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Unformatted text preview: a way that PE decreases
You hold a positively charged ball and walk due west in a region that contains an electric field directed due east.
FE E FH dr BB WH is the work done by the hand on the ball
WE is the work done by the electric field on the ball
Which of the following statements is true: 14 A) WH > 0 and WE > 0 B) WH > 0 and WE < 0
C) WH < 0 and WE < 0
D) W < 0 and W > 0 Not a conservative force.
Does not have any ∆ U. Conservative force: ∆ U = - WE FE FH E dr B) WH > 0 and WE < 0
Is ∆ U positive or negative?
positive or negative?
BB 16 B) Negative Two ways to tell:
1 ) ∆ U opposite in sign to W done by E field
2) If released, ball will want to move toward
lower potential energy, or in direction of
force (to the left)
force Example: Two Point Charges
Calculate the change in potential energy for two point charges originally very far apart moved to a separation of “d”.
Work done by q1 on q2 is NEGATIVE, so ∆ U is POSITIVE. Compute magnitude o f wo rk to g e t ∆ U.
d d q1q2
W = ∫ k 2 dr = k
∞ q1 q1q2
∆U = k
d q2 1 q1q2
4πε 0 d C h a rg e d p a rtic le s w/ s a m e s ig n h a ve a n inc re a s e in p o te ntia l e ne rg y wh e n b ro ug h t c lo s e r to g e th e r.
Fo r p o int c h a rg e s o fte n c h o o s e r=infinity a s “ze ro ” p o te ntia l e ne rg y .
2d In case A two negative charges which are equal in magnitude are separated by a distance d. In case B the same charges are separated by a distance 2d. Which configuration has the highest potential energy?
A) Case A
B) Case B 22 ACT Discussion
• As usu...
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- Spring '14