerance

Color

None

±20%

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Example The first band is yellow, so the first digit is

Example

The first band is yellow, so the first digit is

4
The second band is violet, so the second digit is 7
The third band is red, so the multiplier is
Resistor value is
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Metric Units and Conversions Abbreviation Means Multiply unit by Or p

Metric Units and Conversions

Abbreviation Means Multiply unit by Or
p pico

.000000000001 10 -12
n nano .000000001 10 -9
µ micro .000001 10 -6
m milli .001 10 -3
. Unit 1 10 0
k kilo 1,000 10 3
M mega 1,000,000 10 6
G giga 1,000,000,000 10 9
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Digital Multimeter 1 DMM is a measuring instrument An ammeter measures

Digital Multimeter 1

DMM is a measuring instrument
An ammeter measures current
A voltmeter

measures the potential difference (voltage) between two points
An ohmmeter measures resistance
A multimeter combines these functions, and possibly some additional ones as well, into a single instrument
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Digital Multimeter 2 Voltmeter Parallel connection Ammeter Series connection Ohmmeter Without

Digital Multimeter 2

Voltmeter
Parallel connection
Ammeter
Series connection
Ohmmeter
Without any power supplied
Adjust range (start from

highest limit if you don’t know)
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Ammeter Connection Break the circuit so that the ammeter can be

Ammeter Connection

Break the circuit so that the ammeter can be connected

in series
All the current flowing in the circuit must pass through the ammeter
An ammeter must have a very LOW input impedance
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Voltmeter Connection The voltmeter is connected in parallel between two points

Voltmeter Connection

The voltmeter is connected in parallel between two points of

circuit
A voltmeter should have a very HIGH input impedance
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Ohmmeter Connection An ohmmeter does not function with a circuit connected

Ohmmeter Connection

An ohmmeter does not function with a circuit connected to

a power supply
Must take it out of the circuit altogether and test it separately
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Resistors in Series Rtotal=R1+R2 Rtotal=1+1=2kΩ

Resistors in Series

Rtotal=R1+R2
Rtotal=1+1=2kΩ

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Resistors in Parallel

Resistors in Parallel

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Exercise 1

Exercise 1

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Variable Resistor Concept In electrical circuit, a switch is used to

Variable Resistor Concept

In electrical circuit, a switch is used to turn

the electricity on and off just like a valve is used to turn the water on and off.
There are times when you want some water but don’t need all the water that the pipe can deliver, so you control water flow by adjusting the faucet.
Unfortunately, you can’t adjust the thickness of an already thin wire.
Notice, however, that you can control the water flow by forcing the water through an adjustable length of rocks, as shown to the right.
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Variable Resistor Construction To vary the resistance in an electrical circuit,

Variable Resistor Construction

To vary the resistance in an electrical circuit,

we use a variable resistor.
This is a normal resistor with an additional arm contact that can move along the resistive material and tap off the desired resistance.

Stationary contact

Wiper contact

Resistive material

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Variable Resistor Operation The dial on the variable resistor moves the

Variable Resistor Operation

The dial on the variable resistor moves the arm

contact and sets the resistance between the left and center pins. The remaining resistance of the part is between the center and right pins.
For example, when the dial is turned fully to the left, there is minimal resistance between the left and center pins (usually 0Ω) and maximum resistance between the center and right pins. The resistance between the left and right pins will always be the total resistance.
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Variable Resistor: Rotary Potentiometers

Variable Resistor: Rotary Potentiometers

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Variable Resistor: Other Examples Photoresistor Thermistor

Variable Resistor: Other Examples

Photoresistor

Thermistor

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Resistance Formula

Resistance Formula

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Capacitor Concept A capacitor is an energy storage element which is

Capacitor Concept

A capacitor is an energy storage element which is analogous

to the spring element of mechanical systems.
It can store electrical pressure (voltage) for periods of time.
-When a capacitor has a difference in voltage (electrical pressure) across its plate, it is said to be charged.
-A capacitor is charged by having a one-way current flow through it for a period of time.
-It can be discharged by letting a current flow in the opposite direction out of the capacitor.
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Capacitor Construction A capacitor is constructed using a pair of parallel

Capacitor Construction

A capacitor is constructed using a pair of parallel conducting

plates separated by an insulating material (dielectric).
When the two plates of a capacitor are connected to a voltage source as shown, charges are displaced from one side of the capacitor to the other side, thereby establishing an electric field.
The charges continue to be displaced in this manner until the potential difference across the two plates is equal to the potential of voltage source.
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Capacitor Water Pipe Analogy —I In the water pipe analogy, a

Capacitor Water Pipe Analogy —I

In the water pipe analogy, a capacitor

is thought of as a water pipe:
with a rubber diaphragm sealing off each side of the pipe and
a plunger on one end.
When the plunger pushes toward the diaphragm, the water in the pipe forces the diaphragm to stretch until the force of the diaphragm pushing back on the water equals the force on the plunger→pipe is charged!
If the plunger is released, the diaphragm will push the plunger back to its original position →pipe is discharged.
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Capacitor Water Pipe Analogy —II If the rubber diaphragm is made

Capacitor Water Pipe Analogy —II

If the rubber diaphragm is made very

soft, it will stretch out and hold a lot of water but will break easily (large capacitance but low working voltage).
If the rubber diaphragm is made very stiff, it will not stretch far but withstand higher pressure (low capacitance but high working voltage).
By making the pipe larger and keeping the rubber stiff, we can achieve a device that holds a lot of water and withstand high pressure.
So the pipe size is determined from the amount of water to be held and the amount of pressure to be handled.
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Capacitor Water Pipe Analogy —III Water capacitor: a tube with a

Capacitor Water Pipe Analogy —III

Water capacitor: a tube with a rubber

membranne in the middle
Rubber membranne analogous to the dielectric, two chambers analogous to two capacitor plates
When no water pressure is applied on the water capacitor, the two chambers contain same amount of water (uncharged)
When pressure is applied on the top chamber, the membrane is pushed down causing the water to be displaced from the bottom chamber (appearance of current flow → displacement current)
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Capacitor V-I Characteristic The charge accumulated on capacitor plates is directly

Capacitor V-I Characteristic

The charge accumulated on capacitor plates is directly proportional

to voltage applied across the plates.
q ∝V q = CV
where C is the constant of proportionality and is called capacitance (unit: Farad).
V-I characteristic of a capacitor is obtained by computing
Alternatively, integrating the above equation w.r.t. time, and rearranging terms, we get
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Capacitance Formula For a parallel capacitor: - ε0 = permittivity of

Capacitance Formula

For a parallel capacitor:
- ε0 = permittivity of free space
-

A = plate area
- d = separation distance of plate.
Often, we use G = A/d as geometry factor (for other types of capacitors as well).
If a dielectric material with dielectric constant K separates the two plates of the capacitor, then C = Kε0G, where K = dielectric constant. Usually K > 1.
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Capacitor Symbols

Capacitor Symbols

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Capacitor Variations Ceramic capacitors very popular nonpolarized capacitor small, inexpensive, but

Capacitor Variations

Ceramic capacitors
very popular nonpolarized capacitor
small, inexpensive, but poor temperature stability

and poor accuracy
ceramic dielectric and a phenolic coating
often used for bypass and coupling applications

Electrolytic
Aluminum, tantalum electrolytic
Tantalum electrolytic capacitor has a larger capacitance when compared to aluminum electrolytic capacitor
Mostly polarized.
Greater capacitance but poor tolerance when compared to nonelectrolytic capacitors.
Bad temperature stability, high leakage, short lives

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Capacitor Variations Mylar very popular, nonpolarized reliable, inexpensive, low leakage poor

Capacitor Variations

Mylar
very popular, nonpolarized
reliable, inexpensive, low leakage
poor temperature stability

Mica
extremely accurate, low

leakage current
constructed with alternate layers of metal foil and mica insulation, stacked and encapsulated
small capacitance
often used in high-frequency circuits (i.e. RF circuits)
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Capacitor Reading Example —I Thus, we have a 0.1μF capacitor with ±10% tolerance.

Capacitor Reading Example —I

Thus, we have a 0.1μF capacitor with ±10%

tolerance.
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Capacitor Reading Example —II

Capacitor Reading Example —II

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Variable Capacitors Devices that can be made to change capacitance values

Variable Capacitors

Devices that can be made to change capacitance values with

the twist of a knob.
Air-variable or trimmer forms
Air-variable capacitor consists of two sets of aluminum plates (stator and rotor) that mesh together but do not touch. Often used in frequently adjusted tuning applications (i.e., tuning communication receivers over a wide band of frequencies).
A trimmer capacitor is a smaller unit that is designed for infrequent fine-tuning adjustment (i.e., fine-tuning fixed-frequency communications receivers, crystal frequency adjustments, adjusting filter characteristics)
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Inductors For an ideal coil, magnetic flux is proportional to current,

Inductors

For an ideal coil, magnetic flux is proportional to current, so

∝ I or λ = LI
L is constant of proportionality, called inductance (unit: Henry, Wb/Amp).
So, now, the V-I characteristic of an inductor is:
The above V-I characteristics demonstrate that the current through an inductor can not be altered instantaneously.
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Inductor-Water Analogy —I Suppose a turbine is hooked up to the

Inductor-Water Analogy —I

Suppose a turbine is hooked up to the flywheel

and water is supplied to the turbine. The flywheel will start to move slowly. Eventually, the flywheel will move at the same rate as the current.
If the current alternates back and forth, the flywheel/turbine will take some time to build up to the initial direction that the water wants to flow.
As the current moves back and forth, the flywheel creates the extra “resistance” to the change in current flow, but eventually the flywheel/turbine will move in the same direction as the current flow.
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Inductor-Water Analogy —II When switch S contacts A, the field generated

Inductor-Water Analogy —II

When switch S contacts A, the field generated by

the applied positive voltage creates a reverse induced voltage that initially resists current flow
Based on the value of inductance, as the magnetic field reaches steady-state, the reverse voltage decays
A collapsing field is generated when applied voltage is removed (switch S contacts B), creating a forward induced voltage that attempts to keep current flowing
Based on the value of inductance, as the magnetic field reaches zero steady-state, the forward voltage decays

Mechanical inertia and inductor both resist sudden change in their state

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Inductance of a Cylindrical Coil If number of turns per unit

Inductance of a Cylindrical Coil

If number of turns per unit length

is “n”, then N= , so:

A = cross-sectional area of coil.
If a magnetizable material forms the core of coil, then permeability μ will be larger than μ0.

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Inductor Variations —I

Inductor Variations —I

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Inductor Variations —II Antenna coil contains an iron core that magnifies

Inductor Variations —II

Antenna coil
contains an iron core that magnifies magnetic field

effects
used to tune in ultra-high-frequency signals, i.e. RF signals

Tuning coil
screw-like “magnetic field blocker” that can be adjusted to select the desired inductance value
used in radio receivers to select a desired frequency.

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Inductor Variations —III Chokes general-purpose inductors that act to limit or

Inductor Variations —III

Chokes
general-purpose inductors that act to limit or suppress fluctuating

current.
some use a resistor-like color code to specify inductance values.

Toroidal coil
resembles a donut with a wire wrapping
high inductance per volume ratios, high quality factors, self-shielding, can be operated at extremely high frequencies

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Inductor Symbols

Inductor Symbols

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Transformer Isolation acts exclusively as an isolation device; does not increase

Transformer

Isolation
acts exclusively as an isolation device; does not increase or decrease

the secondary voltage
usually come with an electrostatic shield between the primary and secondary. Often come with a three-wire plug and receptacle that can be plugged directly into a power outlet

High Frequency
often come with air or powered-iron cores
used for high frequency applications, i.e. matching RF transmission lines to other devices (transmission line to antenna)

Audio
used primarily to match impedances between audio devices
work best at audio frequencies from 150Hz to 12kHz
come in a variety of shapes and sizes, typically contain a center tap

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Kirchoff’s Voltage Law The algebraic sum of voltage around a loop

Kirchoff’s Voltage Law

The algebraic sum of voltage around a loop is

zero.
Assumption:
Voltage drop across each passive element is in the direction of current flow.

I

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Kirchoff’s Current Law Algebraic sum of all currents entering and leaving

Kirchoff’s Current Law

Algebraic sum of all currents entering and leaving a

node is zero.
At node A:
Current entering a node is assigned positive sign. Current leaving a node is assigned a negative sign.
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Law of Voltage division

Law of Voltage division

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