Physics 1161 Lecture 2
Electric Fields
Three Charges
• Calculate force on +2mC charge due to other two
charges
Q=+2.0mC
4m
– Calculate force from +7mC charge
– Calculate force from –3.5mC charge
– Add (VECTORS!)
Q=+7.0mC
Physics 1161: Lecture 2, Slide 2
6m
Q=-3.5 mC
Electric Force on Electron by Proton
• What are the magnitude and direction of the
force on the electron by the proton?
q=1.6x10-19 C
+
r = 1x10-10 m
Physics 1161: Lecture 2, Slide 3
e-
Comparison:
Electric Force vs. Electric Field
• Electric Force (F) - the actual force felt by a
charge at some location.
• Electric Field (E) - found for a location only –
tells what the electric force would be if a
charge were located there:
F = qE
• Both are vectors, with magnitude and
direction
Physics 1161: Lecture 2, Slide 4
Electric Field
• Charged particles create electric fields.
– Direction is the same as for the force that a + charge
would feel at that location.
E  F/q
– Magnitude given by:
• Field at A due to proton?
q=1.6x10-19 C
+
r = 1x10-10 m
Physics 1161: Lecture 2, Slide 5
A
What is the direction of the electric
field at point A, if the two positive
charges have equal magnitude?
1.
2.
3.
4.
5.
Up
Down
Right
Left
Zero
A y
+
+
B
x
Preflight 2.2
What is the direction of the electric field at
point A?
1) Up
2) Down
3) Left
4) Right
5) Zero
Physics 1161: Lecture 2, Slide 7
A
+
y
B
x
Preflight 2.3
What is the direction of the electric field at point B?
1) Left
2) Right
3) Zero
A
+
Physics 1161: Lecture 2, Slide 8
y
B
x
What is the direction of the electric
field at point C?
1. Left
2. Right
3. zero
y
+
C
-
x
Preflight 2.5
X
A
Y
B
Charge A is
1) positive
Physics 1161: Lecture 2, Slide 10
2) negative
3) unknown
Preflight 2.6
X
A
Y
B
Compare the ratio of charges QA/ QB
1) QA= 0.5QB
Physics 1161: Lecture 2, Slide 11
2) QA= QB
3) QA= 2 QB
Preflight 2.8
X
A
Y
B
The magnitude of the electric field at point X is greater than at point Y
1) True
Physics 1161: Lecture 2, Slide 12
2) False
Compare the magnitude of the
electric field at point A and B
1. EA> EB
2. EA= EB
3. EA< EB
B
A
E inside of conductor
• Conductor  electrons free to move
– Electrons feels electric force - will move until
they feel no more force (F=0)
– F=qE: if F=0 then E=0
• E=0 inside a conductor (Always!)
Physics 1161: Lecture 2, Slide 14
E inside of conductor
• Conductor  electrons free to move
– Electrons feel electric force - will move until
they feel no more force (F=0)
– F=qE: if F=0 then E=0
• E=0 inside a conductor (Always!)
Physics 1161: Lecture 2, Slide
15
Preflight 2.10
X
A
Y
B
"Charge A" is actually a small, charged metal ball (a conductor). The
magnitude of the electric field inside the ball is:
(1) Negative
Physics 1161: Lecture 2, Slide 16
(2) Zero
(3) Positive
Recap
• E Field has magnitude and direction:
– EF/q
– Calculate just like Coulomb’s law
– Careful when adding vectors
• Electric Field Lines
– Density gives strength (# proportional to charge.)
– Arrow gives direction (Start + end on -)
• Conductors
– Electrons free to move  E=0
Physics 1161: Lecture 2, Slide 17
To Do
• ActivPhysics 11.1 – 11.3
– On the MasteringPhysics website
• Read sections 19-6 & 19-7
• Do Prelecture 03 & Preflight 03.
See you next Wednesday!
Remember no class Fri or Mon!
Physics 1161: Lecture 2, Slide 18