Day 93, Monday, 1 February, 2016 Capacitance Electric Fields Electric Potential Coulombs Millikan's Oil Drop Experiment Capacitors Coulomb’s Law

Static Charge Silk on glass+ Fur on plastic -

Number of charges measured in coulombs (C) The smallest charge itself is called  e One coulomb of charge contains 6.2 x charges or 6.2 x e That is +6.2 x (protons) or -6.2 x (electrons) in a coulomb One proton contains x C of charge One electron ( e - ) contains x C of charge

Conductors (charges move) In elevators people go to the walls. Everyone moves to the farthest possible point, so they are all around the walls. In conductors electrons distribute themselves around the surface. Electrons are free to move and stay as far apart as possible

Insulators (charges isolated) Insulators are also called dielectrics because the charges cannot move, so the atoms stretch. This leads to a negative side and a positive side.

Air breakdown voltage 3 million volts Sparks at much lower voltages

Electric field Electric field relates force on a test charge to the size of the test charge E = F/q’units N/C

Electric field lines Direction perpendicular to field Spacing indicates strength Positive charge lines go out Negative charge lines go in

Electric Fields near conductors

Electric Potential Electric potential (also called the electrostatic potential) at a point in space is potential energy divided by charge that is associated with a static electric field. It is a scalar quantity, typically measured in volts.

Electric Potential - Voltage Electric Potential Difference ∆V = W on q´ /q´ Volt1 V = 1 J/C

Coulomb 1 coulomb is the amount of electrical charge in ×10 18 electrons or other elementary charged particles The charge of one electron is equal to × C

Coulomb A coulomb is equal to × elementary charges An electron contains x coulombs of negative charge x C A proton contains x coulombs of positive charge x C

Millikan oil drop experiment – the mass of electron

Storing charge Capacitors C = q/∆V coulomb / volt = farad Named after Michael Faraday 2 conducting plates parallel to each other

Capacitors Start Run

Of tape and Gecko’s feet

The balance of positive and negative charges Coin ≈1 million coulombs negative charge unbalanced charge large force

Insert comb trick here

Coulomb’s Law F = k c q A q B d 2 k c = 8.99 x 10 9 Nm 2 /C 2 (Coulomb’s constant) C = 6.25 x e - or p + (coulomb) 1/ 6.25 x = 1.60 x C/e -

Coulomb’s Constant and Boltzmann’s Constant are different entities Coulomb’s Constant k e or k c It is 8.99 x 10 9 Nm 2 /C 2 Boltzmann’s Constant k or k b It is (13)×10 −23 J K − (78)×10 −5 eV K − (13)×10 −16 erg K −1

Coulomb’s Law Problem One charge of 2.0 C is 1.5 m away from a –3.0 C charge. Determine the force they exert on each other.

Coulomb’s Law Problem worked out F e = k c q 1 q 2 r 2 F e = (8.99 x 10 9 ) (2.0)(−3.0) F e = 2.4 x N (attractive)

Coulomb’s Law Problem # 2 Two balloons are charged with an identical quantity and type of charge: nC. They are held apart at a separation distance of 61.7 cm. Determine the magnitude of the electrical force of repulsion between them.

Problem # 2 Worked Out Q 1 = nC = x C Q 2 = nC = x C d = 61.7 cm = m F elect = [k (Q1 Q2) ]/ d 2 F elect = [(9.0 x 10 9 )(6.25 x )(6.25 x )] / (0.617 ) 2 F elect = 9.23 x N

Credits Gecko Feet Comb Trick Photo science-notebook.com/.../comb-paper.jpg

Credits Capacitor diagram Capacitor photos (big green) Profesanxenxo.iespano.es Capacitor photos (little yellow)

Credits Coulomb’s Law numerical example # 1 df Coulomb’s Law numerical example # 2 tatics/U8L3b.cfm

Credits Electric field lines u8l4c.html u8l4c.html