HW-3Solutions-01-09-08 - Physics 21 Spring 2008 HW-3...

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Physics 21 Spring 2008 January 9, 2008 HW-3 Solutions 3-1 (HRW 22-1) In Fig. 22-30 the electric field lines on the left have twice the separation of those on the right. (a) If the magnitude of the field at A is 40 N/C, what is the magnitude of the force on a proton at A? (b) What is the magnitude of the field at B? Figure 22-30 Solution: (a) We note that the electric field points leftward at both points. Using r r F q E = 0 , and orienting our x axis rightward (so ˆ i points right in the figure), we find ( ) 19 18 N ˆ ˆ 1.6 10 C 40 i ( 6.4 10 N)i C F = + × = − × r which means the magnitude of the force on the proton is 6.4 × 10 –18 N and its direction ˆ ( i) is leftward. (b) As the discussion in §22-2 makes clear, the field strength is proportional to the crowdedness of the field lines. It is seen that the lines are twice as crowded at A than at B , so we conclude that E A = 2 E B . Thus, E B = 20 N/C. 3-2 (HRW 22-13) In Fig. 22-36 , the three particles are fixed in place and have charges and . Distance . What are the (a) magnitude and (b) direction of the net electric field at point P due to the particles?
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  • Spring '08
  • Kim
  • Force, Electric charge, HRW

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