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Midterm 1B Solutions

# Midterm 1B Solutions - PHYSICS 9C MIDTERM 1 LAST NAME FIRST...

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1 PHYSICS 9C MIDTERM 1 April 24, 2006 LAST NAME: FIRST NAME: SECTION: Problem 1 . (2 points) Consider two oppositely charged particles with Q 1 = -1 µ C and Q 2 = 2 µ C. Force between them is: A) Attractive; force on particle 1 is twice larger than force on particle 2. B) Attractive; force on particle 1 is equal to force on particle 2. C) Zero. D) Repulsive, force on particle 1 is twice smaller than force on particle 2. E) Repulsive; force on particle 1 is equal to force on particle 2. Solution: The force is attractive and the same because it depends on the product of two charges and the distance between them, Answer B. Problem 2 . (2 points) A positive charge Q= 2 mC is placed inside a spherical conducting shell with the inner radius a and the outer radius b which has an extra charge of 4 mC. Find the charges at the inner and outer surfaces of the shell. Solution. Taking a Gaussian surface inside the conductor Q in +Q=0 therefore Q in =-2mC Charge conservation condition of the conductor tells us that Q in +Q out = Q extra Q out =Q extra -Q in =4 mC+2 mC = 6 mC Problem 3 . (2 points) Find the electric flux through a cubic Gaussian surface of the side a 5 mm surrounding a charge of 3 mC.

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