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PHYS117B: Lecture 2

PHYS117B: Lecture 2. 3 things to remember about the electric charge Electric charge is a fundamental property of matter Electric charge is quantized: the smallest unit is q e = 1.6 x 10 -19 C

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PHYS117B: Lecture 2

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  1. PHYS117B: Lecture 2 • 3 things to remember about the electric charge • Electric charge is a fundamental property of matter • Electric charge is quantized: the smallest unit is qe = 1.6 x 10-19 C • The net electric charge is conserved. Even if you create new particles converting energy to mass (like in the picture from RHIC) you always produce +/- particle pairs. Au+Au ~ thousand charged particles , but the NET charge is conserved J.Velkovska , PHYS117B

  2. Last lecture: • There are 2 and only 2 types of electric charge. We proved that by observing that neutral bodies do not interact with neutral bodies, while all other combinations (+/-,+/+,-/-,+/0,-/0) did. Note that this is NOT a trivial statement. Gravity has only 1 type of mass, the strong force that acts between the quarks and gluons inside the protons and the neutrons has color charges: there are red, blue, green and anti-red, anti-blue, anti-green (total of 6 color charges). • We learned how to charge an object ( by rubbing it we break some molecular bonds and sweep electrons from one body to the other) or by touching a neutral conductor with a charged conductor • We learned about polarization (in insulators) and induced charge( in conductors) J.Velkovska , PHYS117B

  3. Electric Force and Electric Field • Now it is time to get quantitative • How big is the force between two charges? • How does it compare to other forces in nature ? • What if we have more than 2 bodies (charges) interacting ? How does the force change ? • Principle of superposition • How does a charge “feel” another charge at a distance ? Introduce the electric field. • Calculate the electric field for different charge configurations ( we’ll start today and do more on Monday) J.Velkovska , PHYS117B

  4. Let’s start with the force: J.Velkovska , PHYS117B

  5. How big is the electric force compared to gravity ? • Calculate the ratio of: Fc/Fg for 2 protons • Calculate the ratio of Fc/Fg for 1 proton and the Earth • Calculate Fc/Fg for 2 objects like the Earth • Done on the blackboard . See also example 21.1 in the book J.Velkovska , PHYS117B

  6. The electric force is a vector • It has magnitude and direction (have to consider both) • When more than 1 force act simultaneously on 1 body, the forces add following the rules for vector addition • There are applets on MP : you can play with an applet to get a feeling of how the electric force depends on distance, charge and how vectors add together. J.Velkovska , PHYS117B

  7. Electric field • Every charge creates electric field in every point in space. This is a VECTOR field J.Velkovska , PHYS117B

  8. J.Velkovska , PHYS117B

  9. Field of an electric dipole: Use Coulomb’s law and the principle of superposition to find the electric field in points a, b and c. J.Velkovska , PHYS117B

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