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Understanding Electric Charge and Fields in Physics

Explore the fundamental properties of electric charge, its quantization, conservation laws, and interactions with forces. Learn about Coulomb's law, electric fields, and vector addition of forces in the context of physics concepts.

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Understanding Electric Charge and Fields in Physics

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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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