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em04_testsol

# em04_testsol - PHY138Y Electricity and Magnetism Quarterly...

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PHY138Y – Electricity and Magnetism – Quarterly Test Solutions February 8, 2005 Multiple Choice Questions 1. Two point charges, Q and q , are a distance r apart. The electric field is zero at a point P between the charges, on the line segment connecting them. Then (A) Q and q must have the same magnitude and sign; (B) Q and q must have the same sign, but may have different magnitudes; (C) Q and q must have equal magnitudes and opposite signs; (D) Q and q must have opposite signs, and may have different magnitudes; (E) the point P must be midway between Q and q . Remember the electric field points away from a positive charge and towards a negative charge. If the charges have opposite sign (choices C and D here) then some electric field lines will point directly from the positive charge to the negative charge in the region in between. If Q and q have the same sign and magnitude then there will be a zero electric field half way between the charges (choices A and E ) so these are possible solutions – but because there are two of them you should have realised you need to go deeper. In fact the need not have the same magnitude, nor does the point where
E = 0 be half way in between. As long as the charges have the same sign then there will be a point with
E = 0 somewhere along the line segment connecting the charges B is the right answer. 2. An electron is placed in a uniform electric field directed toward the right side of the page. What is the direction of the force that field exerts on the electron? (A) toward the right; (B) toward the left; (C) upwards; (D) downwards; (E) perpendicular to the plane of the page. Remember the electric field direction is defined as the direction a positive charge would be pushed. Thus the electron, that has a negative charge, will be pushed in the opposite direction: bf to the left.

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em04_testsol - PHY138Y Electricity and Magnetism Quarterly...

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