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Coulomb’s Law

Coulomb’s Law. Physics 102 Professor Lee Carkner Lecture 10. Direction of Forces. but, opposites attract, like repel On a coordinate system +F is to +x or +y and –F is to –x or -y. Units of Charge. For most electrostatic problems use coulombs, meters and newtons 1 e = 1.60 X10 -19 C

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Coulomb’s Law

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  1. Coulomb’s Law Physics 102 Professor Lee Carkner Lecture 10

  2. Direction of Forces • but, opposites attract, like repel • On a coordinate system +F is to +x or +y and –F is to –x or -y

  3. Units of Charge • For most electrostatic problems use coulombs, meters and newtons 1 e = 1.60 X10-19 C • Charge is represented by the variable q

  4. Electric Force • The force between two charged objects is represented by Coulomb’s law: F = k q1 q2/r2 • Where: • q1 and q2 are the two charges (in coulombs)

  5. Inverse Square Law • The formula for the gravitational force is: • Both are inverse square laws • Distance is more important than charge or mass • Decreasing the distance will greatly strengthen a force, increasing the distance will greatly weaken the force

  6. Computing Electrical Force • Force is a vector quantity • Direction depends on charge • Repulsive if charges are same sign • equal and opposite

  7. Resultant Force • To find the net force from several forces add the force from each vectorially: • Put an x-y axis on the system • F1, F2, F3 …. • F1x, F1y … • Fx = F1x + F2x … • F2 = Fx2 + Fy2

  8. X and Y • Remember vector addition rules: • Components along axis computed from: • Total F: • Angle to x axis: F Fy = F sin q q q Fx = F cos q

  9. Today’s PAL • What is the magnitude and direction of the force on charge q1? q3 = 4 C 2 m q1 = -2 C q2 = 5 C 3 m

  10. Spherical Charge Distribution • Coulomb’s law applies only to point charges • Non-point charges exert complex forces

  11. The Electric Field • Electrical and gravitational forces act at a distance • The area near the charge is said to be occupied by an electric field • The test charge is small enough so that its field does not affect the main one

  12. Defining the Electric Field E = F/q0 E = k q/r2 • i.e., the electric field at any point a radial distance r away from a charge q • Field exists whether there is a charge near it or not

  13. Calculating the Electric Field • Need to find the magnitude and direction of the field from each charge and add vectorially

  14. Next Time • Read 16.8-16.10 • Homework Ch. 16: P 12, 14, 17, 26

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