Current
OBJECTIVES
After
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•
•
•
•
•
•
•
completing this chapter, the student will be able to:
State the two laws of electrostatic charges.
Define coulomb.
Identify the unit used to measure current flow.
Define the relationship of amperes, coulombs, and time through a formula.
Describe how current flows in a circuit.
Describe how electrons travel in a conductor.
Define and use scientific notation.
Identify commonly used prefixes for powers of ten.
See accompanying
Chapter 2.
CD for interactive
The atom has been defined as the smallest particle of an element. It is composed of electrons,
protons, and neutrons.
Electrons breaking away from atoms and
flowing through a conductor produce an electric
current.
This chapter examines how electrons break
free from atoms to produce a current flow and
how to use scientific notation. Sdentific notation
expresses very large and small numbers in a form
of mathematical shorthand.
10
presentations
and tutorials
relating to
BII ELECTRICAL
CHARGE
Two electrons together or two protons together
represent "like" charges. Like charges resist being
brought together and instead move away from
each other. This movement is called repelling. This
is the first law of electrostatic charges: like
charges repel each other (Figure 2-1). According
to the second law of electrostatic charges, unlike
charges attract each other.
CHAPTER
FIGURE 2-1
Basic laws of electrostatic charges.
NEGATIVE
CHARGE
2 CURRENT
of potential exists between two charged bodies
connected by a conductor, electrons will flow
along the conductor. This flow of electrons is
called current.
2-1 QUESTIONS
1. What are the two laws of electrostatic
charges?
2. What does an electrical charge represent?
POSITIVE
CHARGE
LIKE CHARGES
3. Define coulomb.
REPEL EACH OTHER
mJ CURRENT
UNLIKE CHARGES
ATTRACT EACH OTHER
The negative electrons are drawn toward the
positive protons in the nucleus of an atom. This
attractive force is balanced by the centrifugal force
caused by the electron's rotation about the nucleus. As a result the electrons remain in orbit and
are not drawn into the nucleus.
The amount of attracting or repelling force
that acts between two electrically charged bodies
depends on two factors: their charge and the distance between them.
Single electrons have a charge too small for
practical use. The unit adopted for measuring
charges is the coulomb (C), named for Charles
Coulomb. The electrical charge (Q) carried by
6,240,000,000,000,000,000
electrons (six quintillion, two hundred forty quadrillion, or 6.24 X
1018) represents one coulomb.
1 C = 6.24 X 1018 electrons
Electrical charges are created by the displacement of electrons. When there is an excess of
electrons at one point and a deficiency of electrons at another point, a difference of potential
exists between the two points. When a difference
FLOW
An electric current consists of the drift of electrons from an area of negative charge to an
area of positive charge. The unit of measurement for current flow is the ampere (A). An ampere represents the amount of current in a
conductor when one coulomb of charge moves
past a point in one second. The relationship between amperes and coulombs per second can
be expressed as:
I
= Q
t
where: I
Q
t
=
=
=
current measured in amperes
quantity of electrical charge in
coulombs
time in seconds
What is the current in amperes if 9
coulombs of charge flow past a point in an electric
circuit in 3 seconds?
EXAMPLE:
Given:
Solution:
I =?
I=
Q
t
Q
= 9 coulombs
t
=
3 seconds
9
1=-
3
I
=
3 amperes
SECTION 1 DC CIRCUITS
EXAMPLE: A circuit has a current of 5 amperes.
How long does it take for one coulomb to pass a
given point in the circuit?
Given:
I
=
Q
=
5 amperes
1 coulomb
=?
t
Solution:
I= Q
t
1
5=-
t
5~_;f"1
- »">;-(cross
1
(1)(1)
multiply)
t
(5)(t)
1
5t
1
..8t
-( divide both sides by 5)
5
1
5
0.2 seconds
!if
t
t
Electrons, with their negative charge, represent the charge carrier in an electric circuit. Therefore, electric current is the flow of negative
charges. Scientists and engineers once thought
that current flowed in a direction opposite to electron flow. Later work revealed that the movement
of an electron from one atom to the next created
the appearance of a positive charge, called a hole,
moving in the opposite direction (Figures 2-2,
2- 3). Electron movement and current were found
to be the same.
If electrons are added to one end of a conductor and provision is made to take electrons from
the other end, an electric current flows through
the conductor.
As free electrons move slowly
through the conductor, they collide with atoms,
knocking other electrons free. These new free
electrons travel toward the positive end of the
conductor and collide with other atoms. The electrons drift from the negative to the positive end of
the conductor because like charges repel. In addition, the positive end of the conductor, which represents a deficiency in electrons, attracts the free
electrons because unlike charges attract.
The drift of electrons is slow (approximately
an eighth of an inch per second), but individual
electrons ricochet off atoms, knocking other electrons loose, at the speed of light (186,000 miles
per second). For example, visualize a long, hollow
tube filled with Ping-Pong balls (Figure 2-4). As a
ball is added to one end of the tube, a ball is forced
out the other end of the tube. Although an indi-
FIGURE 2-2
As electrons move from one
atom to another, they
create the appearance of
a positive charge, called
a hole.
NEGATIVE
POTENTIAL
POSITIVE
POTENTIAL
ELECTRON
CHAPTER 2 CURRENT
FIGURE 2-3
Electron movement occurs in the opposite direction to hole movement.
NEGATIVE
\OTENT~l
POSITIVE
POTENTIAL
CONDUCTOR
-HOLEFLOW
FIGURE 2-4
Electrons in a conductor react like Ping-Pong balls in a
hollow tube.
ELECTRON
FLOW -
CURRENT
FLOW -
moves them horn the other end of the conductor
(the positive terminal) is called the voltage source. It
can be thought of as a kind of pump (Figure 2-5).
2-2
QUESTIONS
1. Define electric current.
~
2. What is the unit for measuring flow?
3. What is the relationship of current,
coulombs, and time?
4. What is the current if 15 coulombs of
charge flow past a point in a circuit in 5
seconds?
5. How long does it take for 3 coulombs to
move past a point in a circuit if the circuit
has 3 amperes of current flow?
6. What makes electrons move through a
conductor in only one direction?
vidual ball takes time to travel down the tube, the
speed of its impact can be far greater.
The device that supplies electrons from one
end of a conductor (the negative terminal) and re-
1m SCIENTIFIC
NOTATION
In electronics, it is common to encounter very
small and very large numbers. Scientific notation is
a means of using single-digitnumbers plus powers
SECTION 1 DC CIRCUITS
FIGURE 2-5
A voltage source can be considered a pump that supplies electrons to the load and recycles the excess electrons.
VOLTAGE
-
SOURCE~
I
~I
CONDUCTOR
-
I
I
I
IL_
-
-
-
-
-
-
•
-
.,
I
I
I
I
I
I
_
LOAD
of ten to express large and small numbers. For
example, 300 in scientific notation is 3 X 102.
The exponent indicates the number of decimal places to the right or left of the decimal point
in the number. If the power is positive, the decimal point is moved to the right. For example:
3
X
103
=
3.0
X
103
=
3.000
=
3000
U
3 places
If the power is negative, the decimal point is
moved to the left. For example:
3 X 10-6 = 3.0 X 10-6 = 0000003.
•
=
0.000003
I
6 places
Figure 2-6 lists some commonly used powers
of ten, both positive and negative, and the prefixes
and symbols associated with them. For example,
an ampere (A) is a large unit of current that is not
FIGURE 2-6
Prefixes commonly used in electronics.
Giga-
G
109
1,000,000,000
Mega-
M
10
1,000,000
Kilo-
k
103
1,000
Milli-
m
10-3
0.001
Micro-
!l
10-6
0.000001
Nano-
n
10-
Pico-
P
10-12
6
9
0.000000001
0.000000000001
often found in low-power electronic circuits. More
frequently used units are the milliampere (mA) and
the microampere (/-1A).A milliampere is equal to
CHAPTER2
one-thousandth (111000)of an ampere or 0.001 A.
In other words, it takes 1000 milliamperes to equal
one ampere. A microampere is equal to onemillionth (Ill ,000,000) ofan ampere or O. 00000lA;
it takes 1,000,000 microamperes to equal one
ampere.
EXAMPLE: How many milliamperes are there in
2 amperes?
Solution:
1000 mA
lA
\~(1000
mA
=
1 A)
X
1
2
X
EXAMPLE: How many amperes are there in
50 micro-amperes?
Solution:
1,000,000
J-tA
50 J-tA
XA
lA
1,000,000
1
(1)(50)
50
1,000,000
0.00005
0.00005 A
50
X
(1,000,000) (X)
X
X
X
QUESTIONS
1. Define scientific notation.
2. In scientific notation:
a. What does a positive exponent mean?
b. What does a negative exponent mean?
3. Convert the following numbers to
scientific notation:
a. 500
b.3768
d.0.l05
e. 356.78
4. Define the following prefixes:
a. Millib. Micro5. Perform the following conversions:
a. 1.5 A =
IDA
b. 1.5 A
=
fLA
=
A
d. 750 fLA=
A
(1000)(2)
2000mA
(l)(X)
2- 3
c.0.0056
c. 150 IDA
1000
CURRENT
SUMMARY
.
• Laws of electrostatic charges: like charges
repel, unlike charges attract.
• Electrical charge (Q) is measured in
coulombs (C).
• One coulomb is equal to 6.24 X 1018
electrons.
• An electric current is the slow drift of
electrons from an area of negative charge
to an area of positive charge.
• Current flow is measured in amperes.
• One ampere (A) is the amount of current
that flows in a conductor when one
coulomb of charge moves past a point in
one second.
• The relationship between current,
electrical charge, and time is represented
by the formula:
1= Q
t
• Electrons (negative charge) represent the
charge carrier in an electrical circuit.
• Hole movement (positive charge) occurs in
the opposite direction to electron
movement.
• Current flow in a circuit is from negative
to positive.
SECTION 1 DC CIRCUITS
• If the power-of-ten exponent is positive,
the decimal point is moved to the right.
• If the power-of-ten exponent is negative,
the decimal point is moved to the left.
• The prefix milli- means one-thousandth.
• The prefix micro- means one-millionth.
• Electrons travel very slowly through a
conductor, but individual electrons move
at the speed of light.
• Scientific notation expresses a very large
or small number as a numeral from 1 to 9
to a power of ten.
C HAP T E R 2 SELF - T EST
.~,.,
;.-
:<'\
'~~,.;
_ '~,--<..:
i
1. How much current is in a circuit if it takes 5 seconds for 7 coulombs to flow past a given
point?
2. Describe how electrons flow in a circuit with reference to the potential in the circuit.
3. Convert the following numbers to scientific notation:
a. 235
b.0.002376
c. 56323.786
4. What do the following prefixes represent?
a. Millib. Micro-
j~;~
;if-?'