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112topic1 - Copy - Copy - Copy - Copy - Copy - Copy - Copy - Copy - Copy - Copy

# 112topic1 - Copy - Copy - Copy - Copy - Copy - Copy - Copy - Copy - Copy - Copy

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SAMPLE PROBLEMS: 112- Topic 1: ELECTRIC FORCES & ELECTRIC FIELDS. 1) Charges of - 6 and + 4 nC are 3 m apart. Determine the force they exert on each other, and the electric field at a point midway between them. Solution: This is an electrostatics problem (stationary charges). Since the charges are of opposite sign, they will attract each other. The forces are indicated in the figure. We may immediately apply Coulomb's Law to determine the strength of these forces: q 2 + - q 1 o r F 1 on 2 F 2 on 1 F elec = k q 1 q 2 /r 2 = (9 x 10 9 )(6 x 10 -9 )(4 x 10 -9 )/3 2 = 24 x 10 -9 = 24 nN. In the second part of the problem we wish to determine the electric field at point 'P', halfway between the two charges. Each source charge produces its own field at 'P'. Since q 1 is negative, its field E 1 is directed toward q 1 , while the field E 2 is directed away from charge q 2 since it is positive. q 2 + - q 1 o 3 m E 1 E 2 The magnitudes of these fields can be immediately calculated: E 1 (at P) = k q 1 /(r 1 ) 2 = (9 x 10 9 )(6 x 10 -9 )/(1.5 2 ) = 24 N/C E 2 (at P) = k q 2 /(r 2 ) 2 = (9 x 10 9 )(4 x 10 -9 )/(1.5 2 ) = 16 N/C Electric fields are vector quantities and we must add as vectors. In this case the addition is simple since both fields point in the same direction. Thus: |E net | = |E 1 | + |E 2 | = 24 + 16 = 40 N/C. The direction of E net is, of course, toward the negative charge. 2). The strength of the electric field in the region between two parallel deflecting plates of a cathode-ray oscilloscope is 25 kN/C. Determine the force exerted on an electron passing between these plates. What acceleration will the electron experience in this region?

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