Testing fox vbtool thing. Btw, look at the amazing Graphics: Electric Field demo

[kedaman]

Put it all in a form, run and enjoy:

Option Explicit
'KedamansChargesandElectricFieldDemo

' 1. mouse down where you want to have a chage
' 2. drag to set the size
' 3. hold shift down for positive chages

Const Grid! = 50 'Decrease this value (not below 15) for more more gridpoints ( _
	cool), increase to see the field lines (also cool) to 100 or 150
Const ShowCharges As Boolean = True
Const Escale! = 2000000 'Field Strength Scale Constant

Private Type P2S
	 X As Single
	 Y As Single
End Type
Private Type Charge
	 Mag As Single
	 Pos As P2S
End Type
Private Enum enmmode
	 None
	 DrawCharge
End Enum
Dim mode As enmmode, current As Charge, _
	charges() As Charge, chargescount As Long

Private Sub Draw()
On Error Resume Next
Dim x!, y!, n&, sumx!, sumy!, dx!, dy!, m!, q!
	 Cls
	 For Y = ScaleTop To ScaleTop + ScaleHeight Step Grid
		  For X = ScaleLeft To ScaleWidth Step Grid
				sumx = 0
				sumy = 0
				For N = 0 To chargescount - 1
					 With charges(N)
						  dx = .Pos.X - X
						  dy = .Pos.Y - Y
						  M = (dx * dx + dy * dy)
						  Q = Sqr(M)
						  If Q > 10 And M > 10 Then
								sumx = sumx + dx * .Mag / (M * Q) * Escale
								sumy = sumy + dy * .Mag / (M * Q) * Escale
						  End If
					 End With
				Next N
				M = sumx * sumx + sumy * sumy
				Q = Sqr(M)
				'if you want you could make a function that gradiently shifts between colors
				
				If Q Then Line (X, Y)-Step( _
	sumx / Q * 100, sumy / Q * 100), RGB((M / 1000) And 255, _
	(M / 100) And 255, (M / 10000) And 255)
				
				'this is another version, doesn't look cool though, red and logaritmic
				'If q Then Line (x, y)-Step(sumx / q * 100, sumy / q * 100), (Log(m) * 10)
		  
				'If m < 10000000 Then Line (x, y)-Step(sumx / 10, sumy / 10), RGB((Log(m) * 2) And 255, _
	(Log(m) * 4) And 255, (Log(m) * 6) And 255)
		  
		  
		  Next X
	 Next Y
	 If ShowCharges Then
		  For N = 0 To chargescount - 1
				With charges(N)
					 FillColor = QBColor(Sgn(.Mag) * 1.5 + 2.1)
					 Circle (.Pos.X, .Pos.Y), Abs(.Mag)
				End With
		  Next N
	 End If
End Sub

Private Sub Form_Load()
	 Caption = "Kedamans Electric field and chages demo: mouse down where you want to have a chage, drag to set the size, _
	hold shift down for positive chages"
	 FillStyle = vbDiagonalCross
	 WindowState = vbMaximized
	 'BackColor = 0
End Sub

Private Sub Form_MouseDown(Button As Integer, Shift As Integer, _
	X As Single, Y As Single)
	 current.Pos.X = X
	 current.Pos.Y = Y
	 mode = DrawCharge
End Sub

Private Sub Form_MouseMove(Button As Integer, Shift As Integer, _
	X As Single, Y As Single)
Dim dx!, dy!
	 If mode = DrawCharge Then
		  With current
				dx = .Pos.X - X
				dy = .Pos.Y - Y
				FillColor = QBColor(Shift * 3 + 1)
				Circle (.Pos.X, .Pos.Y), Sqr(dx * dx + dy * dy)
		  End With
	 End If
End Sub

Private Sub Form_MouseUp(Button As Integer, Shift As Integer, _
	X As Single, Y As Single)
Dim dx!, dy!
	 dx = current.Pos.X - X
	 dy = current.Pos.Y - Y
	 current.Mag = Sqr(dx * dx + dy * dy) * Sgn(Shift - 0.5)
	 mode = None
	 ReDim Preserve charges(chargescount)
	 charges(chargescount) = current
	 chargescount = chargescount + 1
	 Draw
End Sub

Private Sub Form_Resize()
	 Draw
End Sub


'Code improved by vBulletin Tool (Save as...)