1. Precalculus V. J. Motto Exponential/Logarithmic Part 5

2. Exponential and Logarithmic Functions4

3. Modeling with Exponential and Logarithmic Functions
4.5

4. Modeling with Exponential Functions
Many processes that occur in nature can be modeled using exponential functions.
Population growth
Radioactive decay
Heat diffusion

5. Modeling with Logarithmic Functions
Logarithmic functions are used in models for phenomena such as:
Loudness of sounds
Intensity of earthquakes

6. Exponential Models of Population Growth

7. Exponential Models of Population Growth
Biologists have observed that the population of a species doubles its size in a fixed period of time.
For example, under ideal conditions, a certain bacteria population doubles in size every 3 hours.
If the culture is started with 1000 bacteria, after 3 hours there will be 2000 bacteria, after another 3 hours there will be 4000, and so on.

8. Exponential Models of Population Growth
If we let n = n(t) be the number of bacteria after t hours, then

9. Exponential Models of Population Growth
From this pattern, it appears that the number of bacteria after t hours is modeled by the function n(t) = 1000 · 2t/3

10. Exponential Models of Population Growth
In general, suppose the initial size of a population is n0 and the doubling period is a.
Then, the size of the population at time t is modeled by: n(t) = n02ct where c = 1/a.
If we knew the tripling time b, the formula would be: n(t) = n03ct where c = 1/b.

11. Exponential Models of Population Growth
These formulas indicate that the growth of the bacteria is modeled by an exponential function.
However, what base should we use?

12. Exponential Models of Population Growth
The answer is e.
Then, it can be shown (using calculus) that the population is modeled by: n(t) = n0ert where r is the relative rate of growth of population, expressed as a proportion of the population at any time.
For instance, if r = 0.02, then at any time t, the growth rate is 2% of the population at time t.

13. Population Growth & Compound Interest
Notice that the formula for population growth is the same as that for continuously compounded interest.
In fact, the same principle is at work in both cases.

14. Population Growth & Compound Interest
The growth of a population (or an investment) per time period is proportional to the size of the population (or the amount of the investment).
A population of 1,000,000 will increase more in one year than a population of 1000.
In exactly the same way, an investment of $1,000,000 will increase more in one year than an investment of $1000.

15. Exponential Growth Model
A population that experiences exponential growth increases according to the model
n(t) = n0ert
where:
n(t) = population at time t
n0 = initial size of the population
r = relative rate of growth (expressed as a proportion of the population)
t = time

16. Exponential Models of Population Growth
In the following examples, we assume that:
The populations grow exponentially.

17. E.g. 1—Predicting the Size of a Population
The initial bacterium count in a culture is 500.
A biologist later makes a sample count of bacteria in the culture and finds that the relative rate of growth is 40% per hour.
(a) Find a function that models the number of bacteria after t hours.
(b) What is the estimated count after 10 hours?
(c) Sketch the graph of the function n(t).

18. E.g. 1—Predicting Population Size
Example (a)
We use the exponential growth model with n0 = 500 and r = 0.4 to get: n(t) = 500e0.4t where t is measured in hours

19. E.g. 1—Predicting Population Size
Example (b)
Using the function in part (a), we find that the bacterium count after 10 hours is: n(10) = 500e0.4(10) = 500e ≈ 27,300

20. E.g. 1—Predicting Population Size
Example (c)
The graph is shown here.

21. E.g. 2—Comparing Different Rates of Population Growth
In 2000, the population of the world was 6.1 billion and the relative rate of growth was 1.4% per year.
It is claimed that a rate of 1.0% per year would make a significant difference in the total population in just a few decades.

22. E.g. 2—Comparing Different Rates of Population Growth
Test this claim by estimating the population of the world in the year 2050 using a relative rate of growth of:
(a) 1.4% per year
(b) 1.0% per year

23. E.g. 2—Comparing Different Rates of Population Growth
Graph the population functions for the next 100 years for the two relative growth rates in the same viewing rectangle.

24. E.g. 2—Diff. Rates of Popn. Growth
Example (a)
By the exponential growth model, we have n(t) = 6.1e0.014t where:
n(t) is measured in billions.
t is measured in years since 2000.

25. E.g. 2—Diff. Rates of Popn. Growth
Example (a)
Since the year 2050 is 50 years after 2000, we find: n(50) = 6.1e0.014(50) = 6.1e ≈ 12.3
The estimated population in the year is about 12.3 billion.

26. E.g. 2—Diff. Rates of Popn. Growth
Example (b)
We use the function n(t) = 6.1e0.010t.
We find: n(50) = 6.1e0.010(50) = 6.1e ≈ 10.1
The estimated population in the year is about 10.1 billion.

27. E.g. 2—Diff. Rates of Popn. Growth
These graphs show that:
A small change in the relative rate of growth will, over time, make a large difference in population size.

28. E.g. 3—Finding the Initial Population
A certain breed of rabbit was introduced onto a small island about 8 years ago.
The current rabbit population on the island is estimated to be 4100, with a relative growth rate of 55% per year.
(a) What was the initial size of the population?
(b) Estimate the population 12 years from now.

29. E.g. 3—Finding Initial Population
Example (a)
From the exponential growth model, we have: n(t) = n0e0.55t
Also, we know that the population at time t = 8 is: n(8) = 4100

30. E.g. 3—Finding Initial Population
Example (a)
We substitute what we know into the equation and solve for n0:
Thus, we estimate that 50 rabbits were introduced onto the island.

31. E.g. 3—Finding Initial Population
Example (b)
Now that we know n0, we can write a formula for population growth:
n(t) = 50e0.55t
Twelve years from now, t = 20 and n(20) = 50e0.55(20) ≈ 2,993,707
We estimate that the rabbit population on the island 12 years from now will be about 3 million.

32. Exponential Models of Population Growth
Can the rabbit population in Example 3 (b) actually reach such a high number?
In reality, as the island becomes overpopulated with rabbits, the rabbit population growth will be slowed due to food shortage and other factors.
One model that takes into account such factors is the logistic growth model.

33. E.g. 4—World Population Projections
The population of the world in was 6.1 billion, and the estimated relative growth rate was 1.4% per year.
If the population continues to grow at this rate, when will it reach 122 billion?

34. E.g. 4—World Population Projections
We use the population growth function with: n0 = 6.1 billion r = n(t) = 122 billion
This leads to an exponential equation, which we solve for t.

35. E.g. 4—World Population Projections
The population will reach 122 billion in approximately 214 years—in the year = 2214.

36. E.g. 5—The Number of Bacteria in a Culture
A culture starts with 10,000 bacteria, and the number doubles every 40 min.
(a) Find a function that models the number of bacteria at time t.
(b) Find the number of bacteria after one hour.
(c) After how many minutes will there be 50,000 bacteria?
(d) Sketch a graph of the number of bacteria at time t.

37. E.g. 5—Bacteria in a Culture
Example (a)
To find the function that models this population growth, we need to find the rate r.
Thus, we use the formula for population growth with: n0 = 10, t = n(t) = 20,000
Then, we solve for r.

38. E.g. 5—Bacteria in a Culture
Example (a)
We can now write the function for the population growth: n(t) = 10,000e t

39. E.g. 5—Bacteria in a Culture
Example (b)
Using the function we found in part (a) with t = 60 min (one hour), we get:
n(60) = 10,000e (60) ≈ 28,287
The number of bacteria after one hour is approximately 28,000.

40. E.g. 5—Bacteria in a Culture
Example (c)
We use the function we found in part (a) with n(t) = 50,000 and solve the resulting exponential equation for t.

41. E.g. 5—Bacteria in a Culture
Example (c)
The bacterium count will reach 50,000 in approximately 93 min.

42. E.g. 5—Bacteria in a Culture
Example (d)
The graph of the function n(t) = 10,000e t is shown.

43. Radioactive Decay

44. Radioactive substances decay by spontaneously emitting radiation.
Radioactive Decay
Radioactive substances decay by spontaneously emitting radiation.
The rate of decay is directly proportional to the mass of the substance.
This is analogous to population growth, except that the mass of radioactive material decreases.

45. Radioactive Decay
It can be shown that the mass m(t) remaining at time t is modeled by the function
m(t) = m0e–rt
where:
r is the rate of decay expressed as a proportion of the mass.
m0 is the initial mass.

46. Half-Life
Physicists express the rate of decay in terms of half-life—the time required for half the mass to decay.
We can obtain the rate r from this as follows.

47. Radioactive Decay
If h is the half-life, then a mass of 1 unit becomes ½ unit when t = h.
Substituting this into the model, we get:
The last equation allows us to find the rate r from the half-life h.

48. Radioactive Decay Model
If m0 is the initial mass of a radioactive substance with half-life h, the mass remaining at time t is modeled by the function m(t) = m0e–rt where

49. Polonium-210 (210Po) has a half-life of 140 days.
E.g. 6—Radioactive Decay
Polonium-210 (210Po) has a half-life of 140 days.
Suppose a sample has a mass of 300 mg.
(a) Find a function that models the amount remaining at time t.
(b) Find the mass remaining after one year.
(c) How long will it take for the sample to decay to a mass of 200 mg?
(d) Draw a graph of the sample mass as a function of time.

50. E.g. 6—Radioactive Decay
Example (a)
Using the model for radioactive decay with m0 = 300 and r = (ln 2/140) ≈ we have: m(t) = 300e t

51. We use the function we found in part (a) with t = 365 (one year).
E.g. 6—Radioactive Decay
Example (b)
We use the function we found in part (a) with t = 365 (one year).
m(365) = 300e (365) ≈
Thus, approximately 49 mg of 210Po remains after one year.

52. E.g. 6—Radioactive Decay
Example (c)
We use the function we found in part (a) with m(t) = 200 and solve the resulting exponential equation for t.

53. The time required for the sample to decay to 200 mg is about 82 days.
E.g. 6—Radioactive Decay
Example (c)
The time required for the sample to decay to 200 mg is about 82 days.

54. A graph of the function m(t) = 300e-0.00495t is shown.
E.g. 6—Radioactive Decay
Example (d)
A graph of the function m(t) = 300e t is shown.

55. Newton's Law of Cooling

56. Newton’s Law of Cooling
Newton’s Law of Cooling states that:
The rate of cooling of an object is proportional to the temperature difference between the object and its surroundings—provided the temperature difference is not too large.
Using calculus, the following model can be deduced from this law.

57. Newton’s Law of Cooling
If D0 is the initial temperature difference between an object and its surroundings, and if its surroundings have temperature Ts , then the temperature of the object at time t is modeled by the function T(t) = Ts + D0e–kt where k is a positive constant that depends on the type of object.

58. E.g. 7—Newton’s Law of Cooling
A cup of coffee has a temperature of 200°F and is placed in a room that has a temperature of 70°F.
After 10 min, the temperature of the coffee is 150°F.
(a) Find a function that models the temperature of the coffee at time t.
(b) Find the temperature of the coffee after 15 min.

59. E.g. 7—Newton’s Law of Cooling
(c) When will the coffee have cooled to 100°F?
(d) Illustrate by drawing a graph of the temperature function.

60. E.g. 7—Newton’s Law of Cooling
Example (a)
The temperature of the room is: Ts = 70°F
The initial temperature difference is: D0 = 200 – = 130°F
So, by Newton’s Law of Cooling, the temperature after t minutes is modeled by the function T(t) = e–kt

61. E.g. 7—Newton’s Law of Cooling
Example (a)
We need to find the constant k associated with this cup of coffee.
To do this, we use the fact that, when t = 10, the temperature is T(10) = 150.

62. E.g. 7—Newton’s Law of Cooling
Example (a)
So, we have:

63. E.g. 7—Newton’s Law of Cooling
Example (a)
Substituting this value of k into the expression for T(t), we get: T(t) = e t

64. E.g. 7—Newton’s Law of Cooling
Example (b)
We use the function we found in part (a) with t = 15.
T(15) = e (15) ≈ 133 °F

65. E.g. 7—Newton’s Law of Cooling
Example (c)
We use the function in (a) with T(t) = 100 and solve the resulting exponential equation for t.

66. E.g. 7—Newton’s Law of Cooling
Example (c)
The coffee will have cooled to 100°F after about half an hour.

67. E.g. 7—Newton’s Law of Cooling
Example (d)
Here’s the graph of the temperature function.
Notice that the line t = 70 is a horizontal asymptote.
Why?

68. Logarithmic Scales

69. Logarithmic Scales
When a physical quantity varies over a very large range, it is often convenient to take its logarithm in order to have a more manageable set of numbers.

70. We discuss three such situations:
Logarithmic Scales
We discuss three such situations:
The pH scale—which measures acidity
The Richter scale—which measures the intensity of earthquakes
The decibel scale—which measures the loudness of sounds

71. Other quantities that are measured on logarithmic scales include:
Light intensity
Information capacity
Radiation

72. The pH Scale
Chemists measured the acidity of a solution by giving its hydrogen ion concentration until Sorensen, in 1909, proposed a more convenient measure.
He defined: pH = –log[H+] where [H+] is the concentration of hydrogen ions measured in moles per liter (M).

73. He did this to avoid very small numbers and negative exponents.
The pH Scale
He did this to avoid very small numbers and negative exponents.
For instance, if [H+] = 10–4 M then pH = –log10(10–4) = –(–4) = 4

74. Solutions with a pH of 7 are defined as neutral.
pH Classifications
Solutions with a pH of 7 are defined as neutral.
Those with pH < 7 are acidic.
Those with pH > 7 are basic.
Notice that, when the pH increases by one unit, [H+] decreases by a factor of 10.

75. E.g. 8—pH Scale and Hydrogen Ion Concentration
(a) The hydrogen ion concentration of a sample of human blood was measured to be: [H+] = 3.16 x 10^-8
Find the pH and classify the blood as acidic or basic.

76. E.g. 8—pH Scale and Hydrogen Ion Concentration
(b) The most acidic rainfall ever measured occurred in Scotland in Its pH was 2.4.
Find the hydrogen ion concentration.

77. A calculator gives: pH = –log[H+] = –log(3.16 x 10–8) ≈ 7.5
E.g. 8—pH Scale
Example (a)
A calculator gives: pH = –log[H+] = –log(3.16 x 10–8) ≈ 7.5
Since this is greater than 7, the blood is basic.

78. E.g. 8—Hydrogen Ion Concentration
Example (b)
To find the hydrogen ion concentration, we need to solve for [H+] in the logarithmic equation log[H+] = –pH
So, we write it in exponential form: [H+] = 10–pH
In this case, pH = 2.4; so, [H+] = 10–2.4 ≈ 4.0 x 10–3 M

79. The Richter Scale
In 1935, American geologist Charles Richter (1900–1984) defined the magnitude M of an earthquake to be where:
I is the intensity of the earthquake (measured by the amplitude of a seismograph reading taken 100 km from the epicenter of the earthquake)
S is the intensity of a “standard” earthquake (whose amplitude is 1 micron = 10-4 cm).

80. The magnitude of a standard earthquake is:
The Richter Scale
The magnitude of a standard earthquake is:

81. Richter studied many earthquakes that occurred between 1900 and 1950.
The Richter Scale
Richter studied many earthquakes that occurred between 1900 and 1950.
The largest had magnitude 8.9 on the Richter scale.
The smallest had magnitude 0.

82. This corresponds to a ratio of intensities of 800,000,000.
The Richter Scale
This corresponds to a ratio of intensities of 800,000,000.
Thus, the scale provides more manageable numbers to work with.
For instance, an earthquake of magnitude 6 is ten times stronger than an earthquake of magnitude 5.

83. E.g. 9—Magnitude of Earthquakes
The 1906 earthquake in San Francisco had an estimated magnitude of 8.3 on the Richter scale.
In the same year, a powerful earthquake occurred on the Colombia- Ecuador border and was four times as intense.
What was the magnitude of the Colombia-Ecuador earthquake on the Richter scale?

84. E.g. 9—Magnitude of Earthquakes
If I is the intensity of the San Francisco earthquake, from the definition of magnitude, we have:
The intensity of the Colombia-Ecuador earthquake was 4I.
So, its magnitude was:

85. E.g. 10—Intensity of Earthquakes
The 1989 Loma Prieta earthquake that shook San Francisco had a magnitude of 7.1 on the Richter scale.
How many times more intense was the 1906 earthquake than the 1989 event?

86. E.g. 10—Intensity of Earthquakes
If I1 and I2 are the intensities of the 1906 and 1989 earthquakes, we need to find I1/I2.
To relate this to the definition of magnitude, we divide numerator and denominator by S.

87. E.g. 10—Intensity of Earthquakes
Therefore,
The 1906 earthquake was about 16 times as intense as the 1989 earthquake.

88. The ear is sensitive to an extremely wide range of sound intensities.
The Decibel Scale
The ear is sensitive to an extremely wide range of sound intensities.
We take as a reference intensity I0 = 10–12 W/m2 (watts per square meter) at a frequency of 1000 hertz.
This measures a sound that is just barely audible (the threshold of hearing).

89. The Decibel Scale
The psychological sensation of loudness varies with the logarithm of the intensity (the Weber-Fechner Law).
Hence, the intensity level B, measured in decibels (dB), is defined as:

90. The intensity level of the barely audible reference sound is:
The Decibel Scale
The intensity level of the barely audible reference sound is:

91. E.g. 11—Sound Intensity of a Jet Takeoff
Find the decibel intensity level of a jet engine during takeoff if the intensity was measured at 100 W/m2.
From the definition of intensity level, we see that:
The intensity level is 140 dB.

92. Intensity Levels of Sound
The table lists decibel intensity levels for some common sounds—ranging from the threshold of human hearing to the jet takeoff of Example 11.
The threshold of pain is about 120 dB.