Then rub them together. Measure the charge on each. Discharge the cup and the blue and white paddles. Observe electrometer reading after each step. Determine sign of effect.
Tribology is the study of friction.
Instead of frictional charging, use the Pasco power supply. Ground the black binding post and connect the green binding post to the hollow sphere with a copper wire. Do the same experiment but touch inside the sphere. Since it is not practical to remove a piece of the surface, we place the aluminum paddle flat against the surface and measure the charge on the paddle after it is removed. Repeat step 8 but with paddle perpendicular to the surface instead of flat.