68 Electric Potential*P25.57The plates create uniform electric field to the right in the picture, with magnitude VVdVd0002−−=bg.Assume the ball swings a small distance xto the right. It moves to a place where the voltage createdby the plates is lower by −=−ExVdx20. Its ground connection maintains it at V=0 by allowingcharge qto flow from ground onto the ball, where −+==202VxdkqRqVxRkdee. Then the ballfeels electric force FqEe==4022to the right. For equilibrium this must be balanced by thehorizontal component of string tension according to TmgTecossinθθ4022tanθ4022kdmgxLefor small x. Then VRLe02124=FHGIKJ.If V0is less than this value, the only equilibrium position of the ball is hanging straight down. If V0exceeds this value the ball will swing over to one plate or the other.P25.58(a)Take the origin at the point where we will find the potential. One ring, of width dx, hascharge Qdxhand, according to Example 25.5, creates potential
This is the end of the preview. Sign up
access the rest of the document.