Field lines are a way of drawing something invisible. They start on positive charge, end on negative charge, never cross, and crowd together where the field is strong. Here you place charges and conductors wherever you like and watch the lines redraw themselves around whatever you have built.
Eight objects to place: point charge, line charge, hollow sphere, solid sphere, sphere with a cavity, parallel-plate capacitor, concentric shells, and an infinite sheet. Click one to drop it in, then click it again to select it and set its sign and magnitude, from q up to 5q. Drag objects around and the lines follow. Ready-made setups under Presets: single charge, dipole, two like charges, unequal charges, concentric shells. Under Scene you can turn on equipotential surfaces, cut a sphere open to see inside, switch the camera, and clear everything.
The lines themselves. Watch what happens inside a hollow conductor — nothing, because the field there is zero. Inside a solid charged sphere the field grows steadily from the centre outward, and the lines show that. Put two opposite charges near each other and the lines swing from one to the other.
Charges are given as multiples of q, not in coulombs. The constant k is taken as 1. There are no numbers on screen — this sim is about shape, not size.
You should be able to sketch the field lines for a new arrangement before you place it, and check yourself.