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Common and Natural Logarithms

    what you'll learn...


This page explains

 •  common logarithm or logarithm of base 10

 •  natural logarithm or logarithm of base e

The number e is also introduced in a thought-provoking and ingenious way.

base 10

In decimal system, the numbers are represented in the base 10

The place value is in units, tens, hundreds, etc. This is in powers of 10.

We learned that

 •  logarithm can be specified in any base. eg: log23,log103

 •  a logarithm of one base can be converted to logarithm of another base. eg: log23=log73÷log72 (which will be explained later)

Knowing these, the base of logarithm can be standardized to a numerical value for uniformity and simplicity.

One of the good choices is "log of base 10, as base of decimals is 10".

common or natural

Logarithm of base 10 is called common logarithm.

If the logarithm is specified as log3 (without the base), then it represents logarithm to the base 10.

In coming up with a standard for base of logarithms, the considerations were only related to being uniform with the base of decimal number system. The application scenarios and the practical uses of logarithm were not taken in consideration.

Let us examine an application scenario and explore an use of logarithm. This discussion will make "natural logarithm" evident.

The logic is little involved. Students are advised to go through this.

linear and proportional

Consider a medicine being absorbed by cells in body.

Consider 100mg of medicine in the blood. Let us examine two mechanisms (out of several under different levels and environment) by which the medicine is absorbed.

In the first case, a constant amount of medicine comes in contact with cells and is absorbed. 20mg of medicine comes in contact with the cells in 1 hour and so every 1 hour 20mg of medicine is absorbed.
It is modeled as x(t)=100(1-0.2t),
that is if
t=0, x(0)=100mg,

In the second case, only part of the medicine comes in contact with the cells and that is absorbed. If the amount of medicine is more, then the amount absorbed is more. As the amount reduces, proportionally the amount absorbed is lesser, because the medicine do not come in contact.

In 1 hour 100mg is reduced to 1001.25=80mg.
And in next 1 hour, it is reduced to 801.25=64mg


This absorption is explained with intervals of 1 hour. But, it does not happen in discrete 1 hour durations, the absorption happens continuously over time. And it is modeled as

first order

We derived a model for first-order-absorption of medicine.

This model applies to several other applications.

 •  the decay of radio-active material

 •  the growth of bacteria when no constraints on resources

 •  the compound interest on money charged continuously (not at discrete 3 months or 1 year durations).

These are modeled as
x(t)=x0a-t or

Let us examine this further.

rate of change

We derived the model x(t)=x0a-t.

The equation takes different values for a for different medicines. For one medicine, a=10 and another a=15.

It is noted that any number a can be written as a=bk, where b does not change and k changes for different a.
eg :10=23.322 and 15=23.907.

Thus, we are set to define a standard form for x(t)=x0b-kt

Obviously, in the model of a-t, the variable a varies. But in the model of b-kt, the variable k varies. It does not seem to change anything. A value for b is chosen keeping in mind the rate-of-change calculations done in these kind of problems.

Consider k=1 and so, x(t)=b-t. When the value of b is 2 or 2.2, the magnitude of rate of change is lower than the x(t).
and when the value of b is 3 or more, the magnitude of rate of change is higher than the x(t)
There is a value between 2 and 3, e.

For that value b=e in b-t, the magnitude of rate-of-change of the quantity is proportional to the quantity itself. That happens only when e=2.71828.

The value of e is numerically computed, such that rate of change of et equals et.

It is chosen, especially because, it helps in finding rate-of-change(differentiation) and in finding aggregate-of-change (integration). These are explained in later classes.


students may skip this page
If the mathematical model is

Then the rate of change is

The constant is derived to be loge(a)

If the a is represented with a=ek
the equation becomes

The difference between representing the model with a and with e is that,

 •  when a is used as the base, the rate of change involves loge(a), leading to base of e

 •  when e is used as the base, the rate of change is readily in the constant k.

The latter is considered to be clean.

natural ln

Natural Logarithm : Logarithm of base e is called the natural logarithm.
The natural logarithm is specified as lnx.


The common logarithm of 1000 is "3".

Common logarithm is to the base 10.

If natural logarithm of 10 is 2.3, What is the natural logarithm of 100?
The answer is "4.6".

Natural logarithm is to the base e.
given that ln10=2.3


Common Logarithm : Logarithm of base 10 is called the common logarithm.
If the base is not specified then the logx is taken to be common logarithm log10x.

Natural Logarithm : Logarithm of base e is called the natural logarithm.
The natural logarithm is specified as lnx.


The outline of material to learn "Exponents" is as follows. Note: click here for detailed outline of Exponents s

    →   Representation of Exponents

    →   Inverse of exponent : root

    →   Inverse of exponent : Logarithm

    →   Common and Natural Logarithms

    →   Exponents Arithmetics

    →   Logarithm Arithmetics

    →   Formulas

    →   Numerical Expressions

    →   PEMA / BOMA

    →   Squares and Square roots

    →   Cubes and Cube roots