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?
Q1Q2
F k 2
r
1 Q1Q2
F
2
 4 r
How do particles Interact?
Charged Particles Emit “Fields” proportional to their charge
Strength of field drops of as 4r2. (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/r2.
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)
Ek
Q1 1 Q1

2
r
 4 r 2
• Electric Force on a charged object (#2)
F  EQ2
F
1 Q1Q2
 4 r 2
Equations go Both Ways
• Electric Field from charged object (#2)
Ek
Q2 1 Q2

r 2  4 r 2
• Electric Force on a charged object (#2)
F  EQ1
F
1 Q2Q1
 4 r 2
Gravity
• What is gravitational field at sea level?
• g = 9.8 m/s2
• Gravity Interesting in that Force is
proportional to mass
• Acceleration always proportional to mass
– F=ma
Generic Force Fields
• Electric : E = kQ/r2
• Gravitational: EG = GM/r2
• 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 Charges
Like 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 eField 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: EG = GME/rE2 = 9.8 m/s2 =“g”
• Gravitational force of any object near earth
surface is equal to FG=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  109 N
•
•
•
•
Elephant weighs 104 Newtons
Airplane ~ 106 Newtons
Skyscraper ~ 109 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 mC
3m
5 mC
+ 10 mC
4m
Find the Force
• Vector Practice
- 10 mC
3m
+ 10 mC
4m
5 mC
Find the Force
• Vector Practice
5 mC
3m
+ 10 mC
4m
- 10 mC
Paperwork
• Today
– Chapter 21
• Tuesday
– Quiz?
– Problem Solving, Skip Lab4
• Wed
– Review Exam
• Fri
– Exam 1 (Labs 1-3, Chapters 17-20)