Calculus and its Application in Radio

July 1932 Radio News Article

## July 1932 Radio News[Table of Contents]These articles are scanned and OCRed from old editions of the Radio & Television News magazine. Here is a list of the Radio & Television News articles I have already posted. All copyrights are hereby acknowledged. |

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0 | 0 | 0 |

By J. E. Smith*

In order to apply the formulas for differentiating, the student should become familiar with the following examples with reference to the standard forms (1)-(5) inclusive (which were shown in the last lesson

Differentiate the following:

1) y = x

Here, formula (5) applies where x = v, and the exponent 4 = u.

Solution:

(A) , but from the formula (1), dx/dx = 1,

therefore A becomes: dy/dx = 4x

2) y = x

Here, formulas (3) and (5) apply, where a and b = c of formula (3), and the exponents 3 and 5 respectively = n.

Solution:

= 3ax

7) y = ax

The derivative of a constant is zero; i.e., dc/dx = 0.

Solution:

= 20x

12) y = 3cx

Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 |

Electrical and radio theory are very often advanced by the use of calculus, which takes the liberty of performing the necessary operations on the sine and cosine functions. Let us apply the theory which was outlined above in finding the derivative of a sine function. That is, if a sine function is expressed by y = sin x, it is the purpose of this analysis to determine its proper derivative.

In the previous texts it has been shown how to plot the function y = sin x, and this is shown in Figure 1 for the values of x from 0-180 degrees. In order to study the derivative of this function, reference is made to Figures 2 and 3, where the values of x have been plotted from 0-90 degrees. Applying the theory which has been outlined above, we see that when Δx approaches zero, Δy/Δx finally approaches a limiting value, and it is remembered that this limiting value is called the derivative of y with respect to x, This limit, represented by dy/dx, has been shown above to be equal to the tangent θ.

With reference to Figure 4, it is noticed that the angle θ at the point A is somewhat larger than the values of θ for the points Band C. Finally, at the point D, the tangent line TD becomes parallel to the abscissa, and it is noticed that the angle θ has become increasingly smaller until it has approached the value of zero.

Now, the derivative of the function y = sin x at the point A is by definition equal to the tangent of θ. In like manner, the derivatives of the function at points B, C and D are again equal respectively to the tangents of the angles θ.

By close analysis of the relative sizes of these angles, we arrive at the following table:

For point A, θ equals about 45°

For point B, θ equals about 35°

For point C, θ equals about 17°

For point D, θ equals about 0°

But the tangent of these angles is by definition equal to the derivative of the function at its respective points; thus:

dy/dx at point A is about 1

dy/dx at point B is about 0.70

dy/dx at point C is about 0.31

dy/dx at point D is about 0

Plotting these values of dy/dx at point A, B and C, respectively, we have the graph of Figure 5. But this dy/dx curve is further noticed to be the same as the function y = cos x, so it is apparent that:

d (sin x)/ax = cos x

or the derivative of the sin x is equal to the cos x.

Figure 6 shows the functions y = sin x and y = cos x plotted together.

The following formulas are a continuation of the ones listed above from (1-5) inclusive.

6. d/dx (sin v) = cos v (dv/dx)

The derivative of a sine function is equal to its cosine function, times the derivative of the function.

7. d (cos v) / dx = -sin v (dv/dx)

8. d (tan v) / dx = sec

9. d (cot v) / dx = -csc

See v and csc v above are the abbreviations for secant and cosecant, respectively. The application of the above differential forms to radio theory is now quite readily presented.

1) 4x

2) 6x

3) 2x

4) 9x

5) 5x

6) 3ax

7) 4ax

8) 6ax

9) 7ax

10) 1 - 3ax

11) 20x

12) 6cx - 8d

13) 4ax + 8x - 5a

*President National Radio Institute.

+Note - Many of the examples have been taken from the book, "Elements of the Differential and Integral Calculus,"

by W. A. Granville, published by Ginn and Company, N. Y.

Posted August 22, 2013