Transcript Ch33 - Siena College

Reading Quiz
1. What is the SI unit
of charge?
2. A charge alters the space
around it. What is this
alteration of space called?
A.Coulomb
B. Faraday
C. Ampere
D. Ohm
E. Volt
A.Charged plasma
B. Charge sphere
C. Electric ether
D. Electric field
E. Electrophoresys
3. If a negative charged rod is
held near a neutral metal
ball, the ball is attracted to
the rod. This happens
4. The electric field of a
charge is defined by the
force on
A. because of magnetic effects.
B. because the ball tries to pull
the rod’s electrons over to it.
C. because the rod polarizes the
metal.
D. because the rod and the ball
have opposite charges.
A.an electron.
B. a proton.
C. a source charge.
D. a test charge.
What is the SI unit of charge?
A. Coulomb
B. Faraday
C. Ampere
D. Ohm
E. Volt
A charge alters the space around it.
What is this alteration of space called?
A. Charged plasma
B. Charge sphere
C. Electric ether
D. Electric field
E. Electrophoresys
If a negative charged rod is held near a
neutral metal ball, the ball is attracted to
the rod. This happens
A. because of magnetic effects.
B. because the ball tries to pull the
rod’s electrons over to it.
C. because the rod polarizes the metal.
D. because the rod and the ball
have opposite charges.
The electric field of a charge is defined
by the force on
A. an electron.
B. a proton.
C. a source charge.
D. a test charge.
The Electric Dipole
Coulomb’s Law - Magnitude
K q1q2
Fe 
2
r
• K = 9.0 x 109 N m2/C2
– Coulomb’s contant
• q1, q2 = charges

– measured in Coulombs (C)
– 1 C is a lot of charge!
• r = distance between q1 and q2
q1q2
Fe 
2
40 r
1
ε0 =permittivity of free space
(more relevant to future chapters)
Electric Force
Gm1m2
Fg 
r2
K q1q2
Fe 
2
r
• Similar form as gravitational force (weird, huh?),
except
– positive and negative charges


• Like charges repel; opposite charge attract
Net Force from Multiple Charges
• An electrostatic force exists between each pair
of charges according to Coulomb’s law
Fe  Fe,12  Fe,13  Fe,23
k q1q2 k q1q3 k q2q3



2
2
2
r
r
r
• Add components of forces to get net force
when adding multiple charges
The Electric Field
We begin our investigation of electric fields by postulating
a field model that describes how charges interact:
1.Some charges, which we will call the source
charges, alter the space around them by creating an
electric field.
2.A separate charge in the electric field experiences a
force exerted by the field.
Suppose probe charge q experiences an electric force Fon q
due to other charges.
The units of the electric field are N/C. The magnitude E of
the electric field is called the electric field strength.
The Electric Field
Group Problems
Group Problems
• Workbook Volume 4
• Pages 26-9 – 26-12
Group Problems
1. What is the net force on the 48 C charge? Field?
Q1
Q2
+y
+x
Q4
Q3
2. Find net force
on Q1. Field?