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Electrostatics

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A set of slides created to teach Electrostatics to students following the South African National Science Curriculum (NSC CAPS) in Cape Town.

A set of slides created to teach Electrostatics to students following the South African National Science Curriculum (NSC CAPS) in Cape Town.

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  • 1. Electrostatics K Warne
  • 2. THE NEUTRAL ATOM
    • The atom consists of a nucleus containing protons and neutrons surrounded by a cloud of electrons .
    • Number of protons = number of electrons in a neutral atom.
    - - - - - - - - + + + + + +
  • 3. Rubbing materials
    • Electron clouds from the atoms interact with each other.
    • Atoms from different substances usually attract electrons with different strengths.
    • Electrons from the atoms in one substance can get rubbed off onto the atoms of the other substance.
    • The substance receiving electrons becomes NEGATIVE the substance loosing electrons becomes POSITIVE .
    - - - - - - - - - + - + ELECTRON TRANSFERRED CHARGED ATOMS (IONS) FORMED - - - - - - - + - - - - - - - - + - - - - - - - - +
  • 4. Separating Charge
    • If a polythene rod is rubbed with cloth electrons are transferred to the rod .
    • The total number of negatives and positives are still equal.
    • Charges have simply been separated – charge has not been created.
    • The Law of Conservation of charge states that:
    • CHARGE CANNOT BE CREATED OR DESTROYED ONLY TRANSFERRED FROM ONE PLACE TO ANOTHER.
    + + + + + - - - - - (+5) + (-5) = 0
  • 5. A charged object experiences a force in the vicinity of a second charged object. We say there is an electric field present in the space around the charged object. An electric field is defined as a space in which a charged object will experience an electrostatic force . Electrostatics + + Force of repulsion Electric field
  • 6. Factors affecting force
    • Size of the charge: +1, +2 etc.
      • As the charge INCREASES the force INCREASES.
    • Distance between charges.
      • As the distance INCREASES the force DECREASES.
    d More charge = bigger force! Bigger distance = smaller force! + + Force of repulsion F 2+ 2+ F + + F
  • 7. Charges and Force
    • Like charges repel
    • Unlike charges attract.
    + + + - REPEL ATTRACT!!
  • 8. Polarization
    • An uncharged object is ATTRACTED by a charged object.
    • This is due to POLARISATION.
    • The charged object attracts opposite charges in the object.
    • This causes a CHARGE SEPARATION or POLARISATION in the object.
    • An electrostatic force is then established between the two objects.
    - - - - + + + + + + + + + +
  • 9. Polarization
    • An uncharged object is ATTRACTED by a charged object.
    • This is due to POLARISATION.
    • The charged object repels like charges in the object.
    • This causes a CHARGE SEPARATION or POLARISATION in the object.
    • An electrostatic force is then established between the two objects.
    - - - - + + + + - - - - - -
  • 10. The distribution of charge on a conductor depends on its shape. Electrostatics Electric charge is concentrated around a point. - - - - - - - - - - - - - - - - - - - - - - - - - - -
  • 11. 1 Coulomb of charge is an _______ charge. 1 electron has a charge of ___________ - this is called the _______________ charge. The number of electrons in 1 C is: Electrostatics
  • 12. 1 Coulomb of charge is an enormous charge. 1 electron has a charge of -1,6 x 10 -19 C - this is called the elementary charge. The number of electrons in 1 C is: Electrostatics = 6,25 x 10 18 electrons
  • 13. 1 Coulomb of charge is an enormous charge. 1 electron has a charge of 1,6 x 10 -19 C - this is called the elementary charge. The number of electrons required to make a charge C is: Electrostatics = No. of electrons in the charge
  • 14. When the collecting sphere of a large ____________ generator is charged sufficiently and the spark is 10 cm long, there is only ________ on the sphere. Typical everyday charges are of the order of __________ (_C)(____) or _____________ (_C) (____C). Electrostatics
  • 15. When the collecting sphere of a large Van der Graaff generator is charged sufficiently and the spark is 10 cm long, there is only 0,01 C on the sphere. Typical everyday charges are of the order of pico coulombs ( p C)( x 10 -12 C) or nano coulombs ( n C) (x10 -9 C). micro coulombs (  C) (x10 -6 C) Electrostatics
  • 16. Sharing Charge Charged Neutral (polarised) Each! Final charge on each sphere = ½(-3x10 -6 ) = -1.5 x 10 -6 C How many electrons were transferred onto the neutral sphere??? No. Of e - = charge/(-1.6x10 -19 ) = -1.5x10 -6 /(-1.6x10 -19 ) = 9.4x10 12 electrons! - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
  • 17. Electroscopes
    • Gold leaf electroscopes are used to detect the presence of electrostatic charge .
    • When the electroscope is charged the gold leaf is repelled by the shaft (LIKE CHARGES) and it stands out to the side.
    Metal disk Metal shaft Gold foil (leaf) Container with glass windows Simplified diagram UNCHARGED CHARGED ! + + + + INSULATION
  • 18. Charging Electroscopes
    • If a positively charged object is brought close to the electroscope plate - electrons would be attracted to the plate leaving the bottom of the shaft and gold leaf, positive .
    • The gold leaf would now repel the shaft.
    • THE POSITIVE CHARGES DO NOT MOVE!!!!
    + + + + + + + + + + + + - - - - - - + + + + + + + + - - - - - - + + + + Some of the electrons are now transferred to the rod. The electroscope has now acquired a positive charge. + + + + - - - + + + + + + +
  • 19. Charging Electroscopes
    • If a negatively charged object is brought close to the electroscope plate - electrons would be repelled by the plate making the bottom of the shaft and gold leaf, negative .
    • The gold leaf would now repel the shaft.
    • THE POSITIVE CHARGES DO NOT MOVE!!!!
    - - - - - - - - - ++++++ - - - - - - - - - ++++++ Some of the electrons are now transferred to the ELECTROSCOPE. The electroscope has now acquired a NEGATIVE charge. - - - - + + + - - - - - - - - - - - - - - - - - - - - -
  • 20. Milikan’s Experiment
    • Milikan used a microscope to observe the oil droplets between the plates.
    • These droplets are charged as they are forced out of the nozzle.
    • As the plate voltage increases some of the drops fall more and more slowly until the drops stop moving.
    • At this point the electric fore is equal to the weight of the oil droplet.
    • The electric force on the object is equal to the weight.
    • qV = mg
    • d
    • q=Droplet Charge V= Holding Voltage  d= Distance between plates  m=droplet mass g= Acceleration due to gravity.
    • From his experiments Milikan determined that the charge on an electron was 1.6×10-19 C.
    • By timing how long it takes for a droplet to fall with the plates switched off he could calculate the mass of the droplet.
  • 21. V d Graaf Generator
    • The brushes transfer electrons to and from the belt.
    • These electrons then accumulate on the metal dome.
    • The insulating stand keeps the charge from being transferred to the ground.
    brushes Insulating stand motor e - e - e - e - e - e - e - e - e - e -

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