Download 1. Electrostatics

Electricity
Charge and Field
Presentation 2003 R. McDermott
Static Electricity
•
•
•
•
•
Non-moving charges.
Characteristic of insulators.
Positive and negative charges.
Protons in a solid can’t move.
Changes in charge are due to electron
transfer.
• Friction produces static electricity.
Two Kinds of Charge
•
•
•
•
Unlike charges attract
Like charges repel
Both charges attract neutral objects
Removing electrons results in positive
charge (silk on glass).
• Adding electrons results in negative
charge (fur on plastic).
Conservation of Charge
• Total charge is constant.
• Electrons transfer from more negative to less negative
object.
• Most everyday objects are neutral (most atoms are
neutral).
• Identical objects in contact develop the same charge.
Atoms Contain Charge
• Protons inside nucleus are
positive.
• Electrons outside nucleus
are negative.
• Proton and electron have
same charge magnitude.
• Electrons can move within
solid material
The Smallest Charge
• The charge on an electron (-1), or
proton (+1) is one “elementary” charge.
• Which equals 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
Producing Localized Charges
• Rubbing
• Inducing a nonsymmetrical
charge distribution.
• Conduction
• Grounding (Induction)
– Connect with wire
leading to ground
Inducing a nonsymmetrical
charge distribution.
In the picture to the right, a
negatively charged rod brought
close causes electrons in the object
to move away (downward).
Even though the object is still
neutral, its top is now slightly
positive, and its leaves slightly
negative as a result.
Inducing a nonsymmetrical
charge distribution.
In the picture to the right, a
positively charged rod brought close
causes electrons in the object to
move toward it (upward).
Even though the object is still
neutral, its top is now slightly
negative, and its leaves slightly
positive as a result.
Charging by Conduction
• An object is charged by
conduction if you touch a
charged object to it, allowing
electrons to be transferred.
• Using a negative object results in
a negative charge. Using a
positive object results in a
positive charge, as shown to the
right:
Charging by Induction
• An object is charged by induction
if you touch a neutral object
(ground) to it, while holding a
charged object nearby.
• Using a negative object results in
a positive charge. Using a
positive object results in a
negative charge, as shown to the
right:
Electroscope
• Detects charge with leaves that repel or
needle that rotates.
Charging by Induction
Charging by Induction
Insulators and Conductors
• Conductors carry charge easily
– Metals have many free electrons
– Ionic liquids
– Plasmas
• Insulators conduct poorly
– Dry gases
– Wood, paper, cloth, glass, etc
• Semiconductors (silicon, germanium) are
insulators that can be altered to conduct.
Coulomb’s Law
• Electric force is 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 the permittivity
of free space
• Applies to point charges
Example 1 – Calculate
Coulomb Force
• Find the force between two objects
with charge 1 Coulomb at a
separation of one meter.
Ans. 9x109 N
• If their mass is 1 kg each, what is
the gravitational force between
Ans. 6.7x10-11 N
them?
• How do the two forces compare?
Example 2
• Find force between two, 1 -coulomb
(10-6 coulomb) charges at separation of
20 cm
F = kQ1Q2/r2
F = (9x109 N-m2/C2)(1x10-6 C)(1x10-6 C)/(0.20 m)2
F = 0.225 N
Electrostatic Force and
Vectors
• Fnet = F1 + F2 + F3 + …
• Principle of superposition of forces
• Use component method of vector
addition
• Fx = F1x + F2x
Fy = F1y + F2y
Recall:
• F1x = F1cosq
• F1y = F1sinq
• F = (F1 + F2)1/2
• Tanq = Fy/Fx
Electric Field
• Force acting at a distance vs. field
concept.
• Field, E, at a point is the force on a
positive charge at that point divided
by magnitude of that charge.
• Direction is the same as the
direction of the force on a positive
charge.
• The spacing of field lines indicates
the strength of E.
Electric Field Strength
• Units: newtons per coulomb
• Defined as E = F/q
• E = kQ/r2 is the field strength due to a
point charge Q as measured at some
point ‘r’ from Q.
Problem Solving
• Draw careful diagrams
• Apply Coulomb’s Law to get magnitude of
forces or fields
• Determine direction of forces by considering
like and unlike charges
• Show and label each vector force or field
• Add vectorially to get resultant
• Use symmetry when possible
Three Charges in a Line
• Each charge will experience two electric
fields and therefore two forces; one
from each of the other two charges.
• The net field (and force) will be the
vector sum of those two fields (and
forces).
Field due to 2 Like Charges
Field due to Unlike Charges
Fields and Conductors
•
•
•
•
Field inside conductor is zero.
If not, force F=qE would make charges move.
Charge spreads out optimally on surface.
Charge +Q inside spherical uncharged shell
induces -Q on inside surface of shell.
• +Q then exists on outside surface of shell.
• Electric field just outside a conductor is
always perpendicular to the surface.
Acknowledgements
•
•
•
•
Zoomschool.com
Glenbrook South Physics
Fizzics Fizzle at Thinkquest.com
Dr. Philip M. Dauber