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IB DP Maths AA SL Study Notes

1.3.3 Applications

Exponential Growth and Decay

Exponential Growth

Exponential growth describes a scenario where the rate of change of a quantity is proportional to the quantity itself.

  • Definition: Exponential growth is defined as the increase of a quantity (P) with respect to time (t) that is proportional to its current value, mathematically expressed as dP/dt = rP, where r is the rate of growth.
  • Example: If you start with one bacterium that doubles every hour, after one hour, you will have two, after two hours, you will have four, after three hours, eight, and so on. This is a classic example of exponential growth, where the rate of growth is proportional to the current population.

Exponential Decay

Conversely, exponential decay describes a process where a quantity decreases at a rate proportional to its current value.

  • Definition: Exponential decay is described by a similar equation to growth but with a negative rate, expressed as dN/dt = -λN, where λ is the decay constant.
  • Example: Radioactive decay is a classic example of exponential decay, where the remaining quantity of a radioactive element decreases by a consistent percentage over a fixed time period, known as the half-life.

pH Calculations

The concept of pH is pivotal in chemistry and biology, providing a measure of the acidity or basicity of a solution.

  • Definition: pH is defined as the negative logarithm (base 10) of the concentration of hydrogen ions, H+, in a solution. Mathematically, it is expressed as pH = -log[H+].
  • Example: If the H+ concentration in a solution is 1 x 10-7 M, the pH is 7, which is neutral on the pH scale. A pH less than 7 indicates an acidic solution, while a pH greater than 7 indicates a basic solution.
  • Application: The pH scale is crucial in various scientific domains, helping scientists and researchers understand the acidic or basic nature of solutions and maintaining desired conditions in experiments and industrial processes.

Richter Scale

The Richter scale is a logarithmic scale that quantifies the magnitude of earthquakes, enabling scientists to manage the vast range of energies released by seismic activities.

  • Definition: The Richter magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded by seismographs. Mathematically, it is expressed as ML = log10(A) - log10(A0), where A is the maximum excursion of the Wood-Anderson seismometer and A0 is the standard amplitude.
  • Example: An earthquake that measures 5.0 on the Richter scale has a shaking amplitude 10 times larger than one that measures 4.0. The energy release is approximately 31.6 times greater, as the energy release is proportional to the 1.5 power of the shaking amplitude.
  • Application: The Richter scale allows scientists to quantify and compare the energies released by different earthquakes, aiding in research, and planning for future seismic activities.

Example Question 1: Exponential Growth

Consider a bacteria culture initially has 500 bacteria and the number of bacteria doubles every 2 hours. Find the number of bacteria after 5 hours.

Solution: Using the formula P(t) = P0 * e(rt), and given that the bacteria doubles every 2 hours, we can find the rate, r, by substituting P(2) = 1000 (since it doubles): 1000 = 500 * e(2r) e(2r) = 2 2r = ln(2) r = ln(2)/2

Now, to find the number of bacteria after 5 hours, substitute t = 5 into the formula: P(5) = 500 * e(5(ln(2)/2)) P(5) = 500 * e(5ln(sqrt(2))) P(5) = 500 * (sqrt(2))5 P(5) = 500 * 11.3137 P(5) approximately equals 5657

Thus, after 5 hours, there will be approximately 5657 bacteria.

Example Question 2: pH Calculation

Calculate the pH of a solution with [H+] = 1 x 10-5 M.

Solution: Using the formula pH = -log[H+]: pH = -log(1 x 10-5) pH = -(-5) pH = 5

Thus, the pH of the solution is 5, indicating that it is acidic.

These applications of exponential and logarithmic functions underscore their significance in various scientific fields, offering a mathematical lens through which we can comprehend and quantify natural phenomena and processes. Understanding these applications not only enriches our mathematical knowledge but also enhances our ability to navigate and engage with the world around us in a scientifically informed manner.


In biology, exponential growth is often used to describe a population's growth under unlimited resource conditions. The formula for exponential growth in populations is N(t) = N0 * e(rt), where N(t) is the population size at time t, N0 is the initial population size, r is the intrinsic growth rate, and e is the base of the natural logarithm. This model assumes that resources are unlimited and that populations can grow indefinitely, which is often not the case in real-world scenarios. However, it provides a useful approximation for populations experiencing rapid growth and where resources are not yet limiting.

The pH of a solution is calculated using the formula pH = -log10([H+]), where [H+] is the concentration of hydrogen ions in the solution. The pH scale is logarithmic to accommodate the wide range of hydrogen ion concentrations encountered in chemistry, which can vary over 14 orders of magnitude. The logarithmic scale compresses this range into a more manageable size and also linearises the response of the hydrogen ion concentration, making calculations and comparisons of acidity more straightforward. A one-unit change in pH represents a tenfold change in the hydrogen ion concentration, which helps in easily understanding and categorizing the acidity or basicity of solutions.

Exponential growth is fundamental in calculating compound interest in finance, which is interest calculated on the initial principal, which also includes all of the accumulated interest from previous periods on a deposit or loan. The formula for compound interest that is compounded continuously is A = P * e(rt), where A is the amount of money accumulated after n years, including interest, P is the principal amount, r is the annual interest rate (decimal), and t is the time the money is invested or borrowed for, in years. This formula allows for the calculation of interest which is compounded continuously, providing a mathematical model for growth where your balance continually earns interest.

The Richter scale is logarithmic, specifically a base-10 logarithmic scale, and it's used to express the magnitude of earthquakes. The formula to calculate the magnitude M on the Richter scale is M = log10(A/A0), where A is the maximum amplitude of seismic waves measured by a seismograph and A0 is a reference amplitude. The logarithmic scale is used because the energy release during earthquakes can vary over a vast range. A logarithmic scale allows this wide range to be compressed into a more manageable scale, and it also allows for easier comparison between the energies of two earthquakes. A one-unit increase in the Richter scale corresponds to a tenfold increase in amplitude and roughly 31.6 times more energy release.

Exponential decay models are widely used to describe radioactive decay. Radioactive decay is a random process, but when a large number of atoms are considered, it follows a predictable pattern described by the equation N(t) = N0 * e(-lambda * t), where N(t) is the quantity that remains after time t, N0 is the initial quantity, lambda is the decay constant, and e is the base of the natural logarithm. The decay constant lambda is specific to the decaying element. This model is applicable because radioactive decay is a process where the probability of decay is constant over time, making the decay rate proportional to the number of atoms present, which aligns with the properties of exponential decay.

Practice Questions

Exponential Growth in Finance

A bank offers an annual interest rate of 5% compounded continuously. If you deposit £1000, how much money will be in the account after 10 years?

To solve this problem, we use the formula for continuous compound interest, which is A = P * e(rt), where P is the principal amount (£1000), r is the rate of interest (0.05), t is the time in years (10), and e is the base of the natural logarithm (approximately 2.71828). Substituting these values in, we get A = 1000 * e(0.05*10) = 1000 * e0.5. Calculating this out, A = 1000 * 1.64872 = £1648.72. So, after 10 years, the account will have £1648.72.

Calculating pH Levels

A solution has a hydrogen ion concentration of [H+] = 1 x 10-8 M. Calculate the pH of the solution and determine whether it is acidic, basic, or neutral.

The pH of a solution is calculated using the formula pH = -log[H+]. Substituting the given hydrogen ion concentration into the formula, we get pH = -log(1 x 10-8) = -(-8) = 8. The pH scale ranges from 0 to 14, with 7 being neutral. A pH less than 7 is acidic, and a pH greater than 7 is basic. Therefore, a pH of 8 indicates that the solution is basic. It's crucial to note that the pH scale is logarithmic, meaning each whole pH value below 7 is ten times more acidic than the next higher value. Similarly, each whole pH value above 7 is ten times less acidic than the one below it.

Dr Rahil Sachak-Patwa avatar
Written by: Dr Rahil Sachak-Patwa
Oxford University - PhD Mathematics

Rahil spent ten years working as private tutor, teaching students for GCSEs, A-Levels, and university admissions. During his PhD he published papers on modelling infectious disease epidemics and was a tutor to undergraduate and masters students for mathematics courses.

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