What are i: The Mysteries Behind This Mathematical Constant

The constant i is the square root of negative one, also known as the imaginary unit. It is a fundamental concept in mathematics and has important applications in fields such as physics and engineering. In this article, we will explore the mysteries behind i and its significance in various mathematical formulas and equations.

Complex Numbers

Complex numbers are numbers that can be expressed in the form a + bi, where a and b are real numbers and i is the imaginary unit. The real part of the complex number is represented by a, and the imaginary part is represented by bi. For example, 3 + 2i is a complex number, where 3 is the real part and 2i is the imaginary part.

Complex numbers can be added, subtracted, multiplied, and divided just like real numbers. When adding or subtracting complex numbers, the real parts and imaginary parts are added or subtracted separately. When multiplying complex numbers, the distributive property is used to find the product.


(3 + 2i) * (2 + 5i) = (3 * 2) + (3 * 5i) + (2i * 2) + (2i * 5i) = 6 + 15i + 4i + 10i^2 = -4 + 19i

The Imaginary Unit

The imaginary unit i is defined as the square root of -1, which means that i^2 equals -1. This may seem like a paradoxical concept, since the square of any real number is always positive. However, the square root of a negative number is considered an imaginary number, and i is the standard symbol for the unit of imaginary numbers.

Since i is not a real number, it cannot be plotted on a number line. However, complex numbers can be plotted on a complex plane, which is a two-dimensional coordinate system where the horizontal axis represents the real part of the complex number and the vertical axis represents the imaginary part. The point where the two axes intersect is the origin (0, 0).

Euler’s Formula

Euler’s formula is a mathematical identity that relates the exponential function to trigonometric functions. The formula is expressed as:

e^(i*x) = cos(x) + i*sin(x)

where x is a real number and e is the base of the natural logarithm. The left-hand side of the equation contains the imaginary unit, which allows for the conversion of trigonometric functions into exponential functions. This formula is widely used in engineering, physics and mathematics.


If we plug in x = pi, we get:

e^(i*pi) = cos(pi) + i*sin(pi) = -1

This is known as Euler’s identity, which is a special case of Euler’s formula where x = pi. It is considered one of the most beautiful equations in mathematics, due to the simplicity of its components and its far-reaching implications.

Applications in Electrical Engineering

The concept of complex numbers and imaginary units is crucial in electrical engineering, particularly in the analysis of AC (alternating current) circuits. Since AC voltage and current are sinusoidal functions that vary with time, complex numbers and phasors are used to represent them in mathematical equations. A phasor is a complex number that represents the amplitude and phase angle of a sinusoidal function at a single frequency.

Phasors can be added, subtracted, and multiplied just like complex numbers. The magnitude of a phasor is the amplitude of the sinusoidal function, and the angle is the phase shift relative to a reference point. By using phasors, engineers can analyze AC circuits and determine the currents, voltages, and power at any point in the circuit.


If a circuit has an AC voltage of 120 V at a frequency of 60 Hz and a phase angle of 30 degrees, the voltage can be represented by the phasor:

Real Part (V) Imaginary Part (V) Magnitude (V) Angle (degrees)
103.92 60 120 30

Complex Analysis

Complex analysis is a branch of mathematics that deals with the properties and behavior of complex functions. Complex functions are functions that have complex numbers as inputs or outputs, and they play a vital role in the study of fluid dynamics, quantum mechanics, and other fields.

The principles of complex analysis are based on the theory of complex numbers and complex functions, and they involve concepts such as analytic functions, singularities, residues, and conformal mappings. Analytic functions are functions that can be expressed as a power series of complex numbers, and they have a number of important properties, such as differentiability and holomorphy.


The complex function f(z) = z^2 has the first derivative f'(z) = 2z and the second derivative f”(z) = 2. Since f'(z) exists for all values of z, f(z) is an analytic function.

Applications in Quantum Mechanics

The principles of complex numbers and imaginary units are central to the field of quantum mechanics, which is the study of the behavior of subatomic particles. In quantum mechanics, wave functions are used to describe the probability distributions of particles, and these wave functions are complex functions that involve the imaginary unit i. The square of the wave function gives the probability density of finding the particle in a particular location.

The Schrödinger equation, which is the fundamental equation of quantum mechanics, is a complex partial differential equation that involves the imaginary unit i. This equation describes the evolution of the wave function over time, and it plays a critical role in predicting the behavior of subatomic particles.


If we consider a simple particle in a one-dimensional box, the wave function can be expressed as:

ψ(x) = √(2/L) * sin(n*pi*x/L)

where L is the length of the box, n is a positive integer, and x is the position of the particle within the box. The wave function is normalized so that the probability of finding the particle within the box is equal to 1. By using the principles of complex numbers and the Schrödinger equation, physicists can determine the energy levels and quantum states of the particle.


Overall, the imaginary unit i is a powerful tool in mathematics and has significant applications in fields such as electrical engineering and quantum mechanics. The concept of complex numbers and functions, as well as the principles of complex analysis and functional analysis, are based on the properties of i and its relationship to the real number system. By gaining a greater understanding of the mysteries behind i, we can unlock new insights and discoveries in multiple areas of research and innovation.

Common Questions and Answers:

  • What is the value of i? The value of i is the square root of -1.
  • What is the difference between a real number and an imaginary number? Real numbers are numbers that can be plotted on a number line and are represented without the use of the imaginary unit i. Imaginary numbers are numbers that involve i and cannot be plotted on a number line.
  • What is Euler’s formula, and why is it important? Euler’s formula is a mathematical identity that relates the exponential function to trigonometric functions. It is important because it allows for the conversion of trigonometric functions into exponential functions, which has applications in engineering, physics, and mathematics.
  • What is the significance of the Schrödinger equation in quantum mechanics? The Schrödinger equation is the fundamental equation of quantum mechanics and describes the evolution of the wave function over time. It is used to predict the behavior of subatomic particles and has significant implications for the field of physics.


  • Boas, R. P. (2006). Mathematical methods in the physical sciences. John Wiley & Sons.
  • Nahvi, M., & Edminister, J. A. (2014). Electric circuits. McGraw Hill Professional.
  • Srednicki, M. (2007). Quantum field theory. Cambridge University Press.

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