Coulomb’s Law

• Coulomb’s Law• Electrostatic vs. Gravitational • Coulomb’s Law Examples• The Electric Field • Electric Field Examples

Coulomb’s Law• Fundamental property charge• Positive and negative charge• Units Coulombs (C)• Force Law

Coulomb’s Law animation

• Fundamental Electrostatic Force

https://sites.google.com/site/physicsflash/home/coulomb

You’d never get this much force from gravity!

Coulomb.swf

Compare with gravitation

• Fundamental property mass• Positive mass only• Units kilograms (kg)• Force Law

Electrostatic vs. Gravitational Electrostatic Gravitational

Basic Law

Similarities

a) force along line joining

a) product of charge (C)

b) inverse square law

a) force along line joining

a) product of masses (kg)

b) inverse square law

Differences

a) positive and negative charge

b) attractive and repulsive

c) strong (k=9 x 109 N-m2/C2)

a) positive mass only

b) attractive only

c) weak (G =6.67 x 10-11 N-m2/kg2)

Quantization Charge quantized 1.6 x 10-19 C Mass can be anything

Vector Superposition Example 16-4 (page 448) example 5-12 (page 120)

221

r

mGmF

221

r

qkqF

Coulomb’s - Example 16.1

• Fundamental charge

• Force

(check units)

Coulomb’s Example – 1D Vector Addition

• Example 16-3 - Calculate Net force on Q3 due to Q1 and Q2

• Force due to Q1

• Force due to Q2

Coulomb’s Example – 2D Vector Addition (1)

• Example 16-4 – Get magnitudes

Coulomb’s Example – 2D Vector Addition (2)• Example 16-4 – XY Table (did last semester)

Force X-component Y-component

F32 0 N + 330 N

F31 140 N cos30 = 120 N -140N sin30 = -70 N

Total 120 N 260 N

Coulomb’s Example – Problem 13 • Charge Triangle

“Field” concept - Gravity• Newton’s Universal Law of Gravitation

• One-step process (tedious)

• Two-step process (simple)

“Field” concept - Electrical

• Cellphone signal– Multiple users sharing same tower.– Why calculate each phone separately?

• 2 step process – 1. Calculate common “field”.– 2. Calculate each phone’s interaction

with that “field”.

• “Field” equals # of “bars” you have!

Electrostatic vs. Gravitational Field

• In gravitation we calculate

– Bracket part becomes “g”

• In electrostatic we calculate

– Bracket part becomes “E”

• Field line point away from (+), toward (-)• F=qE (+) moves with field, (-) moves against field

Electrostatic vs. Gravitational FieldElectric Field Gravitational Field

Field Concept

Can be anything Usually equal 9.8 m/s2

Force & fieldF=q2E

Force in field/opposite direction

F= m2g

Force in field direction

Field Definition Ratio of Force/chargeRatio of Force/mass,

(simplifies to acceleration)

StrengthElectrostatic so strong appears on

circuit-board scaleGravity so weak only appears on planetary

scale

Comment“E” quite interesting

Varies all over the place“g” usually boring

usually constant 9.8 m/s2

SuperpositionCan superimpose continuously

Wires, electrodes, circuit boards

Can superimpose discretely

Planets, etc

21

r

kqE

21

r

Gmg

Electric Field - Example 16-8 (1)

• Field from Q1

• Field from Q2

Electric Field - Example 16-8 (2)

• Force on proton at P

• Acceleration of proton at P

• Force on electron at P

• Acceleration of electron at P

Electric Field Animation

• 2-D animation

https://ef55311d-a-62cb3a1a-s-sites.googlegroups.com/site/physicsflash/Efield.swf?attachauth=ANoY7crCoduqPW1yrSvMtvv2qNVfA62NqyoNF1X8FY1ldipscty_-KXPmxmMyKYNdrruNd8vROoqOCxqee-i4LdS8Ct27vHfViZ597w8ETuqbnejUPkP7AiKqr4M-S3qn2VLzO2dQNl_KXv-Re0jQ5puGFnzKrPwZvuy0UkMwMU4QhXhJIIX4jWlWsQhQdgzqz-f17N3xQOXvEvfeWC1O-3ObHX89WV72w%3D%3D&attredirects=1

Efield.swf

Electric Field - Example 16-9 (1)• Get magnitudes

Electric Field - Example 16-9 (2)• XY table

Field X-component Y-componentE2 0 N/C +5 x 106 N/C

E1 +1.1 x 106 N/C -0.625 x 106 N/C

Total +1.1 x 106 N/C +4.375 x 106 N/C

Electric Field – Problem 16

• Charge square