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Electric Force

Electric Force. IDS Physics: M Blachly. Fundamental Quantities. There are a few quantities that are fundamental to our universe. We cannot define them in terms of anything else: they just are. Examples Mass Time Length

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Electric Force

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  1. Electric Force IDS Physics: M Blachly

  2. Fundamental Quantities • There are a few quantities that are fundamental to our universe. We cannot define them in terms of anything else: they just are. • Examples • Mass • Time • Length • Velocity is not fundamental, because it can be defined in terms of length and time.

  3. Fundamental Quantities • We now reach a new fundamental quantity: Charge • Discovered by Michael Faraday in 1833 when he proposed that current must be the flow of discrete entities. • The electron is what we now identify as the carrier of a charge. The word comes from the Greek elektron, which means “Amber”. The Greeks discovered that amber rubbed with fur attracted light objects such as feathers.

  4. Charge • There are two types of charge: positive and negative. • When these charges occur in equal ratios, there is no net charge. • Opposite charges attract, like charges repel.

  5. Bohr Model of the Atom • The negative charge orbits around the positive charge. • This is a model: it is not how it actually is, but just a way to think about some of the important concepts

  6. Exploring Charge • We can explore charge using a Van der Graff generator, which removes electrons from the dome.

  7. Things to explore • Pith balls, charged balloons and charged hair

  8. More things to Explore • The Electroscope • Used to detect the presence of charge. • The leaves fly apart if the ball touches anything that contains excess positive or negative charge. • Charged balloons and streams of water.

  9. Measuring Charge • Coulombs became the standard unit of charge before we knew what the source of charge actually was. • 1 electron has a charge of 1.6 x 10-19 C • How many electrons does it take to make 1 Coulomb of charge?

  10. Electric Force • It was known that there was a force between positive and negative charges • Charles-Augustin de Coulomb quantified this relationship

  11. Coulombs Law • Coulombs Law tells us the magnitude of the force between any two charges.

  12. Example Calculation • Two balloons each have a charge of 300 nC and they are 30 cm apart. What is the force of repulsion between them?

  13. Another Example • Consider two charges, one of 3.0 µC at x=5 meter and the other of -5.0 µC at x=1 m. What is the magnitude and direction of the electrostatic force on the 3.0 µC charge?

  14. Superposition Principle • The total force on any charge is the sum of the forces from each of the other charges.

  15. Example • Find the force on the 2 nC charge due to the other charges.

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