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# How will the test charge move immediately after being

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Unformatted text preview: Slide 9 Physics 212 Lecture 2, Slide ­ + 23 ­ + + ­ Physics 212 Lecture 2, Slide 10 Physics 212 Lecture 2, Slide Checkpoint 3 A positive test charge q is released from rest at a distance r away from a charge of +Q and a distance 2r away from a charge of +2Q. How will the test charge move immediately after being released? A. To the left B. To the right C. Stay still D. Other INTERESTING: statement is correct, but given in support of “to the left” !! “although the force is double for the right charge although for being 2Q, the force is also quartered for being 2r away. “ being “The distance is squared in the denominator. The For the +Q, that leaves 1/1. for the +2Q, that leaves 2/4=1/2. The +Q is therefore stronger in this instance.” in “the 2q is 2r away and the q is r away, so the the twos cancel out and the test charge will have the same amount of force from each side” the 12 Physics 212 Lecture 2, Slide 11 Physics 212 Lecture 2, Slide Example +q P d -q “Just some more examples would be cool” What is the direction of the electric field at point P, the unoccupied corner of the square? +q d (A) (B) Calculate E at point P. (E) Need to know d & q Qi ˆ E = ∑ k 2 ri ri i q Ex = k 2 − d 20 (D) (C) E = 0 Need to know d q Ey = k 2 − d ( π cos 2 4 2d ( π sin 4 q q 2d ) ) 2 Physics 212 Lecture 2, Slide 12 Physics 212 Lecture 2, Slide Continuous Charge Distributions “my head is spinning, what does the dQ mean?” Summation becomes an integral (be careful with vector nature) Qi ˆ E = ∑ k 2 ri ri i dq ˆ E = ∫k 2 r r WHAT DOES THIS MEAN ?? Integrate over all charges (dq) r is vector from dq to the point at which E is defined Linear Example: λ = Q/L dE pt for E r charges 25 dq = λdx dq Physics 212 Lecture 2, Slide 13 Physics 212 Lecture 2, Slide Charge Density “What exactly is charge density, how is...
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