Transcript the electric field - Haiku for Ignatius

ELECTROSTATICS
1. Something called "electric charge" exists on matter. We detect
it's presence by attraction or repulsion to other "charge".
2. Two kinds of charge:
1. Positive - which we attribute to a deficit of electrons
2. Negative - which we attribute to an excess of electrons
3. "Electrons" are carriers of negative electric charge
4. Like charges repel; unlike charges attract
5. Charge is conserved in a closed system. The number of
electrons always remains the same
6. Conductors permit electrons to flow; Insulators inhibit the flow
of electrons
ELECTROSTATICS
What makes a good conductor?
SHELL
Sub
shell
Max # of
electrons
K
s
2
L
s
2
p
6
s
2
p
6
d
10
M
Nickel Atom
ELECTROSTATICS
Copper atom
29 Protons
29 Electrons
29 Neutrons
ELECTROSTATICS
Let's introduce some definitions before we continue:
to quantify "electric charge" we label the amount of
charge on a body as:
q
q = quantity of electric charge
We can have -q (negative charge)
or
+q (positive charge)
ELECTROSTATICS
We further define a basic unit of charge (just as
we defined the basic unit of mass as a kilogram) as:
the "Coulomb"
One Coulomb = 1.0 C = 6.242 x 1018 electrons
This means that a SINGLE electron carries a very small
charge. Can you figure out how much charge (in "Coulombs")
are on a single electron?
-____________ C on 1 eThis number is a constant and a very important value. It also
represents the charge on the PROTON ( but + )
ELECTROSTATICS
Typical current in a lightning bolt is 40,000 Amperes
(that’s about 40,000 Coulombs per second) with a voltage
of up to 100,000,000 volts.
ELECTROSTATICS
THE ELECTRIC FIELD
Between two charged bodies there is a force, F, of attraction or
repulsion:
++++++
+
+
+
+++++
+
-
A
We don't understand why; we can only say this is what happens.
We can think of a charged body as changing the nature of the
space surrounding it.
ELECTROSTATICS
The field gets weaker as we move away from the
charge…it follows the “inverse square law” just as
Gravitation did and just as our recent study of
Sound Intensity showed.
Why?
+
+++ +
+
+
A
ELECTROSTATICS
Direction of the Electric Field
Outward (away) from a positive
charge
+
These are called “field
arrows”
Inward (towards) a negative
charge
_
ELECTROSTATICS
We draw the number of arrows proportional to the
charge…more charge, more arrows. Say the charges are
in “mCoulombs” (that’s micro-coulombs, or 10-6
Coulombs)
6
12
?
ELECTROSTATICS
We draw the number of arrows proportional to the
charge…more charge, more arrows. Say the charges are
in “mCoulombs” (that’s micro-coulombs, or 10-6
Coulombs)
+6
+12
-8
-10
ELECTROSTATICS
When charges get near each other, these fields
interact
For unlike charges, the arrows go from the
positive charge to the negative charge:
+6
-4
ELECTROSTATICS
For like particles the arrows are repelled:
-4
-4
The field arrows never cross in either case
ELECTROSTATICS
Coulomb’s Law
quantifies the attractive and repulsive behavior we observe in electrostatics
Coulomb used a torsion balance (similar to the one Cavendish used to determine
the gravitational constant)
k|q1q2|
F=
r2
+
-
degrees of twist
determines force
ELECTROSTATICS
F=
k|q1q2|
r2
Force = Newtons
q1 = charge on particle one (in Coulombs)
q2 = charge on particle two (in Coulombs)
r = distance between particles (in meters)
k = 9 x 109 (N·m2)/C2 (Coulomb's constant)
ELECTROSTATICS
SUPERPOSITION OF ELECTRIC CHARGE
(super - pose: to place on top of; to add)
The electrostatic force is additive
q1
-
q2
1.0 m
+
The force on this charge =
force from q1 + force from q3
q3
1.8 m
+
The force on this charge =
force from q1 + force from q2
ELECTROSTATICS
q1
q2
q3
-1.3 x 10-6 C
+2.3 x 10-6 C
+1.8 x 10-6 C
-
1.0 m
+
The force on this charge =
force from q1 + force from q3
F = k|q1q2| / (r12)2 + k|q2q3|/ (r23)2
1.8 m
+
The force on this charge =
force from q1 + force from q2
F = k|q1q3| / (r13)2 + k|q2q3|/ (r23)2
The subscripts on "r" refer to the particle distance; that is, r12
represents the distance between particles 1 and 2.
ELECTROSTATICS
Just as we defined a gravitational field, we define
an "electric field" in a similar manner:
GmM
F
2
r
GmM
mg 
2
r
GM
g 2
r
This is the gravitational field (Earth = 9.8 m/s2 or 9.8 N/kg)
ELECTROSTATICS
Just as we defined a gravitational field, we define
an "electric field" in a similar manner:
GmM
F
2
r
GmM
mg 
2
r
GM
g 2
r
F
k qQ
r
2
This is the gravitational field (Earth = 9.8 m/s2 or 9.8 N/kg)
ELECTROSTATICS
Just as we defined a gravitational field, we define
an "electric field" in a similar manner:
GmM
F
2
r
GmM
mg 
2
r
GM
g 2
r
F
k qQ
q (?) 
2
r
k qQ
r2
This is the gravitational field (Earth = 9.8 m/s2 or 9.8 N/kg)
ELECTROSTATICS
Just as we defined a gravitational field, we define
an "electric field" in a similar manner:
GmM
F
2
r
GmM
mg 
2
r
GM
g 2
r
F
k qQ
r
q( E ) 
2
k qQ
r2
This is the gravitational field (Earth = 9.8 m/s2 or 9.8 N/kg)
ELECTROSTATICS
Just as we defined a gravitational field, we define
an "electric field" in a similar manner:
GmM
F
2
r
GmM
mg 
2
r
GM
g 2
r
F
k qQ
r
q( E ) 
E
2
k qQ
r
2
kQ
r2
This is the gravitational field (Earth = 9.8 m/s2 or 9.8 N/kg)
ELECTROSTATICS
The general equation for an ELECTRIC FIELD is:
E
kQ
r
2
Newtons N

Coulomb C
(compare this to the equation for the gravitational field)
ELECTROSTATICS
Notice that for gravity,
F = mg
We see that in electrostatics, F = qE
GmM
F
2
r
GmM
mg 
2
r
GM
g 2
r
F
k qQ
r
q( E ) 
E
2
k qQ
kQ
r
2
r
2
ELECTROSTATICS
Here’s our equations to date. At first we will only
deal with POSITIVE charges:
Electric Field
Force
F
k qQ
r
2
F = |q|E
E
kQ
r
2