Chapter 2, 3, & 4
Dr. Farid Farahmand
CET 263
Outline
Reviewing basic concepts: Voltage,
Current, and Resistance
 Ohm’s law
 Power and Energy

Atom Structure

An atom is the smallest
particle of an element that
retains the characteristics of
that element.
 An atom has a nucleus,
consisting of positively
charged particles called
protons, and uncharged
particles called neutrons.
 The basic particles of
negative charge, called
electrons, orbit the nucleus.
Electron Shell and Valence Electrons






Electrons orbit the nucleus at discrete
distances from the nucleus.
Orbits are grouped onto energy bands
known as shells.
Electrons with the highest energy exist
in the outermost shell, known as the
valence shell, and its’ electrons are
called valence electrons.
Valence electrons are relatively loosely
bound to the atom.
If a valence electron acquires enough
external energy to leave the atom, the
process is known as ionization.
The escaped electron is called a free
electron.
Categories of Materials


Conductors readily permit
current flow, due to a large
number of free electrons in the
material.
Insulators have few free
electrons, and tend not to
permit current to flow through
them.


Insulators have no free electrons
Semiconductors have
relatively small number of free
electrons

limited number of electrons can
escape
Electrical Charge

The charge of an electron and that of a proton
are equal in magnitude but opposite in polarity.
 The force acting between charges is called an
Electric Field!
electric field.
+
 Electrical charge (Q) is measured
in coulombs (C).
+
 By definition:
One coulomb is the total charge possessed by
6.25 x 1018 electrons.
Q = (number of electrons)/(6.25 x 1018)
Voltage
The unit of voltage is the volt (V).
 By definition:


One volt is the potential difference (voltage)
between two points when one joule of
energy is used to move one coulomb of
charge from one point to the other.
1 Joule of Energy
1C
The potential
difference is one
Volt!
Voltage Sources

A battery is a type of voltage source that
converts chemical energy into electrical
energy.
 Solar Cells convert light energy into electrical
energy.
 Generators convert mechanical energy into
electrical energy.
 Electronic power supplies?

They do not produce electrical energy, but they
transform the ac voltage from the wall outlet into a
constant dc voltage for use in our circuits.
Current

Current definition
 The movement of free
electrons from negative to
positive is electrical
current (I).
 Electrical current is the
rate of flow of charge
I = Q/t
One C of
charges/sec

The Unit of Current is Amp
 One amp is the amount
of current that exists
when a number of
electrons having a total
charge of one coulomb
move through a given
cross-sectional area in
one second.
Resistance



The property of a
material that restricts the
flow of electrons is
called resistance.
The unit of Resistance is
Ohm ().
By definition

One ohm of resistance
exists if there is one
ampere of current in a
material when one volt
is applied across the
material.
Applying 1 V
1 A of current
is generated
There must be 1 ohm of resistance!
Resistors and Conductance


Resistors are used to limit current or divide
voltage.
Conductance is the reciprocal of resistance.




G = 1/R
The unit of conductance is siemens (S).
Common resistors are carbon-composition,
carbon film, and metal film
Variable resistors are designed so that their
resistance values can be changed with manual or
automatic adjustment.


A potentiometer is a variable resistor used to divide
voltage.
A rheostat is a variable resistor used to control current.
Color-code bands on a resistor
1st band is the first digit of the resistance
value.
 2nd band is the second digit of the resistance
value.
 3rd band is the multiplier (number of zeros).
 4th band indicates the tolerance.

Examples: http://www.ccsu.edu/technology/farahmand/ccsu/courses/cet233/cet_233.htm#Resistors
Resistor color code
Resistor color code
Ohm’s Law
Ohm’s law describes
mathematically how voltage,
current, and resistance are
related. I = V/R
Hence, current and voltage are
linearly proportional.



In resistive circuits, with a
constant resistance, if voltage
increases or decreases by a
certain percentage, so will
current.
On the other hand, current and
resistance are inversely related.


With constant voltage, if
resistance is reduced, current
goes up; when resistance is
increased, current goes down
Ohm’s Law
Given R=10 ohm;
how will this graph look like?
Ohm’s Law
Ohm’s Law
Given V=10 V;
How will this graph
look like?
Ohm’s Law
Voltage and Current Measurements


To measure voltage, the
voltmeter is placed in
parallel across the
component; that is, one
lead is place on each
side of the component.
To measure current, the
ammeter must be placed
in series with the
component; that is, it
must be in line with the
current path.
Voltage and Current Measurements


To measure voltage, the
voltmeter is placed in
parallel across the
component; that is, one
lead is place on each
side of the component.
To measure current, the
ammeter must be placed
in series with the
component; that is, it
must be in line with the
current path.
Analyzing the circuit:
Total resistance = 10+15+22+3.3=50.3K
I=V/R = 0.298 mA
What is the measurement error? About %2 error!
Where does the error come from?
What is the measurement error across 15K resistor?
304.2 μ
4.469
6.509
Check the numbers! Make sure
you understand how we got
them!
Current Measurements
OFF
V
Hz
V
304.2 μ
mV
A
Range
Autorange
Touc h/Hold
4.469
1s
1s
10 A
V
40 m A
COM
Fused
6.509
Voltage Measurements
OFF
V
Hz
V
304.2 μ
mV
A
Range
Autorange
Touc h/Hold
4.469
1s
1s
10 A
V
40 m A
COM
Fused
6.509
Energy and Power

Energy is the ability to do work - joules (J)
 Power is the rate at which energy is used - in
watts (W)
 By definition:
One watt is the amount of power when one
joule of energy used in one second.
Power = energy/time
P = W/t
Kilowatt-hour
The kilowatt-hour (kWh) is frequently
used as a unit of energy. One kWh is
used when one thousand watts is used
for one hour.
 Power utilized over a period of time
represents energy consumption.
W = Pt
 Energy can also be expressed as wattseconds (Ws), watt-hour (Wh).

Power in an Electric Circuit

There is always a certain amount of
power in an electric circuit, and it is
dependant on the amount of resistance
and the amount of current, expressed as:
P=
2
IR
Energy Conversion and Voltage
Drop in Resistance

A resistor used in a circuit must have a power
rating in excess of what it will have to handle.
 As electrons flow through resistors, some of
their energy is given up as heat.
 The same number of electrons entering a
resistor will exit it, only their energy will be
less, so the voltage exiting a resistor is less
than the voltage entering the resistor.

This is called voltage drop across a resistor.
Power Loss and Efficiency


The output power of an electronic power supply is always less
that the input power, because some of the input power is used to
operate the power supply circuitry.
An important characteristic of electronic power supplies is
efficiency, which is the ratio of output power to input power.
POUT = PIN – PLOSS
Efficiency = Pout/Pin