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Electricity and Magnetism

Learn about charged particles, forces, charge distribution, and principles of superposition in Physics Lecture 3. Topics include calculating forces, charge densities, and applications in everyday scenarios.

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Electricity and Magnetism

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  1. Electricity and Magnetism Physics 208 Dr. Tatiana Erukhimova Lecture 3

  2. Quiz A charged particle with positive charge q1is fixed at the point x=a, y=b. d y q2 b q1 c x a What are the x and y components of the force on a particle with positive charge q2 which is fixed at the point x = c, y = d?

  3. A charged particle with positive charge q1is fixed at the point x=a, y=b. A second particle, positive charge q2, is fixed at x=0, y=b. b y q2 q1 a x What are the x and y components of the force on a particle with positive charge q4 which is fixed at the origin?

  4. The negative charge of electron has exactly the same magnitude as the positive charge of the proton. Neutral atom Positive ion Negative ion

  5. Charging of neutral objects By contact: a) b)

  6. How you can make a balloon stick to the wall?

  7. Principle of Superposition (revisited) The presence of other charges does not change the force exerted by point charges. One can obtain the total force by adding or superimposing the forces exerted by each particle separately. Suppose we have a number N of charges scattered in some region. We want to calculate the force that all of these charges exert on some test charge .

  8. How do we calculate the total force acting on the test charge ? We introduce the charge density or charge per unit volume

  9. Let be the unit vector pointing from th chunk to the test charge; let be the distance between chunk and test charge. The total force acting on the test charge is We chop the blob up into little chunks of volume ; each chunk contains charge . Suppose there are N chunks, and we label each of them with some index . This is approximation!

  10. The approximation becomes exact if we let the number of chunks go to infinity and the volume of each chunk go to zero – the sum then becomes an integral: If the charge is smeared over a surface, then we integrate a surface charge density over the area of the surface A: If the charge is smeared over a line, then we integrate a line charge density over the area of the length:

  11. Problem 6 page 10 Suppose a charge were fixed at the origin and an amount of charge Q were uniformly distributed along the x-axis from x=a to x=a+L. What would be the force on the charge at the origin?

  12. Another example on force due to a uniform line charge A rod of length L has a total charge Q smeared uniformly over it. A test charge q is a distance a away from the rod’s midpoint. What is the force that the rod exerts on the test charge?

  13. Have a great day! Hw: All Chapter 2 problems and exercises Reading: Chapter 2

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