10   '
20   '  ELECTRICAL POTENTIAL FOR TWO POINT CHARGES
30   '
40   ' ---------------- 11/8/87 ------------------------
50   '
60   '  This program calculates the electrical potential 
70   '  for 2 point charges at an array of 16 x 21 points 
80   '  in the range (-10,+10) for X and Y.
90   '
100  '            --> LIST OF PROCEDURES <--
110  '
120        GOSUB 220  'INITIALIZATION & INPUT
130        GOSUB 400  'CALCULATE POTENTIAL ON GRID
140        GOSUB 810  'OUTPUT
150        PRINT
160        PRINT
170        INPUT "WANT TO RUN ANOTHER (Y/N)? ", AGAIN$
180        IF AGAIN$ = "Y" OR AGAIN$ = "y" THEN 120
190        END
200  '
210  '------------------------------------------------------
220  'INITIALIZATION & INPUT
230  '
240  DIM V(21,21)
250  '
260  PRINT "INPUT VALUES FOR THE FOLLOWING:"
270  PRINT
280  PRINT "      X CO-ORD FOR Q1: A1"
290  PRINT "      X CO-ORD FOR Q2: A2"
300  PRINT "            CHARGE #1: Q1"
310  PRINT "            CHARGE #2: Q2"
320  PRINT
330  PRINT "SEPARATE THE VALUES WITH COMMAS AND PRESS RETURN"
340  PRINT
350  INPUT A1,A2,Q1,Q2
360  PRINT
370  '
380  RETURN
390  '------------------------------------------------------
400  'CALCULATE POTENTIAL ON GRID
410  '
420  ' -- Loop over 21 Y values
430  '
440  FOR J = 1 TO 21
450     '
460     PRINT " > Working on line";J;"of 21";
470     LOCATE ,1
480     '
490     ' -- Loop over 16 X values
500     '
510     FOR K = 1 TO 16
520      ' 
530      ' -- The X and Y coordinates are calculated from the row
540      '    and column indices J and K
550      '
560      X = K - 11
570      Y = J - 11
580       '
590       ' -- Caculate distances R1 and R2 from the point (X,Y)
600       '    to each of the two point charges.
610       '
620       R1 =  SQR ((X - A1) ^ 2 + Y ^ 2)
630       R2 =  SQR ((X - A2) ^ 2 + Y ^ 2)
640       '
650       ' -- Next two statements are in case R1 or R2 is zero
660       '
670       R1 = R1 + .000001
680       R2 = R2 + .000001
690       '
700       ' -- Calculate the potential at this point.
710       '
720       V(J,K) = ( Q1 / R1 ) + ( Q2 / R2 )
730       '
740       NEXT K
750     '
760     NEXT J
770  '
780  PRINT
790  RETURN
800  '------------------------------------------------------
810  'OUTPUT
820  '
830  PRINT
840  INPUT "PRINTER (Y/N)? ",ANS$
850  PRINT
860  IF ANS$ = "Y" OR ANS$ = "y" THEN OPEN "LPT1:" FOR OUTPUT AS #1
870  IF ANS$ <> "Y" AND ANS$ <> "y" THEN OPEN "SCRN:" FOR OUTPUT AS #1
880  '
890  PRINT #1,"    A1 = ";A1;
900  PRINT #1,":   A2 = ";A2;
910  PRINT #1,":   Q1 = ";Q1;
920  PRINT #1,":   Q2 = ";Q2
930  PRINT #1,
940  '
950 FOR J = 1 TO 21
960     '
970     FOR K = 1 TO 16
980        VP = V(J,K)
990        IF VP >= 10 THEN PRINT #1, " ****";
1000       IF VP >= 1 AND VP < 10 THEN PRINT #1, USING " #.##"; VP;
1010       IF VP >= 0 AND VP <  1 THEN PRINT #1, USING "  .##"; VP;
1020       IF VP > -1 AND VP <  0 THEN PRINT #1, USING " #.##"; VP;
1030       IF VP <= -1 THEN PRINT #1, " ****";
1040       NEXT K
1050    '
1060    PRINT #1,
1070    PRINT #1,
1080    NEXT J
1090 '
1100 CLOSE #1
1110 '
1120 RETURN
1130 '---------------- END OF PROGRAM LINES ----------------