cap.c

 /* Simple capacitor plate area calculator
  * compile with cc -Wall -o cap cap.c -lm
  * 2021 The Lightning Stalker
  */

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <unistd.h>
#include <ctype.h>

const long double e = 0.08854187812813;  // ɛ - electric constant with cm/pF conversion

double roundval(long double inval)
// round to 2 decimal places
{
  int i;
  i = (int)(inval * 100 + 0.5);
  return (double)i / 100;
}

void plate()
{
  // variable declarations
  long double C, d, A, K, l, i;

  /*   C = desired capacitance
   *   d = depth of dielectric material (thickness)
   *   A = plate area
   *   K = dielectric constant
   *   l = length of sides of a square plate
   *   i = intermediate temp variable
   */

   // input section
   puts("Plate type");
   printf("Enter target capacitance (pF): ");
   scanf("%Lf", &C);
   printf("Enter spacing or depth of dielectric material (cm): ");
   scanf("%Lf", &d);
   printf("Enter dielectric K constant of dielectric material: ");
   scanf("%Lf", &K);

   // begin formulas
   i = e * K;
   A = C * d / i;
   i = sqrt(A);
   l = roundval(i);
   i = roundval(A);
   // end formulas

   // output section
   printf("\n \
    The capacitor area shall be %.2Lf cm² such as a square\n \
    having %.2Lf cm length sides.\n\n", i, l);
}

void cylinder()
{
  // variable declarations
  long double C, R1, R2, K, l, i;

  /*   C = desired capacitance
   *   R1 = inner diametr of dielectric
   *   R2 = outer diameter of dielectric
   *   K = dielectric constant
   *   l = length of capacitor
   *   i = intermediate temp variable
   */

   // input section
   puts("cylinder type");
   printf("Enter target capacitance (pF): ");
   scanf("%Lf", &C);
   printf("Enter inner diameter of dielectric (cm): ");
   scanf("%Lf", &R1);
   printf("Enter outer diameter of dielectric (cm): ");
   scanf("%Lf", &R2);
   printf("Enter dielectric K constant of dielectric material: ");
   scanf("%Lf", &K);

   // begin formulas
   i = logl(R2 / R1);
   l = (C * i) / (2 * M_PI * e * K);
   i = roundval(l);
   // end formulas

   // output section
   printf("\nThe capacitor shall be %.2Lf cm in length.\n\n", i);
}

void sphere()
{
  // variable declarations
  long double C, R;

  /*   C = desired capacitance
   *   R = radius of sphere
   */

   puts("sphere type");
   printf("Enter target capacitance (pF): ");
   scanf("%Lf", &C);

   // begin formulas
   R = C / (4 * M_PI * e) * 2;
   // end formulas

   // output section
   printf("\nThe sphere shall have a diameter of %.2Lf cm.\n\n", R);
}

void disc()
{
  // variable declarations
  long double C, R;

  /*   C = desired capacitance
   *   R = radius of disc
   */

   puts("disc type");
   printf("Enter target capacitance (pF): ");
   scanf("%Lf", &C);

   // begin formulas
   R = C / (8 * e) * 2;
   // end formulas

   // output section
   printf("\nThe disc shall have a diameter of %.2Lf cm.\n\n", R);
}

void usage(int status)
{
  // boiler plate
  puts("\n \
  cap is a capacitor calculator.\n \
  cap will find dimensions of the conductors for a capacitor given the target\n \
  capacitance, thickness and K constant of the dielectric material.\n \
  Enter 'cap -?' for a list of command line options.\n \
  \n");

  puts(" \
  Valid options are:\n \
    \n \
    (Specify one)\n \
    -p = parallel plate type\n \
    -c = cylinder type\n \
    -s = sphere type\n \
    -d = disc type\n \
    -? = this usage text\n \
  ");

  // handle exit status
  if (status)
  {
    exit(0);
  }
  else
  {
    exit(EXIT_FAILURE);
  }
}

int main(int argc, char *argv[])
{
  // variable declarations
  char c;

  // get options passed at the command line
  c = getopt(argc, argv, "pcsd?");
  if (getopt(argc, argv, "pcsd?") != -1) usage(1); // multiple opts
		switch(c)
		{
/*      case -1: // Detect the end of the options.
        plate();
        break; */
			case 'p':
				plate(); // plate type
				break;
			case 'c':
				cylinder(); //cylinder type
				break;
      case 's':
        sphere(); // sphere type
        break;
      case 'd':
        disc(); // disc type
        break;
			case '?':
				usage(0);
				break;
			default:
        if (!isalnum(c)) c = '?';
        printf ("?? invalid option %c ??\n", c);
        usage(1);
        exit(EXIT_FAILURE);
				break;
		}
    exit(0);
}