Today 2/21

1
Today 2/21

• E-Field and Coulombs Law 18.4-5
• Examples
• HW 2/21 2-D E Field Due Wednesday 2/26

2
Many Charge Example (Like HW)
E kQs /r2
Each Square is ro on a side, Q1 -4qo, Q2
2qo, Q3 -6qo (See how this simplifies the
math)
Q1
Find the E field at Q1s location.
Up, (away from Q2)
EQ2,x
Q2
EQ3,x
Down, (toward Q3)
Q3
FOthers,q1 q1Enet,x
Add vectors to get
Enet,x
Down at the x
3
Many Charge Example (Like HW)
E kQs /r2
Each Square is ro on a side, Q1 -4qo, Q2
2qo, Q3 -6qo (See how this simplifies the
math)
Q1
Find the E field at Q2s location.
Up, (toward Q1)
EQ1,x
Q2
EQ3,x
Down, (toward Q3)
Q3
FOthers,q2 q2Enet,x
Add vectors to get
Enet,x
Down at the x
4
Many Charge Example
E kQs /r2
What is the force on 4qo? (direction also)
qo
4qo
Find the field at 4qo due to the other
charges. These are the source charges.
ro
Eq kQs/r2
k(qo)/(ro)2
Eo
-2qo
-3qo
Direction?
Away from so right
E-2q kQs/r2
Eo
k2qo/2(ro)2
k(qo)/(ro)2
E-3q kQs/r2
k(3qo)/(ro)2
3Eo
3kqo/(ro)2
5
Many Charge Example
E kQs /r2
What is the force on 4qo? (direction also)
qo
4qo
Eq Eo
E-2q Eo
E-3q 3Eo
ro
Now add E-Field vectors
-2qo
Ex Eq,x E-2q,x
-3qo
Ex Eo - Eosin45
Ey E-2q,y E-3q,y
Ex 0.29Eo
Ey Eosin45 3Eo
? tan-1(0.29/3.7) 4.5
Ey 3.7Eo
Fon 4q (4qo)3.71Eo 14.84qoEo
3.71Eo
Fon 4q 14.84kqo2/ro2 same direction as Etotal