Electrostatic

1. Electrostatic Adapted from original slide at home.skhtst.edu.hk/~physics/resources/Electrostatics/Electrostatics.ppt

2. Static Electricity Events : • Clothes tumble in the dryer and cling together. • You walk across the carpeting to exit a room and receive a door knob shock. • You pull a wool sweater off at the end of the day and see sparks of electricity. • During the dryness of winter, you step out of your car and receive a car door shock as you try to close the door. • Sparks of electricity are seen as you pull a wool blanket off the sheets of your bed. • You stroke your cat's fur and observe it standing up on its end. • Bolts of lightning dash across the evening sky during a spring thunderstorm. • You have a bad hair day.

3. The structure of matter

4. The structure of matter The diameter of a nucleus is in the order 10-15 m. The diameter of an atom is in the order 10-10 m

5. All material objects are composed of atoms. • An atom consists of a nucleus and a vast region of space outside the nucleus. Electrons are present in the region of space outside the nucleus. They are negatively charged and weakly bound to the atom. Electrons are often removed from and added to an atom by normal everyday occurrences. • The nucleus of the atom contains positively-charged protons and neutral neutrons. These protons and neutrons are tightly bound in the nucleus and are not removable or disturbable by usual everyday methods. It would require some form of high-energy nuclear events to disturb the nucleus and subsequently dislodge its positively-charged protons. These high-energy events are fortunately not an everyday event. One sure truth is that the protons and neutrons will remain within the nucleus of the atom; electrostatic phenomenon can never be explained by the movement of protons.

6. Charged versus Uncharged Objects • Positively-Charged Object possesses more protons than electrons. Electrons have been removed from the originally neutral object. • Negatively-Charged Objectpossesses more electrons than protons. Electrons have been added to the originally neutral object. • Uncharged (Neutral) Objectpossesses equal numbers of protons and electrons

7. Interaction of Electric Charges Electric charges interact through electric force. Electric force is a kind of non-contact force (action-at-a-distance force) Contact forces come into action only when two objects are in contact with each other.

8. Unlike charges attract A B FB on A FA on B Like charges repel A B FB on A FA on B Like charges repel A B FB on A FA on B

9. Unlike charges attract A B FB on A FA on B Like charges repel A B FB on A FA on B FB on A and FA on B form an action reaction pair. According to Newton’s Third Law, the two forces are equal in magnitude, opposite in direction and acting on different objects. FB on A = FA on B = F

10. Interaction between charged and neutral objects They must attract each other. The electric forces between them form an action-reaction pair. B (neutral) A F F B (neutral) A F F This can be explained in terms of polarization.

11. A plastic ruler charged by rubbing with animal fur (or woolen cloth) will attract neutral paper bits which are insulators. A plastic ruler charged by rubbing with animal fur (or woolen cloth) will attract neutral tin foils which are conductors. This can be explained in terms of polarization.

12. If electric forces of repulsion exist between two objects, they must carry the same type of electric charges. If electric forces of attraction exist between two objects, there are two possibilities : • The two objects carry opposite electric charges. • One of the object is charged and the other is neutral.

13. Two balloons charged by rubbing with fur repel each other

14. Two balloons rubbed on human hair will become negatively charged and have an attraction interaction with the hair. If the hair is removed, the balloons repel.

15. When the generator is switched on, the cloth streamers share the same type of charges with the metal dome and repel each other. Cloth streamers lying on the metal dome of the Van de Graaff generator before it is switched on.

16. The man standing on insulated platform shares the same type of charges with the Van de Graaff generator.

17. (a) When the charged rod is held too far from the charged balloon, the electric repulsive force is less than the weight of the balloon. The balloon accelerates downward. (b) When the charged rod is held too close to the charged balloon, the electric repulsive force is stronger than the weight of the balloon. The balloon accelerates upward. (c) When the charged rod is not directly beneath the charged balloon, there is a sideways acceleration.

18. Conductors and Insulators SilverCopperAluminiumIronMercuryCarbonWaterGermaniumSiliconDry AirWoodGlassRubber Conductor Increasing conductivity Semi-conductor Insulator

20. Charged Conductors Electrons are free to move in conductors. Excess electrons placed on a conductor repel each other and try to stay as far apart as possible. To do this they have all to stay on the surface of the conductor.

21. Charged Conductors The quantity of charge per unit area on the surface of the conductor depends on the curvature of the surface. The surface charge density is higher at sharp points. Pear-shaped conductor There is a uniform surface charge density on a metal sphere.

22. Point Action Ionization of air particles Electric wind is set up and may blow a candle flame. The charged object with the sharp point on it will be neutralized in a short time. Point action can be maintained if the sharp point is mounted on the metal dome of a Van de Graaff generator.

23. Mounting a conductor (pop cans) on insulating platforms (styrofoam cups) prevents charge from escaping to the surroundings. It also makes for a convenient handle.

24. Water Water is conductor. Water vapour in air tends to remove excess electric charges from objects. If the humidity is high, it is very difficult to have large amount of electric charges accumulated on an object.

25. A video showing a Van de Graaff generator attracting a small neutral metal sphere http://www.hk-phy.org/iq/van_de_graaff/van_de_graaff_high.rm

26. Soap bubbles are attracted towards a Van de Graff generator and move away after touching the charged metal dome. http://faraday.physics.uiowa.edu/movies/MPEG/5a40.24.mpg

27. A plastic ruler charged by rubbing with animal fur (or woolen cloth) will attract neutral tin foils which are conductors. This can be explained in terms of polarization of conductors.

28. A charged balloon may be attracted to a neutral wooden board. The balloon will not be repelled on touching the wooden board. Why ?

29. Attraction of a water stream by a charged object …showing that water molecules are polar and can align themselves to be attracted to charged objects. …be deflected by the presence of a charged object…. A stream of water falling vertically can ….. Note that the entire water stream remains neutral.

30. A video showing water stream attracted by a charged rod http://faraday.physics.uiowa.edu/movies/MPEG/5a40.40.mpg

31. Charging a metal plate by induction

32. Electrostatic Hazards

33. Electric Sparks Electrons jump between A and B if the amount of charge on A is large enough or the gap between A and B is small enough. B A Earthed metal sphere Van de Graaff generator As the electrons jump across the gap, electrical potential energy is converted to light (sparks can be seen) and heat energy. Air is heat up. It expands and then contracts, resulting in sound waves.

34. Electric Sparks Sparks may lead to the explosion of inflammable gas. Electric charges may accumulate on the truck as it moves. When the amount of electric charges is large enough, sparks may form as electric charges jump between the truck and the earth.

35. Electric Sparks Sparks may lead to the explosion of inflammable gas. The electric charges on the truck can be earthed through the metal chain or through the tyres if they are made of conductive rubber.