Transcript Electricity - TeacherWeb

Electricity
Charge and Field
Static Electricity
•
•
•
•
•
•
Comb picks up paper
Balloon sticks to wall
Sparks when combing hair
Shock when touch doorknob
“static cling” in dryer
lightning
Two Kinds of Charge
•
•
•
•
Unlike charges attract
Like charges repel
Positive on rubbed glass rod
Negative on rubbed plastic rod
Conservation of Charge
• Net amount of electric charge produced
in any process is zero
• When positive and negative combine
they neutralize
• Most every day objects are neutral
• Ordinary atoms are neutral
Atoms Contain Charge
•
•
•
•
Protons inside nucleus are positive
Electrons outside nucleus are negative
Charge on proton same as on electron
Electrons can move within solid material
Methods of Charging
• Rubbing
• Induction
– Separates charge
• Contact
• Grounding
– Connect with wire
leading to ground
Charging by Induction
Insulators and Conductors
• Conductors carry charge easily
– Metals have many free electrons
– Ionic liquids
– Plasmas
• Insulators conduct poorly
– Dry gases, pure molecular liquids
– Wood, paper, cloth, glass, etc
• Semiconductors like silicon, germanium
Electroscope
• Detects charge with leaves that repel
or rotating needle
Coulomb’s Law
• Electric force proportional to product of
charges divided by square of distance
between them
• Q in coulombs
• k is Coulomb constant
• k = 8.988x109 Nm2/C2
• constant e0 is permittivity
of free space 8.85x10-12 C2/Nm2
• Applies to point charges
Electrostatic force vs. gravity
force
• FE = kq1q2/r2
k = 9 x 109 Nm2/C2
• FG =Gm1m2/r2 G = 6.67 x 10-11 Nm2/kg2
• Electrostatic force both attractive and
repulsive
• Gravitational force only attractive (per
Newton, not so certain today)
The Smallest Charge
• Charge on electron or “elementary”
charge qe or e
• e = 1.602 x 10-19 Coulombs
• Charge is quantized
• Discrete amounts only
• 1e, 2e, 3e, 4e etc
• Quarks of sub-atomic physics have 1/3
and 2/3 charge
Example – Calculate Coulomb
Force
• Find the force between two objects with
charge 1 Coulomb at a separation of
one meter
• F = kq1q2/r2 = 9 x 109 x 1/1 = 9 x 109 N
If objects are 100Kg students, what will
be initial acceleration?
•
a = F/m = 9 x 107 m/s/s
• Estimate speed reached in one second
Example(2) – Coulomb Force
• Find force between two 1 micro (10-6)
coulomb charges at separation of 20 cm
• F = kq1q2/r2 = 9 x 109x 10-6 x 10-6 /(0.20)2
• =9/(.04) x 10-3
2.25 x 10 -1 N
Electrostatic Force and
Vectors
• Fnet = F1 + F2 + F3 + …
• Called principle of superposition of
forces
• Use component method of vector
addition
• Fx = F1x + F2x
Fy = F1y + F2y
Component Method Review
•
•
•
•
•
F1X = F1cosq
F1Y = F1sinq
FX = F1X+F2X
FY = F1Y+F2Y
F = (FX2 + FY2)1/2
• TanQ = FY/FX
F1
q
F1X
F1Y
Three Charges in a Line
Assuming the magnitude of all three charges
is equal, what is the direction of the net force
on the positive charge?
To the right
Three Charges in a Line
Assuming the magnitude of all three charges
is equal, what is the direction of the net force
on the red positive charge?
To the right
Three Charges in a Line
Assuming the magnitude of all three charges
is equal, what is the direction of the net force
on the red positive charge?
To the left
Electric Field kq/r2
• Force acting at a distance vs. field
concept
• Field E is force on tiny positive test
charge divided by magnitude of charge
• Direction same as force on + charge
• Field line spacing shows strength of E
Electric field lines
outward from + charge
Electric field lines
inward to - charge
Field due to 2 Like Charges
Field due to Unlike Charges
You Predict
• What would be the direction of the force on a
positively charged pith ball near a positively
charged Van de Graaf sphere?
• What about a negatively charged pith ball?
• If the force on a positively charged pith ball is
toward the sphere, what must be the field
direction?
• What is the charge on the sphere?
Electric Field Strength
• Units newtons per coulomb
• E = F/q
F = Eq q is “test charge”
• E = kQ/r2 due to charge Q
Just Coulomb’s law
without the q2
kq1q 2
2
r
Problem Solving
• Draw careful diagram
• Apply Coulomb’s Law to get magnitude of
forces or fields
• Determine direction of net forces by
considering like and unlike charges
• Show and label each vector force or field
• Add vectorially to get resultant
• Use symmetry when possible
Fields and Conductors
•
•
•
•
Field inside conductor is zero(static)
If not force F=qE would make charges move
Charge spreads out maximally on surface
Charge Q inside spherical uncharged shell
induces –Q on inside surface of shell
• Positive charge Q exists on outside
• Electric field just outside a conductor is
always perpendicular to the surface
Why Field Outside Conductor is
Perpendicular to Surface
• Ask what if there were a parallel
component just outside
• There would also be one just inside
• Electrons would move until equilibrium
• Then there could be no field
• Contradiction
• Therefore the original statement is true
Problem
• Charged conducting spheres with +3Q
and –Q are initially at a distance of L
meters apart. They are brought
together briefly, then moved back to
their original positions. What happens
to the force between them?
+3Q
-Q
Problem
• Charged conducting spheres with +3Q
and –Q are initially at a distance of L
meters apart. They are brought
together briefly, then moved back to
their original positions. What happens
to the force between them?
+3Q
-Q
Problem
• Charged conducting spheres with +3Q
and –Q are initially at a distance of L
meters apart. They are brought
together briefly, then moved back to
their original positions. What happens
to the force between them?
+Q
+Q
Problem
• Charged conducting spheres with +3Q
and –Q are initially at a distance of L
meters apart. They are brought
together briefly, then moved back to
their original positions. What happens
to the force between them?
+Q
+Q
Solution
• On contact there is neutralization
leaving a total of 2Q; each charge gets
Q. Upon putting the charges back
where they were, the force changes
from attractive to repulsive and is 1/3 as
strong.
Question
• What would happen if a positive charge
is placed at the center of a neutral
conducting spherical shell?
+
Draw the electric field
lines
Negative charges induced, inside shell, positive outside
as shown on NEXT SLIDE