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5B10.40 - Projection of Electric Field Lines

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​5B10.40 - Projection of Electric Field Lines

Title5B10.40 - Projection of Electric Field Lines

A two-dimensional model is used for the projection of the field around point charges, parallel plates, etc.

Assembly Instructions
  1. Pull out an aluminum grounding plate, a piezoelectric squeezer, and a grounding plug. 
  2. If this demo is being used in a non-lecture hall classroom, pull down the demo lab overhead with on/off switch (lecture halls are equipt with doc cams that also are back lit, so skip this step for those classrooms). 
  3. Pull out the special glass dish that is located with the demonstration on the shelf and make sure it is clean. 
  4. Fill the dish with approximately 1/4" of oil.  (can use canola, vegetable, mazola, etc)
  5. Sprinkle a small amount of Kentucky blue grass seed in the oil, a fraction of the amount necessary to cover the bottom of the dish.  (too much grass seed and you won't be able to see what is going on) **It helps to mix the oil and grass seed well before they are to be used so that the seeds soak up some of the oil, but no more than 24 hours ahead of time or the seeds become too heavily soaked and won't respond as well to the electric field.**
  6. Attach the necessary electrodes to the glass dish.  If using the point charges, it is helpful to use an outer grounding ring for a more obvious effect.  Provide an alligator clip attached to a banana cable that runs to the grounding plate as  your connection to the outer grounding ring to ensure this ring is indeed grounded.
  7. Test that the apparatus is working.  Inbetween each charging the grass seed has to be manually randomized.  To do this, carefully swish the oil around in the dish to mix it up while touching each electrode to make sure it is grounded. 
    **Be sure to tell the instructor to do the same thing during classroom operation.**

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.


Setup Time10
Operation Time10
Preview Time10
Operation Instructions

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. 

Demo on DimeNo
PIRA 200Yes
Export Instructions (if different)

​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.

Category5 Electricity and Magnetism
Subcategory5B - Electric Fields and Potential
Keywordselectrostatics, static electricity, electric field, potential, image
Construction Information
A large, clear glass dish is needed to contain the demo.  Do not store oil and grass seed in glass dish.  To replace electrode attachments, an alligator clip must be flattened so as to not scratch the glass dish.
jar of grass seed - Kentucky Blue Grass
oil - vegetable
projection chamber (Plexiglas)
piezo charger
grounding plate with grounding plug
field forms - various shapes (point source, parallel planes, rings, etc.)
overhead projector