5B10.40 - Projection of Electric Field Lines
A two-dimensional model is used for the projection of the field around point charges, parallel plates, etc.
ALWAYS clean all the oil off of the apparatus before it is stored! Do not attempt to store oil and grass seed in small quantities in the dish as the oil tends to polymerize, becoming useless and difficult to clean up. Alcohol may be used, but avoid using acetone on the plastic. Attempt to save as much of the oil without grass seeds in it before cleaning the apparatus.
Two electrodes are placed in a small dish which contains fine Kentucky blue grass seed suspended in oil. Place the tip of the Piezo charger on one electrode and your free hand on the second electrode (or make sure the grounded alligator clip is attached to the other electrode), then, squeeze the charger. The grass seeds align themselves along the field lines and are concentrated where the field is strongest. If you run this demonstration more than once, you must carefully randomize the grass seed in between each applied electric field. To do this, simply touch each electrode then carefully swish the oil around the bowl to allow the grass seeds to randomly suspend in the oil. Then repeat the experiment with different electrodes.
An electric field is a vector field that associates to each point in space the Coulomb force experienced by a unit electric charge. The direction of the field is taken to be the direction of the force it would exert on a positive test charge. Electric fields converge and diverge at electric charges. The electric field is radially outward from a positive charge and radially in towards a negative point charge. A charge q would be subject to a force F = q*E for electrostatics, or more generally F = q*E + (q*v x B). The field strenght decreases with distance and increases with the size of the charge producing the field. In the demo, the grass seed align with the electric field lines produced by the charged object, which is where a positive test charge would accelerate if placed upon the line. Direction of the electric field lines are not obvious in the image of these forms.