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Electric Field Strength

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EFS (by definition) The electric field strength, E, is the force, F elec, per unit charge, q, at a point. The equation for EFS is (arrows show the units for each symbol) E = F elec q N/C N C

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Electric Field Strength Steps to measure EFS, E, at some point. 1. Place small + test charge (q) at that point 2. Measure the force (F elec ) exerted on the test charge 3. Divide force by the size (amt.) of test charge

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Example Problems If the electric field strength due to some charged object is 1.5E3 N/C, find the force on a positive charge due to 2E-4 C placed in the field. Known: E = 1.5E3 N/C & q = 2E-4 C Unknown = F elec Equation: E = F elec / q (solve for F elec )

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Example Problems (cont.) F elec = (1.5E3 N/C) x (2E-4 C) = 0.30 N If the charge on the object in example #1 was doubled, does that change the electric field strength? Hint: what change would increasing q have on F elec ?

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Example Problem (cont.) Increasing q will increase F elec so ratio between the two is constant. Conclusion = size of test charge doesn’t effect “E”

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EFS Lines of Flux Directions & Strength of Electric Field (on board) Facts: 1. More flux lines indicates a stronger electric field 2. Lines of flux should always be drawn so they are perp. to surface of the charge 3. Lines of flux should never cross

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Coulomb’s Law Review Coulomb’s Law: the electric force is directly proportional to the product of their charges and inversely proportional to the square of the distance between their charges. F elec = K x q 1 x q 2 d 2

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Coulomb’s Law Review (cont.) Facts: 1. Increasing the amount of charge will increase the electric force 2. Increasing the distance between the charges will decrease the electric force 3. F elec is + if the 2 charges are alike indicating that they will repel each other 4. F elec is – if the 2 charges are different indicating that they will attract each other

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