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Electrostatics

Physics 3/25/10. Electrostatics. What does that word even mean?. Electrostatics = electricity at rest Electrostatics involves electric charges, the forces between them, and their behavior in materials Reminder: Protons (+) Electrons (-) Neutrons (no charge). Charges.

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Electrostatics

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  1. Physics 3/25/10 Electrostatics

  2. What does that word even mean? • Electrostatics = electricity at rest • Electrostatics involves electric charges, the forces between them, and their behavior in materials • Reminder: • Protons (+) • Electrons (-) • Neutrons (no charge)

  3. Charges • The fundamental rule at the base of all electrical phenomena is that like charges repel and opposite charges attract • An object that has unequal numbers of electrons and protons is electrically charged.

  4. Conservation of Charge • The principle that electrons are neither created nor destroyed but are simply transferred from one material to another is known as conservation of charge. When electrons are transferred from the fur to the rod, the rod becomes negatively charged.

  5. Coulomb’s Law • For charged particles or objects that are small compared with the distance between them, the force between the charges varies directly as the product of the charges and inversely as the square of the distance between them. • 1 C (coulomb) is the charge of 6.24 billion billion electrons • k = 9.00 * 109Nm2/C2

  6. q1 and q2 are measured in C • k = proportionality const = 9.00 * 109 Nm2/C2 • d = distance between the charges

  7. Example: • Consider two small charged objects, one with a charge of 5 C and the other of unknown charge. When they are separated by a distance of 1.2 m, each exerts a force of 2.8 N on the other. What is the charge of the second object? q2= 2.8 * (1.2)2 (9*109) * 5 F = k * q1 * q2 d2 2.8 =(9*109) * 5 * q2 (1.2)2

  8. Conductors and Insulators • Materials through which electric charge can flow are called conductors • Metals are good conductors • In insulators, electrons are tightly bound and remain with a particular atom (they are not free to wander about) • Rubber and glass are good insulators and poor conductors of electricity • Semiconductors are materials that can be made to behave sometimes as insulators and sometimes as conductors. • Atoms in a semiconductor hold their electrons until given small energy boosts.

  9. Charging by Friction and Contact • Electrons transferred by friction: • We can stroke a cat’s fur and hear the crackle of sparks that are produced • Comb our hair in front of a mirror in a dark room and see as well as hear the sparks of electricity • Scuff our shoes across a rug and feel the tingle as we reach for the doorknob • Electrons transferred by contact: • A charged rod is placed in contact with a neutral object, some charge will transfer to the neutral object

  10. Charging by Induction • Induction – charging of an object without direct contact • If a charged object is brought near a conducting surface, even without physical contact, electrons will move in the conducting surface

  11. Grounding • When we allow charges to move off (or onto) a conductor by touching it, it is common to say that we are grounding it • When we touch the metal surface with a finger, charges that repel each other have a conducting path to a practically infinite reservoir for electric charge—the ground.

  12. Charge Polarization • Charging by induction is not restricted to conductors • Charge polarization can occur in insulators that are near a charged object. One side of the atom or molecule is induced to be slightly more positive (or negative) than the opposite side, and the atom or molecule is said to be electrically polarized

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