Transcript Physics 1161 Lecture 2 Electric Fields
Physics 1161 Lecture 2 Vectors & Electric Fields
Three Charges • Calculate force on +2 m C charge due to other two charges –
F
1,3 –
F
1,3 Calculate force from +7 m C charge
k
C
C
6 6 3
m
4
m
2 2 Calculate force from –3.5
m C charge
k
3.5 10 6
C
2 2 6
C
3
N
3
N
Q=+2.0
m C 3
N
– Add (VECTORS!) Q=+7.0
m C 6 m 3
N
Q=-3.5 m C
Three Charges •
F
1,3
x
Resolve each force into x and y components 3
N
cos53
o
3
N F
1,3
y
F
2,3
x
3
N
sin 53
o
3
N
cos307
o
3
N
3
N
3
N
sin 307
o
3
N F
2,3
y
3
N
•
F x
Add the x-components & the y-comp.
3
N
3
N
3
N
Q=+2.0
m C 3
N
53 o 53 o
F y
• 3
N
3
N
2.03 10 3
N
Use Pyth. Theorem & Trigonometry to express in R,θ notation Q=+7.0
m C 6 m Q=-3.5 m C
Three Charges • Use Pyth. Theorem & Trigonometry to express in R,θ notation
F
3
N
3
N
2 3
N F
φ
F
4.9 10 3
N
3
N
arctan
F y F x
arctan 3
N
4.54 10 3
N
24
o
Since resultant is in first quadrant, θ = φ 24
o
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 +
F
r = 1x10 -10 m
F F
kq q
1 2
r
2 9 2
C
2 19
C
10 10
m
2 8
N
19
C
Toward the left 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
Electric Field • • Charged particles create electric fields. – – Direction is the same as for the force that a + charge would feel at that location.
Magnitude given by: E F/q Field at A due to proton?
E
E
E
kq
r
2 9 2 10
C
2 10
m
2 11
N C
q=1.6x10
-19 C 19
C
+ r = 1x10 -10 m Toward the right A
What is the direction of the electric field at point A, if the two positive charges have equal magnitude?
1. Up 2. Down 3. Right 4. Left 5. Zero
A
y
0% 1 0% 2 0% 3 0% 4 0% 5 + + B
x
What is the direction of the electric field at point A, if the two positive charges have equal magnitude?
1. Up 2. Down 3. Right 4. Left 5. Zero
A
y
0% 1 0% 2 0% 3 0% 4 0% 5 + + B
x
Preflight 2.2
What is the direction of the electric field at point A?
2) Down 0% 3) Left 0% 4) Right 40% 5) Zero 30%
+ A
y
B
x
Preflight 2.3
What is the direction of the electric field at point B?
1) Left 70% 2) Right 30% 3) Zero
A
y
+ B
x
What is the direction of the electric field at point C?
1. Left 2. Right 3. zero
+
y
C
x
1 0% 0% 2 0% 3
Electric Field Applet
• http://www.cco.caltech.edu/~phys1/java/phys 1/EField/EField.html
Preflight 2.5
X A Y B Charge A is Field lines start on positive charge, end on negative .
1) positive 80% 2) negative 0% 3) unknown 20%
Preflight 2.6
X A Y B Compare the ratio of charges Q A / Q B # lines proportional to |Q| 1) Q A = 0.5Q
B 20% 2) Q A = Q B 30% 3) Q A = 2 Q 40% B
Preflight 2.8
X A Y B
The electric field is stronger when the lines are located closer to one another.
The magnitude of the electric field at point X is greater than at point Y 1) True 10% 2) False 90% Density of field lines gives E
Compare the magnitude of the electric field at point A and B
1. E A > E B 2. E A = E B 3. E A < E B B
1 2 3
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 18
• • 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 19
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 40% (2) Zero 10% (3) Positive 50%
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 21