Download 8_Electronics & Ohm`s Law

Technician
License Class
Gordon West
Technician Class Manual
Pages 142-149
Volts & Amps
Presented by
Heart Of Texas
Amateur Radio Club
(HOTARC)
Electrical Voltage & Current
Current is the flow
of electrons in an
electric circuit.
The unit of measure
R
is amperes or amps.
We measure current
with an ammeter.
Voltage is the “force” that makes electrons
move—causing current.
We measure voltage with a voltmeter.
V
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Resistance
Resistance is what opposes current, or the flow
of electrons. All materials have some resistance.
The unit of measurement for resistance
is the ohm.
Materials with very low resistance, allowing
electrons to move easily, are conductors.
Copper, for example, is a good conductor.
Materials with very high resistance, and do not
allow electrons to flow easily, are insulators.
Glass, for example, is a good insulator.
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Water Flow is a Good Analogy to Electrical
Voltage, Current, and Resistance
Battery supplies voltage,
like pump supplies pressure.
Wire carries current,
like pipes carry water.
Resistor slows down current,
like clogs in pipe slow down flow.
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T4A13 (pg 142)
What instrument is used to measure
Electromotive Force (EMF)
between two points such as
the poles of a battery?
A. Magnetometer
B. Voltmeter
C. Ammeter
D. Ohmmeter
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T4A06 (pg 142)
How much voltage does an automobile
battery usually supply?
A. About 12 volts
B. About 30 volts
C. About 120 volts
D. About 240 volts
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T4A03 (pg 142)
What is the name for the
flow of electrons in an electric circuit?
A. Voltage
B. Resistance
C. Capacitance
D. Current
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7
Two Basic Kinds of Current
When electrons flow in only one direction,
it is called Direct Current (DC).
Batteries are a common source of DC.
Most electronic devices are powered by DC.
When electrons flow alternatively in one
direction, then in the opposite direction, it
is called Alternating Current (AC).
Household current is AC.
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8
Direct Current vs Alternating Current
DC delivers a constant
voltage, like from a
battery.
AC voltage regularly
reverses direction,
positive to negative,
between a maximumminimum, at a certain
frequency.
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T4A04
What is the name of a current that flows only
in one direction?
A. An alternating current
B. A direct current
C. A normal current
D. A smooth current
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T4A01 (pg 143)
Electrical current is measured in which of
the following units?
A. Volts
B. Watts
C. Ohms
D. Amperes
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T4A12 (pg 143)
What instrument is used to measure the flow
of current in an electrical circuit?
A. Frequency meter
B. SWR meter
C. Ammeter
D. Voltmeter
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T4A09 (pg 143)
Which of the following is a good electrical
conductor?
A. Glass
B. Wood
C. Copper
D. Rubber
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T4A08 (pg 143)
What is the name of a current that reverses
direction on a regular basis?
A. An alternating current
B. A direct current
C. A circular current
D. A vertical current
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T4A11 (pg 143)
What is the term used to describe opposition
to current flow in ordinary conductors such
as wires?
A. Inductance
B. Resistance
C. Counter EMF
D. Magnetism
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T4A07 (pg 143)
What is the basic unit of resistance?
A. The volt
B. The watt
C. The ampere

D. The ohm
Greek letter: Omega
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T4A10 (pg 144)
Which of the following is a
good electrical insulator?
A. Copper
B. Glass
C. Aluminum
D. Mercury
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Current, Voltage, & Resistance
Just like water flowing through a hose,
changes in voltage, current, and
resistance affect each other.
The effect is mathematically expressed in
Ohm's Law:
E=IxR
I=E/R
R=E/I
where E = voltage, I= current, and R = resistance
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Ohm’s Law
Electromotive force:
VOLTS
E
Current, the
flow of electrons:
AMPERES
I R
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Resistance
to current:
OHMS
19
T4D02 (pg 144)
What formula is used to calculate voltage
in a circuit?
A. Voltage (E) equals current (I)
E
multiplied by resistance (R)
I R
B. Voltage (E) equals current (I)
divided by resistance (R)
C. Voltage (E) equals current (I)
added to resistance (R)
D. Voltage (E) equals current (I)
minus resistance (R)
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=

20
T4D07 (pg 144)
What is the voltage across the resistor if
a current of 0.5 amperes flows through a
2 ohm resistor?
A. 1 volt
E
B. 0.25 volts
C. 2.5 volts
I R
D. 1.5 volts
E = I  R = 0.5 A  2  = 1 V
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T4D08 (pg 145)
What is the voltage across the resistor if
a current of 1 ampere flows through a
10 ohm resistor?
A. 10 volts
E
B. 1 volt
C. 11 volts
D. 9 volts
I R
E = I  R = 1 A  10  = 10 V
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T4D09 (pg 145)
What is the voltage across the resistor if
a current of 2 amperes flows through a
10 ohm resistor?
A. 20 volts
B. 0.2 volts
C. 12 volts
D. 8 volts
E
I R
E = I  R = 2 A  10  = 20 V
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T4D01 (pg 145)
What formula is used to calculate current in
a circuit?
A. Current (I) equals voltage (E)
multiplied by resistance (R)
B. Current (I) equals voltage (E)
divided by resistance (R)
C. Current (I) equals voltage (E)
added to resistance (R)
D. Current (I) equals voltage (E)
minus resistance (R)
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E
I R
= —
24
T4D06 (pg 146)
What is the current flow in a circuit with an
applied voltage of 120 volts and a
resistance of 80 ohms?
A. 9600 amperes
E
B. 200 amperes
I R
C. 0.667 amperes
D. 1.5 amperes
I=E
/ R = 120 V / 80  = 1.5 A
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T4D10 (pg 146)
What is the current flowing through a
100 ohm resistor connected across
200 volts?
A. 20,000 amperes
B. 0.5 amperes
E
I R
C. 2 amperes
D. 100 amperes
I=E
/ R = 200 V / 100  = 2 A
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T4D11 (pg 146)
What is the current flowing through
a 24 ohm resistor connected
across 240 volts?
A.
B.
C.
D.
24,000 amperes
0.1 amperes
10 amperes
216 amperes
E
I R
I=E
/ R = 240 V / 24  = 10 A
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T4D03 (pg 146)
What formula is used to calculate resistance
in a circuit?
E
A. Resistance (R) equals voltage (E)
I R
multiplied by current (I)
B. Resistance (R) equals voltage (E)
divided by current (I)
C. Resistance (R) equals voltage
(E) added to current (I)
D. Resistance (R) equals voltage
(E) minus current (I)
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= —
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T4D04 (pg 146)
What is the resistance of a circuit when a
current of 3 amperes flows through a
resistor connected to 90 volts?
E
A. 3 ohms
R
I
B. 30 ohms
C. 93 ohms
D. 270 ohms
R=E
/ I = 90 V / 3 A = 30 
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T4D05 (pg 147)
What is the resistance in a circuit where the
applied voltage is 12 volts and the current
flow is 1.5 amperes?
A. 18 ohms
E
B. 0.125 ohms
I R
C. 8 ohms
D. 13.5 ohms
R=E
/ I = 12 V / 1.5 A = 8 
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Power
When moving electrons do some work,
power is consumed.
The unit of measurement for power is
Watts.
Power (P) is the product of current (I) and
voltage (E).
P=IxE
I=P/E
E=P/I
where P = power, I = current, and E = voltage.
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Watt’s Law
Electric power:
WATTS
P
Current, the
flow of electrons:
AMPERES
I E
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Electromotive
force:
VOLTS
32
T4A02 (pg 147)
Electrical Power is measured in which of the
following units?
A. Volts
W
B. Watts
C. Ohms
D. Amperes
Watts
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T4E01 (pg 147)
What unit is used to describe
electrical power?
A. Ohm
B. Farad
C. Volt
W
D. Watt
Watts
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T4E02 (pg 147)
What is the formula used to calculate
electrical power in a DC circuit?
A. Power (P) equals voltage (E)
multiplied by current (I)
B. Power (P) equals voltage (E)
divided by current (I)
C. Power (P) equals voltage (E)
minus current (I)
D. Power (P) equals voltage (E)
plus current (I)
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P
I E
35
T4E03 (pg 147)
How much power is represented by a
voltage of 13.8 volts DC and a current of 10
amperes?
A. 138 watts
P
B. 0.7 watts
C. 23.8 watts
I E
D. 3.8 watts
P = I  E = 10 A  13.8 V = 138 W
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T4E04 (pg 147)
How much power is being used in a circuit
when the voltage is 120 volts DC and the
current is 2.5 amperes?
A. 1440 watts
B. 300 watts
P
I E
C. 48 watts
D. 30 watts
P = I  E = 2.5 A  120 V = 300 W
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T4E05 (pg 148)
How can you determine how many watts are
being drawn by your transceiver when you
are transmitting?
A. Measure the DC voltage
and divide it by 60 Hz
P
B. Check the fuse in the power
I E
leads to see what size it is
C. Look in the Radio Amateur's Handbook
D. Measure the DC voltage at the
transceiver and multiply by the
current drawn when you transmit
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T4E06 (pg 148)
How many amperes are flowing in a circuit
when the applied voltage is 120 volts DC
and the load is 1200 watts?
P
A. 20 amperes
I E
B. 10 amperes
C. 120 amperes
D. 5 amperes
P
I= /
1200
W
/ 120 V = 10 A
E=
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T4C04 (pg 148)
What device is used to convert the
alternating current from a wall outlet
into low-voltage direct current?
A. Inverter
B. Compressor
C. Power Supply
D. Demodulator
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40
Metric Prefixes (powers of ten)
Prefix Abbrev Power
giga- G
mega- M
kilo- K
(basic unit)
centi- c
milli- m
micro- m
nano- n
pico- p
Number
109 1,000,000,000
106
1,000,000
103
1,000
100
1
1
——
–2
100
10
0.01
1
–3
——
10
0.001
1000
1
————
10–6
0.000001
1,000,000
10–9
0.000000001
10–12
0.000000000001
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T4E07 (pg 149)
How many milliamperes is
the same as 1.5 amperes?
A. 15 milliamperes
B. 150 milliamperes
C. 1500 milliamperes
D. 15000 milliamperes
A “milliamp” is 1/1000 amp.
So, each ampere is 1000 milliamps.
And, thus 1.5 amperes is 1500 milliamps.
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T4E10 (pg 149)
How many volts are equal to one microvolt?
A. one one-millionth of a volt
B. one million volts
C. one thousand kilovolts
D. one one-thousandth of a volt
A microvolt is 1/1,000,000 volt…a very tiny voltage.
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T4E09 (pg 149)
How many volts are equal to one kilovolt?
A. one one-thousandth of a volt
B. one hundred volts
C. one thousand volts
D. one million volts
One kilovolt is 1000 volts.
Remember the prefix “kilo” stands for a thousand.
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T4E11 (pg 149)
How many watts does a hand-held
transceiver put out if the output power
is 500 milliwatts?
A. 0.02 watts
B. 0.5 watts
C. 5 watts
D. 50 watts
A milliwatt is 1/1000 watt.
So, 500 milliwatts = 500/1000 watt = 0.5 watt.
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