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Chapter 3Summary
Voltage is
Review of V, I, and R
the amount of energy per charge available tomove electrons from one point to another in a circuit.
Current is the rate of charge flow and is measured in amperes.
Resistance is the opposition to current and is measured in ohms.
Chapter 3
The most important fundamental law in electronics is Ohm’s law, which relates voltage, current, and resistance.
VIR
What is the current in from a 12 V source if the resistance is 10 ? 1.2 A
It takes one volt to push one amp through one ohm.
In a DC circuit, the current is directly proportional to the voltage and inversely proportional to the resistance.
Chapter 3
Ohm’s law
If you need to solve for voltage, Ohm’s law is:
What is the voltage across a 680 resistor if the current is 26.5 mA? 18 V
V IR VIR
Chapter 3
Ohm’s law
If you need to solve for resistance, Ohm’s law is:VRI
115 V
V
1 s
1 s
40 mA
10 A
COM
RangeAutorangeTouch/Hold
Fused
OFF V
V
Hz
mV
A
What is the (hot) resistance of the bulb? 132
VIR
Chapter 3
Graph of Current versus Voltage
I (m
A)
V (V)00 2 4
4
6 8
8
10
12
14
16
A student takes data for a resistor and fits the straight line shown to the data. What is the resistance and the conductance of the resistor?
The slope represents the conductance.
14.8 mA - 0 mA 1.48 mS10.0 V - 0 V
G
The reciprocal of the conductance is the resistance:
1 1 676 Ω1.48 mS
RG
Chapter 3
Graph of Current versus Voltage
Voltage (V)
Cur
rent
(mA
)
0 10 20 300
2.0
4.0
6.0
8.0
10Notice that the plot of current versus voltage for a fixed resistor is a line with a positive slope. What is the resistance indicated by the graph?
2.7 k
What is its conductance? 0.37 mS
mAmAVV
010027
Chapter 3
Graph of Current versus Resistance
If resistance is varied for a constant voltage, the current verses resistance curve plots a hyperbola.
Determine the voltage source for the curve?
Resistance (k )
Cur
rent
(mA
)
0 1.0 2.0 3.00
2.0
4.0
6.0
8.0
10
(8.0mA)(375Ω) = 3V(2.4mA)(1.25kΩ) = 3V
Chapter 3
0 1 2 3 4 5 6 7 8 9
1
2
3
4
Volts
Am
pere
s
2 +
_0 to 9 Volts
2 1
4
The smaller the resistor, the steeper the slope.
A linear resistance has a constant value of ohms. Its R does not change with the applied voltage, so V and I are directly proportional.
Linear Resistance
Carbon-film and metal-film resistors are examples of linear resistors.
Chapter 3Nonlinear Resistance
– In a nonlinear resistance, increasing the applied Vproduces more current, but I does not increase in the same proportion as the increase in V.
– Example of a Nonlinear Volt–Ampere Relationship:
• As the tungsten filament in a light bulb gets hot, its resistance increases.
Chapter 3
– Another nonlinear resistance is a thermistor. – A thermistor is a resistor whose resistance value
changes with its operating temperature. – As an NTC (negative temperature coefficient)
thermistor gets hot, its resistance decreases.
VoltsA
mpe
res
Thermistor
VoltsA
mpe
res
Thermistor
Nonlinear Resistance
Chapter 3
Application of Ohm’s law
The resistor is green-blue brown-gold. What should the ammeter read?
15V/560Ω = 26.8 mA
Power Supply
+15 V- + - +5 V 2A
DC Ammeter+
-
Gnd
V A
Chapter 3
?
20 V 4 I = 20 V4
= 5 A
1 A
? 12 V = 1A × 12 = 12 V
3 A
6 V ? R = 6 V3 A
= 2
Applying Ohm’s Law VI R
Application of Ohm’s law
Chapter 3Open-Circuit
An open circuit has zero current flow.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Chapter 3
A short circuit has excessive current flow.
As R approaches 0, I approaches .
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Short-Circuit
Chapter 3Summary
TroubleshootingAPM – Analysis, Planning, Measuring
? 1. Analyze clues or symptoms of the failure.
- Is the power properly applied to circuit.- Has the circuit ever worked. Conditions of failure.- Check fuses or circuit-breakers, if applicable.- Any components burnt, solder splashes, etc.?
2. Planning – Formulate a plan of attack and working knowledge of the circuit.
- Schematics- Instruction manuals
3. Measuring – Narrow the possible failures by isolating sections of the circuit and making carefully thought out measurements.
Chapter 3
Linear
Ohm’s law
Troubleshooting
Characterized by a straight-line relationship.
A law stating that current is directly proportional to voltage and inversely proportional to current.
A systematic process of isolating, identifying, and correcting a fault in a circuit or system.
Selected Key Terms
Chapter 3Quiz
1. Holding the voltage constant, and plotting the current against the resistance as resistance is varied will form a
a. straight line with a positive slope
b. straight line with a negative slope
c. parabola
d. hyperbola
Chapter 3Quiz
1. Holding the voltage constant, and plotting the current against the resistance as resistance is varied will form a
a. straight line with a positive slope
b. straight line with a negative slope
c. parabola
d. hyperbola
Chapter 3Quiz
2. When the current is plotted against the voltage for a fixed resistor, the plot is a
a. straight line with a positive slope
b. straight line with a negative slope
c. parabola
d. hyperbola
Chapter 3Quiz
2. When the current is plotted against the voltage for a fixed resistor, the plot is a
a. straight line with a positive slope
b. straight line with a negative slope
c. parabola
d. hyperbola
Chapter 3Quiz
3. For constant voltage in a circuit, doubling the resistance means
a. doubling the current
b. halving the current
c. there is no change in the current
d. depends on the amount of voltage
Chapter 3Quiz
3. For constant voltage in a circuit, doubling the resistance means
a. doubling the current
b. halving the current
c. there is no change in the current
d. depends on the amount of voltage
VIR
Chapter 3Quiz
4. A four-color resistor has the color-code red-violet-orange-gold. If it is placed across a 12 V source, the expected current is
a. 0.12 mA
b. 0.44 mA
c. 1.25 mA
d. 4.44 mA
Chapter 3Quiz
4. A four-color resistor has the color-code red-violet-orange-gold. If it is placed across a 12 V source, the expected current is
a. 0.12 mA
b. 0.44 mA
c. 1.25 mA
d. 4.44 mA
kV
2712
Chapter 3Quiz
5. If the current in a 330 resistor is 15 mA, the applied voltage is approximately
a. 5.0 V
b. 22 V
c. 46 V
d. 60 V
Chapter 3Quiz
5. If the current in a 330 resistor is 15 mA, the applied voltage is approximately
a. 5.0 V
b. 22 V
c. 46 V
d. 60 V
)330)(15( mA
Chapter 3Quiz
6. The current in a certain 4-band resistor is 22 mA when the voltage is 18 V. The color bands on the resistor are
a. blue-gray-red-gold
b. red-red-brown gold
c. gray-red-brown-gold
d. white-brown-red-gold
Chapter 3Quiz
6. The current in a certain 4-band resistor is 22 mA when the voltage is 18 V. The color bands on the resistor are
a. blue-gray-red-gold
b. red-red-brown gold
c. gray-red-brown-gold
d. white-brown-red-gold
8202.8182218
mAV
Chapter 3Quiz
7. The circuit with the largest current is
a. (a)
b. (b)
c. (c)
d. (d)
R 10 k
+12 V R 15 k
+15 V R 22 k
+18 V R 27 k
+24 V
(a) (b) (c) (d)
Chapter 3Quiz
7. The circuit with the largest current is
a. (a)
b. (b)
c. (c)
d. (d)
R 10 k
+12 V R 15 k
+15 V R 22 k
+18 V R 27 k
+24 V
(a) (b) (c) (d)mAkV 2.1
1012
mAkV 0.1
1515
mAkV 82.0
2218
mAkV 89.0
2724
Chapter 3Quiz
8. The circuit with the smallest current is
a. (a)
b. (b)
c. (c)
d. (d)
R 10 k
+12 V R 15 k
+15 V R 22 k
+18 V R 27 k
+24 V
(a) (b) (c) (d)
Chapter 3Quiz
8. The circuit with the smallest current is
a. (a)
b. (b)
c. (c)
d. (d)
R 10 k
+12 V R 15 k
+15 V R 22 k
+18 V R 27 k
+24 V
(a) (b) (c) (d)mAkV 2.1
1012
mAkV 0.1
1515
mAkV 82.0
2218
mAkV 89.0
2724
Chapter 3Quiz
9. Before troubleshooting a faulty circuit you should find out
a. If the circuit ever worked
b. The conditions that existed when it failed
c. The symptoms of the failure
d. All of the above
Chapter 3Quiz
9. Before troubleshooting a faulty circuit you should find out
a. If the circuit ever worked
b. The conditions that existed when it failed
c. The symptoms of the failure
d. All of the above
Chapter 3Quiz
10. A troubleshooting method that starts in the middle and works toward a fault is
a. short-circuit testing
b. comparison testing
c. half-splitting
d. resistance testing
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