Name: P. J. Ouseph Address: Physics Department University of Louisville Louisville, KY 40292

Phone: 502 852 0918, Fax: 502 852 0742


Lecture-Demonstration or Advanced Laboratory

Walking a charged pith ball perpendicular to an electric field


A system consisting of two aluminum plate electrodes is used to walk a charged pith ball in a direction perpendicular to the electric field. One of the plates is kept horizontal while the second one makes an angle of 20 with the horizontal. When an electric potential difference is applied between the plates the charge walks from the end with small plate separation to the other end.

Support required for the apparatus: none

Approximate size: 30 cm x 30 cm.

Does this apparatus require power: yes, 120 V ac

Will you be present to set up your apparatus: No.

Other support needed for the proper operation of this apparatus: one 100 W light source.

If an uncharged graphite coated pith ball is placed on the bottom plate of a horizontally kept parallel plate capacitor and potential difference (2.5 to 3 kV) is applied to the two plates the pith ball continuously move up and down. As the voltage is applied the pith ball acquires a charge similar in polarity to that of the bottom plate on which the ball is placed. The pith ball then moves up because of the repulsive force between the pith ball and the charge on the bottom plate and the attraction between the ball and the top plate with opposite charge. As son as the ball hits the top plate the ball acquires a charge from the top plate and moves down because the repulsive force between the ball and the top plate. This up and down motion continues as long as there is potential difference between the plates. Any motion the ball may have perpendicular to the direction of the electric field is minimal. However, the charged sphere can be made to move perpendicular to the electric field with a small change in the system consisting of the plates. The modification of the system involves keeping the top plate at angle of 2 degrees from the horizontal plane. The distance between the plates, therefore, increases from one end to the other end. In our set up the distance between the plates changes from 1 cm to 2.5 cm (Fig. 1). The graphite coated pith ball has a mass of 0.005 g and a radius of 0.5 cm. The aluminum electrodes are 25-cm long and 7-cm wide. The voltage applied to the plates is in the range of 2 to 3 kV. The plates are kept in a Plexiglas box to reduce the chance of electric shock to the viewers and operators. The electrical connection to the plates is made through BNC connectors fixed to the box. When the voltage of 2.5 kV or higher is applied to the plates the charged sphere moves up and down with a net displacement in the horizontal direction, and eventually the ball comes out of the electric field. The horizontal motion is from the end of the plates with small separation to the other end. As the ball moves horizontally time between collisions with the two plates increases because of the increasing distance between the plates and the decreasing electric field between the plates. The sound the pith ball makes during collisions can give us a qualitative idea about the time change. Quantitative results about the time between collision and charge on the ball can be obtained by connecting a resistance between the plate at lower potential and the ground and measuring the voltage signals across the resistance with an oscilloscope. The charge on the ball is found to be equal to the charge density on the plates multiplied by the area of the sphere.

Why should the ball move horizontally in the direction the field is decreasing is an interesting question. The motion of the charge can be explained by considering all forces acting on the ball. The calculation involved is somewhat complicated and, therefore, not presented here. In conclusion the equipment described above can be used to demonstrate charging, force on a charged mass in an electric field and motion of a charge in the direction of decreasing field.

Fig. 1