Transcript Electric Field

Electric Field
Force at a Distance
 Electric charges act on each
other at a distance.
+
• Empty space between
charges
• Net force on a charge
-

kQ2
F-  F-  2
r+ +

kQ
F+  F+  2
r+ +
2



FE  F+ + F-
 Force has to act on another
charge.
• Specific charge for a specific
value of force
• Value needed for force
magnitude
Test Charge
 Introduce an arbitrary positive
q
+
-
q
test charge of magnitude q.
 The force can be calculated
from the charges using
Coulomb’s law.
 The test charge can be moved
to other points in space to find
the force.
Field Strength
 The electric field is the electric
q
force per unit test charge.
• Measured in N/C
+
-
q
 The field has a value at every

 FE
E
q
point in space.
• Independent of force value
• Parallel to force direction
Point Charge
 The field of a point charge is just like the vector force that would act at a
point near the charge.
• Magnitude comes from Coulomb’s law
kQ
E 2
r
Superposition
 Two point charges of +10. nC
are separated by 8.0 mm. Find
the electric field at a point 4.0
mm perpendicular to the
midpoint of the charges.

E2
q1
 The electric fields of point
charges add together.
• Vector addition
 The x-components cancel out.
 The y-components are equal.
E y1 + E y 2  2

E1
kq

sin
45
r2
r 2  x2 + y 2
q2
• k = 8.99 x 109 Nm2/C2
• q = 10 x 10-9 C
• x = y = 4.0 x 10-3 m
 Answer: E = 4.0 x 106 N/C
Screened Charge
 Two oppositely charged point
 Only the x components matter.
charges of 10. nC are
separated by 2.0 mm. Find the
electric field at a point 20.0 mm
from the midpoint along the
line of the charges on the
positive side.
- kq
E x1 
r + d 2
Ex 2 
•
•
•
•
q1 q2

E1

E2
+ kq
r - d 2
k = 8.99 x 109 Nm2/C2
q = 10 x 10-9 C
r = 20.0 x 10-3 m
d = 1.0 x 10-3 m
 Answer: E = 4.5 x 104 N/C
Environmental Fields
 Electric fields exist from
charges in the environment.
 Distance affects the field
strength from point sources.
 Typical field strengths in N/C
• Radio waves: 0.1
• Living room: 3
• 30 cm from a stereo: 90
• Fair weather sky: 150
• Shower (water on): 800
• Thunderstorm: 10,000
• Cell membrane: 10,000,000
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