Transcript Electric Current

Basic Electrical
Engineering
Lecture # 01
Course Instructor:
Engr. Sana Ziafat
Grading Policy
 Final Exam:
 Mid term Exam
 Assignments
 Quizzes:
 Labs
40%
20%
5%
10%
25%
• Quizzes may be announced or unannounced.
• Exams are closed-book and extremely time
limited.
• Exams consist of design questions, numerical,
maybe true-false and short answer questions.
TEXT BOOKS
•
Electric Circuits
by J.W. Nilson & Susan A. Rediel
•
Electric Circuits
Schaum’s Outlines
by: M Nahvi
Objectives of the Course
• To impart the basic knowledge of electric circuits
• To know about various measuring instruments
• To introduce basic electrical concepts and laws
Agenda
•
•
•
•
Basics of electric circuits
Measuring system/SI Unit
Ideal Basic circuit element
Basic electric circuit quantities
Measurement
You are making a measurement when you
Check you weight
Read your watch
Take your temperature
What kinds of measurements did you make
today?
Standards of Measurement
When we measure, we use a measuring
tool to compare some dimension of an
object to a standard.
Some Tools for Measurement
Learning Check
From the previous slide, state the tool (s)
you would use to measure
A. temperature
____________________
B. volume
____________________
____________________
C. time
____________________
D. weight
____________________
Solution
From the previous slide, state the tool (s)
you would use to measure
A. temperature thermometer
B. volume
measuring cup,
graduated cylinder
C. time
watch
D. weight
scale
International System Of Units
• Is a decimal system based on 10
• Used in most of the world
• Used by scientists and hospitals
• Based on seven defined quantities
• Defined quantities are combined to
form derived units.
Units in the Metric System
 length
meter
m
 volume
liter
L
 mass
gram
g
 temperature
Kelvin
K
Stating a Measurement
In every measurement there is a
Number
followed by a
 Unit from measuring device
Learning Check
What is the unit of measurement in each
of the following examples?
A. The patient’s temperature is 102°F.
B. The sack holds 5 lbs of potatoes.
C. It is 8 miles from your house to school.
D. The bottle holds 2 L of orange soda.
Solution
A.
°F (degrees Fahrenheit)
B.
lbs (pounds)
C.
miles
D. L (liters)
Learning Check
Identify the measurement in SI units.
A. John’s height is
1) 1.5 yards
2) 6 feet
3) 2
meters
B. The volume of saline in the IV bottle is
1) 1 liters
2) 1 quart
3) 2 pints
C. The mass of a lemon is
1) 12 ounces
2) 145 grams
3) 0.6 pounds
Solution
A. John’s height is
3) 2 meters
B. The volume of saline in the IV bottle is
1) 1 liter
C. The mass of a lemon is
2) 145 grams
Derived Units
• Force equals mass time acceleration:
newton is defined as unbalanced force that
imparts an acceleration of 1 meter per second
equal to a 1 kilogram mass.
• Work results when force works over a
distance: Joule is equivalent to newton-meter
• Power is rate at which power is done:
watt is joule per second
Information at the foundation of
modern science and technology
from the Physics Laboratoryof NIST
Detailed contents
Values of the constantsand related information
Searchable bibliographyon the constants
In-depth information on the SI, the modern
metric system
Guidelinesfor the expression
of uncertainty in measurement
About this reference . Feedback.
Privacy Statement / Security Notice - NIST Disclaimer
Electric Circuits
• Interconnection of electrical components
• Elementary quantities in the analysis of
electric circuits are:
1. electric current
2. voltage
Electric charge and current
• Charge is bipolar, meaning electrical effects
are described in terms of positive and
negative charges.
• Time rate of change of charge is known as electric
current:
i(t) = dq(t)/dt
where i and q are current and charge respectively.
• Basic unit of charge is ampere (columb per second)
• One ampere is equivalent to 1 columb charge
moving across a fixed surface in 1 second.
Volatge
• If a unit positive charge is moved between two
points then the energy required to move it is the
difference in energy level between the two
points and is defined as voltage.
• v= dw/dq, where as ‘w’ and ‘q’ are work and
charge respectively
• Unit of voltage is volts, 1 volt= 1joule/coulomb=
newton-meter/coulomb
Basic Circuit Element
• It has only two terminals
• It cant be further sub divided
• It is described mathematically in terms of
current and voltage
Passive sign convention
• Whenever the direction for the current in an
element is in the direction of the reference
voltage drop across the element use a positive
sign in any expression that relates the voltage to
the current
Circuit Measurements
• Electric Current - Flow of electrons through a material
• Electrical Potential –
▫ Similar to potential energy (lifting something higher against the
force of gravity gives it greater potential to do work, increasing its
potential energy.)
▫ When given the opportunity, objects will move from higher
potential energy to an area of lower potential energy
▫ Electrical potential is related to their electrical fields and not to
height – as electrons build up on one side they “want” to flow to
an area w/ less potential
Voltage
• Voltage – causes current to flow through an electrical circuit
• Volt – unit of measure to measure this potential
• A Voltage Source (battery or generator) is required to maintain the
electrical potential in a circuit.
Electrical Current
• Water flowing thru a pipe depends on more than the angle
of the pipe. It also depends on the length of the pipe,
diameter of the pipe and if the pipe is clogged or open.
• Electrical Current is measured in Amperes
• Amount of Electrical Current ( amps) depends on more
than just Voltage, it depends on the Resistance found in the
circuit.
Energy and power
• Output of a system is often non electrical and is
expressed in terms of energy or power.
p=dw/dt
• Unit is watt and watt= joule/sec
• p=vi….. How?????????
Readings
• Chapter 1: 1.2, 1.4, 1.5, 1.6 (Electric Circuits)
▫ By James W. Nilson
Q&A