The notion of the square root of -1 -1 originated with the quadratic formula: the solution of certain quadratic equations mathematically exists only if the so-called imaginary quantity -1 -1 could be defined. Euler first used ii for the imaginary unit but that notation did not take hold until roughly Ampère's time. Ampère used the symbol ii to denote current (intensité de current). It wasn't until the twentieth century that the importance of complex numbers to circuit theory became evident. By then, using ii for current was entrenched and electrical engineers chose i for writing complex numbers.

An imaginary number has the form i⁢b=−b2 b b 2 . A complex number, zz, consists of the ordered pair (aa,bb), aa is the real component and bb is the imaginary component (the i is suppressed because the imaginary component of the pair is always in the second position). The imaginary number i⁢b b equals (00,bb). Note that aa and bb are real-valued numbers.

Figure 1 shows that we can locate a complex number in what we call the complex plane. Here, aa, the real part, is the xx-coordinate and bb, the imaginary part, is the yy-coordinate.

From analytic geometry, we know that locations in the plane can be expressed as the sum of vectors, with the vectors corresponding to the xx and yy directions. Consequently, a complex number zz can be expressed as the (vector) sum z=a+i⁢b z a b where i indicates the yy-coordinate. This representation is known as the Cartesian form of zz. An imaginary number can't be numerically added to a real number; rather, this notation for a complex number represents vector addition, but it provides a convenient notation when we perform arithmetic manipulations.

Some obvious terminology. The real part of the complex number z=a+i⁢b z a b , written as ℜ⁢z z , equals aa. We consider the real part as a function that works by selecting that component of a complex number not multiplied by i. The imaginary part of zz, ℑ⁢z z , equals bb: that part of a complex number that is multiplied by i. Again, both the real and imaginary parts of a complex number are real-valued.

The complex conjugate of zz, written as z¯ z , has the same real part as zz but an imaginary part of the opposite sign.

Using Cartesian notation, the following properties easily follow.

Complex numbers can also be expressed in an alternate form, polar form, which we will find quite useful. Polar form arises arises from the geometric interpretation of complex numbers. The Cartesian form of a complex number can be re-written as a+i⁢b=a2+b2⁢(aa2+b2+i⁢ba2+b2) a b a 2 b 2 a a 2 b 2 b a 2 b 2 By forming a right triangle having sides aa and bb, we see that the real and imaginary parts correspond to the cosine and sine of the triangle's base angle. We thus obtain the polar form for complex numbers. z=a+i⁢b=r⁢∠⁢θ r=|z|=a2+b2 a=r⁢cosθ b=r⁢sinθ θ=arctanba z a b r ∠ θ r z a 2 b 2 a r θ b r θ θ b a The quantity rr is known as the magnitude of the complex number zz, and is frequently written as |z| z . The quantity θθ is the complex number's angle. In using the arc-tangent formula to find the angle, we must take into account the quadrant in which the complex number lies.

Surprisingly, the polar form of a complex number zz can be expressed mathematically as

To show this result, we use Euler's relations that express exponentials with imaginary arguments in terms of trigonometric functions. sinθ=ei⁢θ−e−(i⁢θ)2⁢i θ θ θ 2 The first of these is easily derived from the Taylor's series for the exponential. ex=1+x1!+x22!+x33!+… x 1 x 1 x 2 2 x 3 3 … Substituting i⁢θ θ for xx, we find that ei⁢θ=1+i⁢θ1!−θ22!−i⁢θ33!+… θ 1 θ 1 θ 2 2 θ 3 3 … because i2=-1 2 -1 , i3=−i 3 , and i4=1 4 1 . Grouping separately the real-valued terms and the imaginary-valued ones, ei⁢θ=1−θ22!+…+i⁢(θ1!−θ33!+…) θ 1 θ 2 2 … θ 1 θ 3 3 … The real-valued terms correspond to the Taylor's series for cosθ θ , the imaginary ones to sinθ θ , and Euler's first relation results. The remaining relations are easily derived from the first. We see that multiplying the exponential in Equation 3 by a real constant corresponds to setting the radius of the complex number to the constant.

Adding and subtracting complex numbers expressed in Cartesian form is quite easy: You add (subtract) the real parts and imaginary parts separately.

To multiply two complex numbers in Cartesian form is not quite as easy, but follows directly from following the usual rules of arithmetic. Note that we are, in a sense, multiplying two vectors to obtain another vector. Complex arithmetic provides a unique way of defining vector multiplication.

Division requires mathematical manipulation. We convert the division problem into a multiplication problem by multiplying both the numerator and denominator by the conjugate of the denominator.

Because the final result is so complicated, it's best to remember how to perform division—multiplying numerator and denominator by the complex conjugate of the denominator—than trying to remember the final result.

The properties of the exponential make calculating the product and ratio of two complex numbers much simpler when the numbers are expressed in polar form.

z 1 z 2 = r 1 ⁢ei⁢ θ 1 r 2 ⁢ei⁢ θ 2 = r 1 r 2 ⁢ei⁢( θ 1 − θ 2 ) z 1 z 2 r 1 θ 1 r 2 θ 2 r 1 r 2 θ 1 θ 2 To multiply, the radius equals the product of the radii and the angle the sum of the angles. To divide, the radius equals the ratio of the radii and the angle the difference of the angles. When the original complex numbers are in Cartesian form, it's usually worth translating into polar form, then performing the multiplication or division (especially in the case of the latter). Addition and subtraction of polar forms amounts to converting to Cartesian form, performing the arithmetic operation, and converting back to polar form.