Paperwork Today –Chapter 21 Tuesday –Quiz? –Lab4 Wed –Review Exam Fri –Exam 1 (Labs 1-3, Chapters 17-20)

Fields Fields are Sources of “Force” –a.k.a. Force Fields Examine Electrical Force between 2 charges Same Equation Implications?

How do particles Interact? Charged Particles Emit “Fields” proportional to their charge Strength of field drops of as 4  r 2. (Thoughts on why?) Other charged particles feel this field. “Feeling” = Force proportional to field and charge of object

Source vs. Object Magnitude of force easy to find. F = kqq/r 2. More care with regards to direction. + charges have fields that push out (as above) - charges have fields that draw in (reverse arrows)

Source vs. Object One object is source of field Field cannot act on its source This field interacts with every other charged object Induces a force on these objects

Field & Force Equations Electric Field from charged object (#1) Electric Force on a charged object (#2)

Equations go Both Ways Electric Field from charged object (#2) Electric Force on a charged object (#2)

Gravity What is gravitational field at sea level? g = 9.8 m/s 2 Gravity Interesting in that Force is proportional to mass Acceleration always proportional to mass –F=ma

Generic Force Fields Electric : E = kQ/r 2 Gravitational: E G = GM/r 2 Need a Constant –Depends on dimensions (CH. 22) Need a distance dependence –Also depends on dimensions Need a field Source –mass for G, charge for E

Superposition Know that forces add Net Force = Sum of all Forces –Vector Addition Forces arise from fields –All do, but some are hard to see (Tension) Fields add as well Fields have strength and direction –Sadly – fields also vectors

Various Field Interactoins Opposite ChargesLike Charges Examine Like Charges Field Lines Cancel Out Between them at some point Will this happen for unlike charges?

Various Field Interactoins Cases on far right involve “Infinite” planes Useful approximation for large objects Example – electron shooting of finger Finger looks “infinitely” large to e - Field Lines Interesting with planes - Dimensionality

Why use fields Once field is calculated, can be used over and over again to calculate force Example: Gravity Sea level: E G = GM E /r E 2 = 9.8 m/s 2 =“g” Gravitational force of any object near earth surface is equal to F G =mg

Question 1… Friday’s penny example –Small fraction of electrons transferred between two pennies can induce HUGE force –Two 1C charges held 1 m apart  10 9 N Elephant weighs 10 4 Newtons Airplane ~ 10 6 Newtons Skyscraper ~ 10 9 Newtons Force between 2 pennies ~ same as weight of skyscraper….

Question 1… Seems electrical force much larger than gravitational force. Why is gravity usually the main influence on ordinary objects? Why are things Neutral? Takes force equal to the weight of a skyscraper to transfer the electrons of the 2 pennies.

Find the Force Vector Practice + 10  C - 10  C 3 m 4 m 5  C

Find the Force Vector Practice + 10  C - 10  C 3 m 4 m 5  C

Find the Force Vector Practice + 10  C - 10  C 3 m 4 m 5  C

Paperwork Today –Chapter 21 Tuesday –Quiz? –Problem Solving, Skip Lab4 Wed –Review Exam Fri –Exam 1 (Labs 1-3, Chapters 17-20)