# Carga electrica

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Physics 4B Lecture Notes

Introduction
Go over the roll and introductory handout.

Review of Physics 4A
Newton's Laws of Motion explain the concept of force. 1. Newton's First Law - The Law of Inertia describes the natural state of motion as constant velocity defines force as the thing that causes changes from this natural state 2. Newton's Second Law - ∑F=ma defines mass as the inertia ofan object quantifies the amount of force required to make a given change in motion 3. Newton's Third Law - The Law of Action/Reaction forces always occur in action/reaction pairs Conservation Laws 1. Conservation of Energy 2. Conservation of Linear Momentum 3. Conservation of Angular Momentum Study of the Gravitational Force 1. The Law of Universal Gravitation Fg = G m1 m2 r2

Preview of theRest of Physics
The Four Fundamental Forces Name of Force Strength Example Gravitational 1 Solar System Electromagnetic 1036 Hydrogen Atom Weak Nuclear 1025 Beta Decay Strong Nuclear 1038 Nuclear Stability The most common force in every day life is the electric force. All contact forces (friction, normal, tension, pushes pulls, etc.) are actually electrical in nature. These electric forces are thetopic for the next 15 weeks.

Chapter 22 - Electric Charge
Problem Set #1 - due: Ch 22 - 3, 5, 8, 10, 12, 20, 28, 29 The fundamental electric interaction is the force exerted and felt by electric charges. Lecture Outline 1. The Properties of Electric Charge 2. The Electric Force and Coulomb's Rule 1. The Properties of Electric Charge Pith Balls, plastic rod, fur Notice: 1) This force is strongerthan gravity. 2) Unlike gravity, there is attraction and repulsion. 3) Objects sometimes feel the force and sometimes don't. 22-1

Physics 4B Lecture Notes This force is called the "electric force." Things that feel (and exert) this force are said to be "charged." Recall that things that feel (and exert) the gravitational force have mass and there is only attraction. There must be two types ofcharge to account for both attraction and repulsion. They are called "positive" and "negative." From the pith ball demonstration it can be seen that like charges repel and opposite charges attract. The Units of Charge: The "fundamental" unit of charge in the universe is the electron's charge, e. The S.I. unit of charge is the "Coulomb." 1 Coulomb = 1C and 1e = 1.602 x 10-19C (Note that e is apositive number, but the charge of an electron is defined to be negative.) Charge comes in integral multiples of the charge on the electron. The charge on any object is a multiple of the charge on the electron (q=Ne). Charge is said to be “quantized.” Summary of the Properties of Electric Charge: 1)Charge is always conserved (Law of Conservation of Charge). 2)There are two types called "positive" and"negative." 3)Charge is quantized (q=Ne). Discuss these properties in terms of the atomic nature of matter. Discuss these properties in terms of bulk properties of conductors and insulators. 2. The Electric Force and Coulomb's Rule Gravitational Force: m
1

m F r F

2

12

21

Newton's Third Law requires that F12 = F 21 . Therefor, m 1 and m2 must show up in a symmetric way (eg. m1 m2or m1 + m2 ). This led us to the Law of Universal Gravitation, r mm Fg = G 1 2 2 ˆ r r Electrical Force: q F
1

q r

2

12

F

21

Again, Newton's Third Law requires that F12 = F 21 . Therefor, q1 qnd q2 must show up in a symmetric way (eg. q 1 q2 or q1 +q2 ). By analogy with the Law of Gravitation, F12 α q1 q2 . You might suspect that the force decreases with the distance, Fα 1 . rnExperiments indicated that (like gravity) n is almost exactly equal to two. In summary,

22-2

Physics 4B Lecture Notes r qq F e = k 122 ˆ r r Coulomb's Rule

where ˆ points from the charge that causes the force toward the charge that feels the force and r 2 2 1 k ≡ 4πε = 9.00x109 N ⋅ m where ε o = 8.85x10−12 C 2 C2 o N⋅ m Example 1: Find the electric force that a proton exerts on an...