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Chapter 21Electric Charge and
Electric Field
Charles Allison © 2000
Qu
estio
nAn particle with a charge +2e and a
mass of 4mp is on a collision course with a proton with a charge +e and mass mp. The only force acting is the electrostatic force. Which of the following statements is true about the magnitudes of the forces on the two particles?
p
Charge
+2e +e
Mass 4mp mp
A) F= Fp
B) 2F= Fp
C) 4F= Fp
D) F= 2Fp
E) F= 4Fp
Charles Allison © 2000
Qu
estio
nAn particle with a charge +2e and a
mass of 4mp is on a collision course with a proton with a charge +e and mass mp. The only force acting is the electrostatic force. Which of the following statements is true about the accelerations of the two particles?
p
Charge
+2e +e
Mass 4mp mp
A) a= ap
B) 2a= ap
C) 4a= ap
D) a= 2ap
E) a= 4ap
Charles Allison © 2000
21-5 Coulomb’s LawExample 21-2: Three charges in a line.
Three charged particles are arranged in a line, as shown. Calculate the net electrostatic force on particle 3 (the -4.0 μC on the right) due to the other two charges.
Charles Allison © 2000
21-5 Coulomb’s LawExample 21-3: Electric force using vector components.
Calculate the net electrostatic force on charge Q3 shown in the figure due to the charges Q1 and Q2.
Charles Allison © 2000
Questio
n
What is the correct FBD for charge 3 where F31 is the force on Q3 due to Q1 and F32 is the force on Q3 due to Q2?
a) b)
c) d)
+ - +
Q1 Q2 Q3
3
F31
F32
3
3 3
F31
F32
F31
F32
F31
F32
Charles Allison © 2000
21-6 The Electric Field
• Gravitational Field
+
-e
•Electric Field
+
+
+
– Units N/C– Direction of E is the direction a
+ test charge would go
Charles Allison © 2000
The electric field is defined as the force on a small charge, divided by the magnitude of the charge:
21-6 The Electric Field
Charles Allison © 2000
For a point charge:
21-6 The Electric Field
ε0 is the permittivity of the free space=8.85x10-12C2/N.m2
Charles Allison © 2000
Electric Field: point charge
If we have a charge (or a system of charges) we can ask ourselves what the electric field is at any point in space, P, a distance r from the charge.
-eP
r
E Units: N/C
+eP
r
E
Charles Allison © 2000
Force on a point charge in an electric field:
21-6 The Electric Field
Charles Allison © 2000
Electric Field: system of charges
If we have a system of charges we can ask ourselves what the electric field is at any point in space, P. In this case we find the Electric field due to each charge in the system at at that point, P, and we do a vector sum of these fields.
q1
P
r3r1
q2
q3
r2
Charles Allison © 2000
Electric Field: Two Charges
We have two charges q1 = +7.0 µC and q2 = -8.0 µC a distance of 0.50 m to the right of q1. What is the electric field at a distance of 0.30 m to the right of q2?
q1P
q2
r=0.5m r=0.30m
k = 8.99x10+9 Nm2/C2 = coulomb’s constant
Charles Allison © 2000
21-6 The Electric FieldExample 21-7: E at a point between two charges.Two point charges are separated by a distance of 10.0 cm. One has a charge of -25 μC and the other +50 μC. (a) Determine the direction and magnitude of the electric field at a point P between the two charges that is 2.0 cm from the negative charge. (b) If an electron (mass = 9.11 x 10-31 kg) is placed at rest at P and then released, what will be its initial acceleration (direction and magnitude)?
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