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Electrical forces

Electrical forces. Physics 100 Chapt 11. Ben Franklin. Matter is filled with a mysterious Fluid-like substance called “electricity .”. When an object has its “normal amount” of electricity , it is electrically “neutral” And doesn’t feel electrical forces.

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Electrical forces

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  1. Electrical forces Physics 100 Chapt 11

  2. Ben Franklin Matter is filled with a mysterious Fluid-like substance called “electricity.” When an object has its “normal amount” of electricity, it is electrically “neutral” And doesn’t feel electrical forces. When an object has more than its normal amount of electricity, it is “positively charged.” When it has less than its normal amount it is “negatively charged.” Charged objects feel electrical forces.

  3. Friction can move “electricity” from one object to another Franklin hypothesized that rubbing a comb through hair moved some “electricity” from the comb to the hair Comb has an “electricity” deficit & thus becomes negatively charged _ _ _ + Hair has a surplus & becomes positively charged + _ + +

  4. Opposite charges attract

  5. Like-sign charges repel _ _

  6. Modern picture of “electricity” Negatively charged electrons orbit around a positively charged nucleus _ _ + + + Electrical attraction between minus electrons & plus nucleus keeps electrons in orbit _ _ + + _ _

  7. Positive Ion An atom with one or more electrons removed _ _ + + + _ _ _ + + _ ”net” charge is positive

  8. “electricity” flow “electricity” doesn’t flow from comb to hair, some electrons from hair atoms get stuck on the comb Comb has an excess of of electrons & is thus negatively charged _ _ _ + The removal of each electron leaves a positive ions in the hair + _ + +

  9. Atomic electrons play the role of“fluid-like electricity” _ _ + + + _ _ + + _ _

  10. positive ion (fixed in crystal) + ElectricalConductors Conduction electron free to move around _ _ _ _ _ _ _ _ _ _ + + + + + + + + + _ _ _ _ _ _ _ _ _ _ + + + + + + + + + _ _ _ _ _ _ _ _ + + + + + + + + + _ _ _ _ _ _ _ _ _ + + + + + + + + + _ _ _ _ _ _ _ _ _ + + + + + + + + +

  11. Charging by induction

  12. Charging by induction (2)

  13. Electroscope

  14. Electrical Force Charles Coulomb 1736-1806

  15. Coulomb’s Law The electrical force between two charged objects is proportional to the charge of each object and inversely proportional to the square of the distance between them.

  16. Coulomb’s law q2 q1 r Fc q1 proportional to the charge of each object Fc q2 Proportionality constant: Inversely proportional to the square of the distance between them 1 r2 Fc “Coulomb’s Constant” q1q2 r2 q1q2 r2 combine: Fc Fc= k

  17. Units & Coulomb’s constant Unit of charge = “Coulomb”(C) q1q2 r2 Fc= k Unit of distance = m k = 9.0x109Nm2/C2 This is a big number

  18. Hydrogen atom electron qe = -1.6x10 –19 C qp = +1.6x10 –19 C qeqp r2 Fc= k 5x10–11 m (1.6x10-19C)2 (5x10-11m)2 =9x109Nm2/C2 proton =9.2x10-8N

  19. Gravitational force in an atom electron me = 9.1x10 –31 kg 5x10–11 m mp = 1.7x10 –27 kg memp r2 proton FN= G 9.1x10-31kgx1.7x10-27kg (5x10-11m)2 =6.7x10-11Nm2/kg2 =4.1x10-47N

  20. Electrical vs gravitation at atomic scales FC=9.2x10-8N _ FN=4.1x10-47N 5x10–11 m FC9.2x10-8N FN 4.1x10-47N + = = 2.2x10+39 Electrical force is 1039x the gravitational force!!

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