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Unformatted text preview: Like charges repel Unlike charges attract each other Note this electrical force is for point charges only. With calculus we can figure out more complicated arrangement of charges based on this relationship. The SI unit for charge q is coulomb. 4 1 πε = k Superposition of Forces Experiment shows that when two charges exert forces simultaneously on a third charge, the total force acting on the third charge is the vector sum of the two forces. F 13 F 23 +q 1 q 3 +q 2 Σ F ei = m 3 a All materials are made of atoms that are in turn made of electrons , protons and neutrons. Electrons have negative charge and protons positive charge. Neutrons are neutral in charge. Electrons and protons are held together in the nucleus of the atom by the strong nuclear force. Mass of electron = m e = 9.11 x 1031 kg Mass of proton = m p = 1.672 x 1027 kg Mass of Neutron = m n = 1.674 x 1027 kg Electric charge of 1 coulomb = 1.67 x 1019 C Question : An alpha particle is the nucleus of a helium atom. It has mass m = 6.64 x1027 kg and charge q = +2e = 3.2 x 1019 C. Compare the force of electrical repulsion between alpha particles with the force of gravity between them 2 2 r q k F e = The magnitude of the repulsive electrical force is given by Coulombs Law: The magnitude of the gravitational force is given by Newton’s gravitational law: 2 2 r m G F g = ( 29 ( 29 35 2 27 2 19 2 2 11 2 2 9 2 2 10 1 . 3 10 64 . 6 10 2 . 3 / 10 67 . 6 / 10 . 9 4 1 x kg x C x kg m N x C m N x m q G F F G e = ⋅ ⋅ = = πε 35 10 1 . 3 x F F G e = Thus we see the gravitational force here is very negligible in comparison to the electrical force. Though this is always true for interactions of atomic and subatomic particles, in our everyday world charge is basically neutral and so the electrical force is much smaller than the gravitational force. Question Two point charges q 1 = 25nC and q 2 = 75nC are separated by a distance of 3.0cm. Find the magnitude and direction of the electric force q 1 exerts on q 2 ; Then find the electric force q 2 exerts on q 1 . Answer: ( 29 ( 29 ( 29 ( 29 2 9 19 2 2 9 2 2 1 12 030 . 10 75 10 25 / 10 . 9 4 1 m C x C x C m N x r q q F + ⋅ = = πε F 12 = 0.019 N Choosing a coordinate system with plus x to the right, since the two charges have opposite signs they attract; one moves in the positive direction the other in the negative direction. Newton’s 3 rd law applies as shown below. This is true even though each charge is different, that is each charge has a different magnitude. Newton’s third law also states that the direction of the force that q 2 exerts on q 1 is exactly opposite the direction of the force q 1 exerts on q 2 . Two point charges are located on the positive xaxis of a coordinate system....
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 Summer '09
 W.CHRISTENSEN
 Physics, Charge, Force, Electric charge, Fundamental physics concepts

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