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. 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.

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

Opposite charges attract

Like-sign charges repel __

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

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

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

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

Electrical Conductors ____ _ _ __ _ _ __ _ _ __ __ __ ____ _ _ __ ____ _ _ __ _ _ _ _ __ _ _ + positive ion (fixed in crystal) _ Conduction electron free to move around _

Charging by induction

Charging by induction (2)

Electroscope

Electrical Force Charles Coulomb

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.

Coulomb’s law q1q1 q2q2 r F c  q 1 F c  q 2 Fc Fc  1r21r2 combine:Fc Fc  q1q2r2q1q2r2 F c = k q1q2r2q1q2r2 Proportionality constant: “Coulomb’s Constant” proportional to the charge of each object Inversely proportional to the square of the distance between them

Units & Coulomb’s constant F c = k q1q2r2q1q2r2 Unit of charge = “Coulomb”(C) Unit of distance = m k = 9.0 x 10 9 Nm 2 /C 2 This is a big number

Hydrogen atom q e = -1.6x10 –19 C electron proton q p = +1.6x10 –19 C F c = k qeqpr2qeqpr2 = 9x10 9 Nm 2 /C 2 (1.6x C) 2 (5x m) 2 = 9.2x10 -8 N 5x10 –11 m

Gravitational force in an atom m e = 9.1x10 –31 kg electron proton m p = 1.7x10 –27 kg F N = G mempr2mempr2 9.1x kgx1.7x kg (5x m) 2 = 4.1x N 5x10 –11 m = 6.7x Nm 2 /kg 2

Electrical vs gravitation at atomic scales + _ 5x10 –11 m F N = 4.1x N F C = 9.2x10 -8 N F C 9.2x10 -8 N F N 4.1x N = = 2.2x Electrical force is x the gravitational force!!