# Edexcel A Level Further Maths: Core Pure:复习笔记8.1.3 Modelling using First Order Differential Equations

### Modelling using First Order Differential Equations

#### Why are differential equations used to model real-world situations?

• A differential equation is an equation that contains one or more derivatives
• Derivatives deal with rates of change, and with the way that variables change with respect to one another
• Therefore differential equations are a natural way to model real-world situations involving change
• Most frequently in real-world situations we are interested in how things change over time, so the derivatives used will usually be with respect to time t

#### How do I set up a differential equation to model a situation?

• An exam question may require you to create a differential equation from information provided
• The question will provide a context from which the differential equation is to be created
• Most often this will involve the rate of change of a variable being proportional to some function of the variable
• For example, the rate of change of a population of bacteria, P, at a particular time may be proportional to the size of the population at that time
• The expression ‘rate of’ (‘rate of change of…’, ‘rate of growth of…’, etc.) in a modelling question is a strong hint that a differential equation is needed, involving derivatives with respect to time t
• So with the bacteria example above, the equation will involve the derivative
• Recall the basic equation of proportionality
• If y is proportional to x, then y = kx for some constant of proportionality k
• So for the bacteria example above the differential equation needed would be
• The precise value of k will generally not be known at the start, but will need to be found as part of the process of solving the differential equation
• It can often be useful to assume that k > 0 when setting up your equation
• In this case, -k will be used in the differential equation in situations where the rate of change is expected to be negative
• So in the bacteria example, if it were known that the population of bacteria was decreasing, then the equation could instead be written
• Often scenarios will involves multiple things than affect the rate
• If something causes the variable to increase then that term will be added to the rate of change
• Such as water flowing into a space
• If something causes the variable to decrease then that term will be subtracted from the rate of change
• Such as water flowing out of space

#### Worked Example

a) In a particular pond, the rate of change of the area covered by algae, A, at any time t is directly proportional to the square root of the area covered by algae at that time.  Write down a differential equation to model this situation.

b) Newton’s Law of Cooling states that the rate of change of the temperature of an object, T, at any time t is proportional to the difference between the temperature of the object and the ambient temperature of its surroundings, Ta , at that time.  Assuming that the object starts off warmer than its surroundings, write down the differential equation implied by Newton’s Law of Cooling.