// ComPhys File: FieldLine.java // Chapter 9: Draw electric field lines in two dimensions import java.applet.*; import java.awt.*; import java.awt.event.*; import java.lang.*; import java.util.*; public class FieldLine extends Applet implements ActionListener { double L = 10; // box has length, width 2*L = 20 cm double a = 0.2; // radius of visual image of charges class Charge { Charge (double q, double x, double y) { this.q = q; this.x = x; this.y = y; } double q; double x; double y; } Vector chargeVector = new Vector(); boolean doAddCharge; double qtotal; int N; Graphics draw_lines_g; double draw_lines_scale; boolean doDrawLines; void draw_lines () { int lpc = 8; // lines per charge double Emin = 0.01; // if E < Emin stop drawing field lines int sign = 1; if (qtotal < 0) sign = -1; double ds_small = 0.01 * sign; double ds_big = sign; Enumeration i = chargeVector.elements(); while (i.hasMoreElements()) { Charge qi = (Charge) i.nextElement(); if (qi.q * sign > 0) { double dtheta = 2 * Math.PI / (lpc * Math.abs(qi.q)); for (double theta = 0; theta < 2 * Math.PI; theta += dtheta) { boolean stop_plot = false; double xline = qi.x + a * Math.cos(theta); double yline = qi.y + a * Math.sin(theta); double r = 0; do { double Ex = 0; double Ey = 0; Enumeration j = chargeVector.elements(); while (j.hasMoreElements()) { Charge qj = (Charge) j.nextElement(); double dx = xline - qj.x; double dy = yline - qj.y; r = Math.sqrt(dx * dx + dy * dy); if (r > 0.9 * a) { double E0 = qj.q / (r * r * r); Ex += E0 * dx; Ey += E0 * dy; } else { stop_plot = true; } } double E = Math.sqrt(Ex * Ex + Ey * Ey); if (E > Emin || r < 20) { xline += ds_small * Ex / E; yline += ds_small * Ey / E; } else { xline += ds_big * Ex / E; yline += ds_big * Ey / E; } if (xline >= 0 && xline < 2 * L && yline >= 0 && yline < 2 * L) { int ix = (int) (xline * draw_lines_scale); int iy = (int) (yline * draw_lines_scale); draw_lines_g.drawLine(ix, iy, ix, iy); } if (Math.abs(xline) > 100 * L || Math.abs(yline) > 100 * L) break; // line is probably headed to infinity } while (!stop_plot); } } } } boolean doClearPicture; class Picture extends Canvas implements MouseListener { int plotSize = 400; Color ivory = new Color(255, 255, 240); Picture () { setSize(plotSize, plotSize); setBackground(ivory); addMouseListener(this); } public void mouseClicked (MouseEvent me) { double scale = 2 * L / plotSize; if (doAddCharge) { double q = 0; try { q = new Double(chargeTextField.getText()).doubleValue(); } catch (NumberFormatException nfe) { } double x = me.getX() * scale; double y = me.getY() * scale; chargeVector.addElement(new Charge(q, x, y)); qtotal += q; ++N; NLabel.setText("N = " + N); totalChargeLabel.setText("Total Q = " + qtotal); repaint(); } } public void mouseEntered (MouseEvent me) { } public void mouseExited (MouseEvent me) { } public void mousePressed (MouseEvent me) { } public void mouseReleased (MouseEvent me) { } public void paint (Graphics g) { update(g); } public void update (Graphics g) { double scale = plotSize / (2 * L); if (doClearPicture) { g.clearRect(0, 0, plotSize, plotSize); doClearPicture = false; } // draw 1 cm grid g.setColor(Color.lightGray); for (int ix = (int) scale; ix <= plotSize; ix += (int) scale) g.drawLine(ix, 0, ix, plotSize); for (int iy = (int) scale; iy <= plotSize; iy += (int) scale) g.drawLine(0, iy, plotSize, iy); // draw charges int r = (int) (a * scale); Enumeration e = chargeVector.elements(); while (e.hasMoreElements()) { Charge q = (Charge) e.nextElement(); if (q.q != 0) { int ix = (int) Math.round(q.x * scale); int iy = (int) Math.round(q.y * scale); if (q.q > 0) g.setColor(Color.blue); else g.setColor(Color.red); g.fillOval(ix - r, iy - r, 2 * r, 2 * r); } } if (doDrawLines) { g.setColor(Color.black); draw_lines_g = g; draw_lines_scale = scale; draw_lines(); doDrawLines = false; } } } Picture picture; Label NLabel; Label totalChargeLabel; TextField chargeTextField; Button addChargeButton; Button drawLinesButton; Button resetButton; public void init () { add(picture = new Picture()); Panel p = new Panel(); p.setLayout(new GridLayout(3, 2)); p.add(NLabel = new Label("N = 0")); p.add(totalChargeLabel = new Label("Total Q = 0")); p.add(addChargeButton = new Button("Add charge q =")); addChargeButton.addActionListener(this); p.add(chargeTextField = new TextField("+1")); chargeTextField.addActionListener(this); p.add(drawLinesButton = new Button("Plot Field Lines")); drawLinesButton.addActionListener(this); p.add(resetButton = new Button("Reset")); resetButton.addActionListener(this); add(p); } public void actionPerformed (ActionEvent event) { if (event.getSource() == addChargeButton) { doDrawLines = false; doAddCharge = true; } else if (event.getSource() == chargeTextField) { doDrawLines = false; doAddCharge = true; } else if (event.getSource() == drawLinesButton) { doAddCharge = false; doDrawLines = true; picture.repaint(); } else if (event.getSource() == resetButton) { doAddCharge = doDrawLines = false; chargeVector.removeAllElements(); N = 0; NLabel.setText("N = " + N); qtotal = 0; totalChargeLabel.setText("Total Q = " + qtotal); doClearPicture = true; picture.repaint(); } } public static void main (String[] args) { FieldLine fieldLine = new FieldLine(); Frame aFrame = new Frame("Electric Field Lines"); // anonymous inner class to destroy window aFrame.addWindowListener(new WindowAdapter () { public void windowClosing(WindowEvent e) { System.exit(0); } }); aFrame.add(fieldLine); fieldLine.init(); aFrame.setSize(700, 450); aFrame.setLocation(50, 50); aFrame.setVisible(true); } }