Today Questions re: HW Electrostatic Force/Coulomb's Law Examples

Transcription

1 Today Questions re: HW Electrostatic Force/Coulomb's Law Examples Honors Physics 1

2 Coulomb's Law: For two charges Q and q, separated by a distance R, there exists a mutual Electrostatic Force, F E, whose magnitude is F E = kqq where k is known as the Electrostatic Constant; in our system of units, the value of k = 9 E9 Nm 2 /C 2. The direction of this force depends on the signs of the charges: if the charges have the same polarity, the force is REPULSIVE, and F E will have a "positive" value; if the charges have opposite polarity, the force is ATTRACTIVE, and F E will have a "negative" value. R 2 Honors Physics 2

3 Example 1: Imagine 2 identical +1.0 C charges are placed 1.0 m apart from each other. a) How many excess protons would each charge have? Does it seem reasonable/possible to have that much net charge? b) What would be the strength of the Electrostatic force between these charges? Is it attractive, or repulsive? (To put this force in perspective, a typical car weighs ~2 tons. The force above would be equivalent to the weight of approximately 1 million cars.) Honors Physics 3

4 Example 2: R The Hydrogen atom is the simplest in structure on the periodic table. In a hydrogen atom, the nucleus is a single proton (q = 1.6 E-19 C), and it is orbited by a single electron (Q = -1.6 E-19 C), at an average distance of 5.3 E-11 m. What is the Electrostatic Force between them? What is meant by the (-) sign? Example 3: The mass of a proton is m p = 1.67 E-27 kg, and the mass of the electron is m e = 9.11 E-31 kg. What is the strength of the gravitational force between them? [F G = Gm p m e /R 2 ] Honors Physics 4

5 Examples 4-8: Two charges Q and q are separated by a distance R. In this set-up, they experience a force F E. 4) Without changing the charges, they are moved apart to a distance (3R). What happens to the Electrostatic Force? [Does it get weaker/stronger/ change direction/no change?] Describe how the "new" force compares to the original. 5) Without changing the charges, they are moved to a distance (¼R) apart. What happens to the Electrostatic Force? [Does it get weaker/stronger/ change direction/no change?] Describe how the "new" force compares to the original. 6) The charges are returned to the same distance R apart. The charge Q is increased in size to (3Q). What happens to the Electrostatic Force? [Does it get weaker/stronger/change direction/no change?] Describe how the "new" force compares to the original. 7) The charges are returned to the same distance R apart. The charge Q is increased in size to (3Q) and the charge q is increased in size to (3q). What happens to the Electrostatic Force? [Does it get weaker/stronger/change direction/no change?] Describe how the "new" force compares to the original. 8) The charge Q is increased to (3Q) and the charge q is increased to (3q). The charges are moved apart so that they are separated by a distance (3R). What happens to the Electrostatic Force? [Does it get weaker/stronger/ change direction/no change?] Describe how the "new" force compares to the original. Honors Physics 5

6 Honors Physics 6

7 Coulomb's Law: [More than two charges] In a case where there are more than 2 charges, you can find the "Net" Electrostatic Force on a single charge by determining the Electrostatic Force due to each of the other charges, and then adding them as vectors. I.e., the net F E on charge q 1 due to charges q 2 and q 3 would be equal to ΣF E1 = F E12 + F E13 *Note: we would have to add these forces as Vectors... In a 1D case, consider if the forces are +/- In a 2D case, consider forces in x- and y-directions Honors Physics 7

8 Example: In the diagram below, q 1 = +3 E-10 C, q 2 = +4 E-10 C, and q 3 = +3 E-10 C. 2 m 1 m q 1 q 2 q 3 What is the ΣF E on q 1? What is the ΣF E on q 2? What is the ΣF E on q 3? Honors Physics 8

9 Today Questions re: HW More Coulomb's Law Coulomb's Law Questions um... Van de Graaff? Honors Physics 9

10 Example: Assume each Q = 2 E-9 C, and R = 0.30 m. Q R Q R Q What is the net Electrostatic Force (magnitude and direction) on the "Green" Q? Honors Physics 10