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Electrostatics

Electrostatics. Electrostatics. Electrostatics is electricity at rest It involves electric charges, the forces between them, and their behavior in material An understanding of electricity takes a step-by-step approach

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Electrostatics

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  1. Electrostatics

  2. Electrostatics • Electrostatics is electricity at rest • It involves electric charges, the forces between them, and their behavior in material • An understanding of electricity takes a step-by-step approach • Electrostatics is the first step in understanding electricity as a whole

  3. Electrical Forces and Charges • You are familiar with the force of gravity • Now imagine a force that acts on you that is billions upon billions of times stronger • Electrical attracting force • This force is so strong it could compress you to the thickness of a sheet of paper • Luckily, there is another force acting on you, equally as strong • Electrical repelling force • These two forces balance so you do not notice their effects at all • These forces are electrical forces

  4. Electrical Forces and Charges • Electrical forces arise from particles in atoms • In atoms, the nucleus is made up partially of protons, which are positively charged • Around the nucleus is a cloud of electrons, which are negatively charged • The positively charged protons are attracted to the negatively charged electrons • This attracting and repelling behavior is attributed to a property called charge • Opposite charges attract; Like charges repel • The number of protons and the number of neutrons in each atom are equal, so there is a balance of charge

  5. Conservation of Charge • Matter is made of atoms, and atoms are made form protons and electrons (neutrons as well) • An object that has an equal number of protons and electrons has no net electric charge • If there is an imbalance in the numbers, however, an object is then electrically charged • An imbalance comes about by adding or removing electrons • The outermost electrons in atoms can easily be removed because they are farther from the positively charged nucleus • How easily electrons are removed varies with different substances

  6. Conservation of Charge • For example, electrons are held more firmly in rubber than in hair • If a rubber balloon is rubbed on human hair, electrons will be physically transferred from the hair to the balloon • The balloon then has an abundance of electrons and is negatively charged • The hair then has a deficiency of electrons and is positively charged • The negatively charged balloon will stick to a neutrally charged wall because of its difference in charge • Electrons are not created or destroyed, only transferred • There is always a conservation of charge

  7. Questions • Beneath the complexities of electrical phenomena, there lies a fundamental rule from which nearly all other effects stem. What is this fundamental rule? • How does a charge of an electron differ from the charge of a proton (magnitude and sign)? • If you scuff electrons onto your feet while walking across a rug, are you negatively or positively charged?

  8. Coulomb’s Law • The relationship between the electrical forces of two objects was discovered by the French physicist Charles Coulomb in the 18th century • Coulomb’s Law states that for charged particles, the force between the charges is directly proportional to the product of the charges, and inversely proportional to the square of the distance between them • Fe = k [(q1q2)/d2] • d is the distance between the charged particles • q1 is the quantity of charge of one particle • q2 is the quantity of charge of the second particle • k is the proportionality constant, 9.0 x 109 • TheSI unit of charge is the Coulomb (C)

  9. Example • What is the electrical force between the proton and the electron of a hydrogen atom? • The distance between the proton and the electron is 5.3 x 10-11 m • The charge of a proton is 1.6 x 10-19 C • The charge of an electron is -1.6 x 10-19 C • Fe = k [(q1q2)/d2] • Fe = -8.2 x 10-8 • It is NOT necessary to carry the negative sign throughout the equation • Just remember likes repel, opposites attract

  10. Conductors and Insulators • Electrons are more easily moved in some materials than in others • Materials with outer electrons that are loosely bound to the nucleus are good conductors of electricity • The electrons are free to move within the material • Metals • Materials with outer electrons that are closely bound to the nucleus are good insulators • Insulators do not conduct electricity well • Rubber, Glass • Semiconductorscan be made to behave sometimes as conductors and sometimes as insulators • I.E. computer chips that are silicon based • Superconductors are materials that have indefinite conductivity once electric current is established by having very low resistance • I.E. Power transmission lines, MRI machines, and Maglev Trains

  11. Charging by Friction and by Contact • Charging by friction occurs when electrons are transferred when one material rubs against another • Scuff socks along a rug in the dark and see and hear sparks • Charging by contact occurs when electrons are transferred from one material to another by simply touching • A charged metal rod comes in contact with a neutral object; some of the charge transfers

  12. Charging by Induction • If a charged object is brought near a conducting surface, even without physical contact, electrons will move in the conducting surface • If a negatively charged metal rod is brought near a can of pop, electrons in the can near the rod are repelled and a separation of charge is induced in the can • The can will roll toward the charged rod! • Why? • As long as the rod doesn’t touch the can, the rod retains its original charge • When we touch the conducting surface with a finger, the charges that repel each other have a conducting path to the ground • This is called grounding the conductor

  13. Charging by Induction • Charging by induction occurs during thunderstorms • The negatively charged bottoms of clouds induce a positive charge on the surface of Earth below • The kind we are most familiar with is the electrical discharge between the clouds and the oppositely charged ground • Most lightning is an electrical discharge between oppositely charged parts of clouds

  14. Lightning • As the negative charges collect at the bottom of the cloud it forces the negative charges in the ground to be forced away from the surface. This leaves the ground as positive. • A streamer of negative charges is repelled by the bottom of the cloud and is attracted by the ground. • As this streamer approaches the ground, a streamer of positive charges is repelled by the ground and attracted to the negative streamer. • When the two streamers connect, they have created a fairly conductive path which allows a sudden down surge of electrons to jump to the ground. This is the lightning. The rapidly moving electrons excite the air along the path so much that it emits light and causes the air to expand so rapidly it causes thunder.

  15. Charged Polarization • Charging by induction is not limited to conductors • When a charged rod is brought near an insulator, there are no free electrons to migrate throughout the insulating material • Instead, there is a rearrangement of the positions of charges within the atoms and molecules themselves • The atom or molecule is then said to be electrically polarized

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