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Electrostatics

Electrostatics . Electricity at rest. Electrostatics . Electric charges, forces between them and their behavior in different materials. The Atom. An Atom consists of protons and neutrons in the nucleus and charged electrons circling the nucleus. . The Atom.

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Electrostatics

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  1. Electrostatics Electricity at rest

  2. Electrostatics • Electric charges, forces between them and their behavior in different materials

  3. The Atom • An Atom consists of protons and neutrons in the nucleus and charged electrons circling the nucleus.

  4. The Atom • Charge – mutual attractions or repulsions between particles is attributed to charge

  5. The Atom4 Facts to keep in mind • Every Atom has positively charged nucleus surrounded by negatively charged electrons • All electrons are the same – same mass, same charge • The nucleus has protons, neutrons – all protons are identical, all neutrons are the same. A proton is nearly 2000x the mass of the electron but the positive charge of the proton is equal to the negative charge of the electron • Atoms have as many protons as electrons, so a neutral atom has a net charge of zero

  6. More facts about the Atom • Charged atom is called an ion. It has either lost or gained an electron • Positive ion has a positive charge • It has lost an electron • Negative ion has a negative charge • It has gained an electron • An object that has unequal numbers of electrons and protons is electrically charged.

  7. Fundamental rule of electricity • Fundamental rule at the base of all electrical phenomena is that like charges repel and opposite charges attract.

  8. All matter is made of atoms • Atoms in the outer shells move fairly freely around • How much energy is required to tear an electron away varies with individual substances

  9. Principle of Conservation of Charge • Electrons are neither created nor destroyed but are simple transferred from on material to another

  10. Coulomb’s Lawanother inverse-square relationship • States that 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. F = k q1q2 d2

  11. SI unit for charge is Coulomb (C) 1 C = 6.24 x 1018 • One Coulomb is the charge of 6.24 x 1018 electrons

  12. Electrical Proportionality Constant (k) k = 9 000 000 000 N= m2/C2 k = 9 x 109 N= m2/C2

  13. Similar equations F = k q1q2 F = G m1m2 d2 d2 Product of masses Product of charges attractive Attractive or Repulsive Large magnitude Small magnitude

  14. Electrical forces usually balance WHY? • Most objects have exactly the same number of protons and electrons • Atoms sometimes share electrons - bonding

  15. Think • How does gravitational force between particles compare to the electrical force? • Hydrogen proton has a mass of 1.7 x 10-27 kg • An electron has a mass of 9.1 x 10-31 kg • Average distance between them is 5.3 x 10-11 m • Both proton and electrons have an equal but opposite charge • 1.6 x 10-19 C Do the work

  16. So…. • Electrical force between the 2 particles is more than 1039 times greater than the gravitational force • So… gravitational force can be completely negated when dealing with electricity

  17. How do electrons move? • Electrons move more freely in some materials than others • Conductor – materials through which electrons move freely • Metals, good conductors of heat • Insulators – materials through which electrons do not move freely • Rubber, glass, poor conductors of heat • Electrons move easily in good conductors and poorly in good insulators

  18. Conductor or InsulatorWhat are the good for? • Substances can be put in a list from good conductivity to poor • The conductivity of metal is more than 1 million trillion times greater than glass • Why are electrical wires insulated?

  19. Semiconductors • Semiconductor – materials that can be made to behave sometimes as a conductor and sometimes as insulator • Atoms in a semiconductor hold their charge until given a small energy boost • Photovoltaic cell – solar panels convert solar power into electrical power

  20. Charging by Friction or Contact • Two ways electrical charge can be transferred • Friction – electrons are being transferred as one material rubs against another • Contact – electrons will transfer simply by two materials touching

  21. 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 • Induced – electric charge has been redistributed on an object because of the presence of a charged object nearby

  22. Charging by induction

  23. Benny Franklin • Thunderstorm is a good example of induction • Most lightening is electrical discharge between oppositely charged parts of clouds • We are most familiar with the electrical discharge between clouds and oppositely charged ground below

  24. Grounding • Grounding – allowing charges to move freely along a connection between a conductor and the ground

  25. Induction by grounding

  26. Charge polarization • Charge polarization can occur in insulators that are near a charged object • When a charged rod is brought near an insulated material the electrons can’t transfer so they move around • One side of the atom is induced to be slightly more negative or positive

  27. Electrically polarized • Electrically polarized – term applied to an atom or molecule in which the charges are aligned so that one side is slightly more positive or negative than the other

  28. Electrically polarized

  29. In summary • Objects are electrically charged in 3 ways • Friction • Contact • Induction • Polarization occurs with an insulated object

  30. Van de Graaff Generator

  31. Electric Fields and Potential Store house of energy

  32. Electric Fields • Storehouse of energy • Just like space around Earth and any other mass is filled with a gravitational field, every charge is filled with an electric field. • Electric Field - force field that surrounds an electric charge or group of charges. An electric field has both magnitude and direction

  33. An electric field has both magnitude and direction • The magnitude (strength) of an electric field can be measured by its effect on charges in that field • The direction of an electric field at any point by convection, is the direction of the electrical force on a small positive test charge placed at that point • So… if the charge that sets up the field is positive the field points away from the charge and vice versus

  34. Electric Field Lines • You can use electric field lines (lines of force) to represent an electric field. • Where the lines are farther apart, the field is weaker • For isolated charges – field lines extend to infinity • Two or more opposite charges, lines emanate from positive charge and land on negative.

  35. Field lines shown by vectors

  36. It is possible to see a similar effect of an electric field. A charged object placed in fine oil with tiny bits of thin thread will cause the thread to line up in the field. It might look like the picture below.

  37. When two opposite charges are placed near each other in fine oil with fine bits of thread, the situation looks like the picture below.

  38. The picture below shows two parallel metal plates that are oppositely charged. Tiny bits of thread are suspended in oil between the plates. Notice that the threads line up parallel to each other between the plates. The threads arrange themselves along curved lines near the edges.

  39. Electric Shielding • If the charge on a conductor is not moving, the electric field inside the conductor is exactly zero • Absence of a charge within a conductor happens when electrons “settle down” and stop moving. Charge is zero • Electrons arrange themselves to ensure zero charge with the material

  40. How to shield an electric field • Surround object with a conducting surface. The electrons will distribute themselves on the surface of conductor so all field contributions on inside cancel themselves out

  41. Electrical potential energy Waiting to happen

  42. Electrical Potential Energy – the energy a charge has due to its location in an electric field • If the charge is released, it will accelerate in a direction away from the field and the potential energy will turn into kinetic

  43. Volt • Electrical potential - electrical potential energy per charge Volt = electrical potential energy/charge • Volt - SI unit for electrical potential

  44. Joule and Coulomb • Remember – potential energy is measured in Joules, charge is measured in Coulomb So… 1 volt = 1 Joule/Coulomb • Voltage – measurement of volts

  45. Electrical Energy Storage • Capacitor - stores electrical energy • Simple capacitor consists of 2 closely spaced metal parallel plates, when connected to a battery, the plates become equally and oppositely charged. • In practicality, they are thin metal strips separated by paper • Capacitors store and hold charges until discharged

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