Transcript Holt Ch 16 Electric Fields & Forces

Electrostatics
Learning Objectives
• The electrostatic force (Coulomb’s Law)
can be either repulsive or attractive
(SOL 12.a)
• The interaction of two particles can be
described as: the creation of a field by
one of the particles and the interaction
of the field with the second particle
(SOL 12.b).
• Magnitude of charge on protons and electrons
are exactly the same
– Protons have a positive charge
– Electrons have a negative charge
• Neutral atoms contain equal numbers of
protons and electrons
Insulators and Conductors
Need to know
• Insulator: electrons are bound very tightly to
the nuclei. Wood and rubber are good
insulators.
• Conductor: electrons are bound very loosely
and can move about freely. They are often
referred to free electrons. Metals are good
conductors.
• Semiconductor: very few free electrons
(silicon, germanium and carbon)
Static Electricity
• You have probably experienced a charge
lately (comb, dryer, carpet, car seat, …)
• An object becomes charged due to a rubbing
process and is said to possess a net electric
charge
• An item containing a net positive charge has
lost electrons
• An item containing a net negative charge has
gained electrons
Law of Conservation of
Electric Charge
Need to know
The net amount of electric charge produced in
any process is zero
If one object or one region of space acquires a
positive charge, then an equal amount of
negative charge will be found in neighboring
areas or objects
Unlike Charges Attract;
Like Charges Repel
Need to know
3 Ways to Charge an Object
Need to know
1. Friction: Rubbing two objects together
with different electron attachment.
Heat generated frees electrons to join
object with stronger attachment.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
3 Ways to Charge an Object
Need to know
2. Conduction: Electrons are transferred
from one object to another by touching.
Usually it involves moving from one
electric potential to another.
John TraVOLTa Demo
3 Ways to Charge an Object
Need to know
3. Induction: Rod does not touch sphere. It pushes
electrons out of the back side of the sphere and down
the wire to ground. The ground wire is disconnected to
prevent the return of the electrons from ground, then
the rod is removed.
Electromagnetism
• One of the four fundamental forces of the
universe (electromagnetism, gravity, weak
nuclear and strong nuclear forces)
• The forces that act between atoms and
molecules to hold them together are electrical
forces
• Elastic, normal and contact forces (pushes
and pulls) result from electric forces acting at
the atomic level
Forces resulting from charges
• Charges push and pull on one another
• Closer the charge the higher the force
• The stronger the charge the higher the
force
Coulomb’s Law
Need to know
The magnitude of the force between charge
qA and charge qB, separated a distance d,
is proportional to the magnitude of the
charge and inversely proportional to the
square of the distance:
F = K qAqB
qA
qB
d2
d
Coulomb’s Law: Key Facts
Need to know
The charge of an electron is:
-1.60 x 10-19 coulombs (C)
The charge of a proton is:
1.60 x 10-19 coulombs (C)
The charge, q, is measured in coulombs. The
distance, d, is measured in meters. The
force, F, is measured in newtons.
The constant, K = 9.0 x 109 Nm2/C2
Problem Solving Strategy
1. Sketch the system showing all distances
2. Diagram the vectors
3. Use Coulomb’s law to find the magnitude
of the force. Note: it is unnecessary to
include the sign of the charges or the
distance. The answer is always positive.
4. Use your diagram along with trigonometric
relations to find the direction of the force
Example Problem 1
Two charges are separated by 3.0 cm.
Object A has a charge of +6.0 C, while
object B has a charge of +3.0 C. What is
the force on object A?
Known:
qA = +6.0 x 10-6 C
qB = +3.0 x 10-6 C
d = 0.030 m
Unknown:
FB on A = ?
Example 1 Solution
F = K qAqB
d2
= (9.0 x 109 Nm2/C2)(6.0 x 10-6C)(3.0 x 10-6C)
(3.0 x 10-2 m)2
FB on A = 1.8 x 102 N
Example 2: Three Charges
• Given:
+6 µC
-2 µC
6 cm
2 µC
2 cm
• Find the net force on the -2 µC charge
• Known:
+6 µC
-2 µC
6 cm
2 µC
2 cm
FA on B= KqAqB
d2
2
9
Nm
-6
-6
= (9x10
C2)(6x10 C)(2x10 C)
(0.06 m)2
= - 30 N
FC on B= KqCqB
d2
2
= (9x109 Nm C2)(2x10-6 C)(2x10-6 C)
(0.02 m)2
= + 90
FNet = FA on B + FC on B = - 30 N + 90 N = 60 N
Example Problem 3
A sphere with a charge 6.0 C is located near
two other charged spheres. A -3.0 C is
located 4.00 cm to the right and a 1.5 C
sphere is located 3.00 cm directly
underneath. Determine the net force on the
6.0 C sphere.
F
F
net
C on A
A
dAC
C
dAB
qA= 6 C
qc = 1.5 C
B
qB = -3 C

FB on A
Example 3 Solution
A
dAC
dAB
qA= 6 C
B
qB = -3 C
Fnet
FC on A

FB on A
C
qc = 1.5 C
FB on A =
FC on A =
Fnet =
 =
Static Charge Generator
Electric Field
Need to know
• An electric field extends outward from
every charge and permeates all of
space
Investigating the Electric Field
• We can quantify the strength of an
electric field by measuring the force on
a small positive test charge
– So small that the force it exerts does not
significantly alter the distribution of the
charges that create the field
qA+
a
+qB
Electric Field
• An electric field, E, at any point is
defined as the force, F, exerted on a
tiny positive test charge at that point
divided by the magnitude of the test
charge:
E = F/qB
qA+
+qB
Electric Field Equation
E = F/qB
E = K qB qA/r2
qB
+qB
E = KqA/r2
qA+
Electric Field Lines
• Drawn so that they indicate the direction
of the force due to the given field on a
positive charge
+qB
qA+
Electric Field Lines
Need to Know
Lines indicate direction of the force due to the
given field on a positive test charge
Properties of Field Lines
Need to know
1. The field lines indicate the direction of the
electric field
2. The lines are drawn so that the magnitude of
the electric field, E, is proportional to the
number of lines crossing unit area
perpendicular to the lines. The closer the
lines, the stronger the field.
3. Electric field lines start on positive charges
and end on negative charges
Electric Potential Difference
Need to know
V = Won q’ = PE: Potential difference often
q’
q’ referred to as Voltage
Electric Potential Difference Units: Volt =J/C
+
g
displacement
W = Fd = mgd
E
displacement
+
Big
Negative
Charge
W = Vq
Typical Voltages
Source
Voltage
Thundercloud to ground
High voltage power line
Power supply for TV tube
Auto ignition
Household outlet
Auto battery
Resting potential across nerve
membrane
Potential changes on skin
(EKG)
108 V
106 V
104 V
104 V
102 V
12 V
10-1 V
10-4 V
Capacitors
Need to Know
• A capacitor is a device that can
store electric charge
• Consists of two conducting
objects placed near each other
but not touching
• They store charge for later use
• Usage: camera flash, energy
back-up for computers and as
surge protectors
Capacitors
• Consists of a pair of parallel plates
of area, A, and separated by a small
distance d.
• In a diagram, they are represented
by the symbol:
• If a voltage is applied to a capacitor,
one plate acquires a negative
charge and the other an equal
amount of positive charge.
Capacitors
Need to Know
• The amount of charge acquired by each plate
is proportional to the potential difference
Q = CV
• Where C is constant and is called the
capacitance of the capacitor
• Unit: Coulombs/Volt = Farad
– Typical capacitor range is 1pF (10-12) to 1F (10-6)
Determining Capacitance
• Constant for a given a capacitor
• Depends on structure and dimensions of he
capacitor itself:
C = o A/d
A = area
d = separation distance between plates
o = 8.85 x 10-12 C2/Nm2
= permittivity of free space