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OVERVIEW
• Green sheet
• HW assignments (tentative)
• Course overview
See www.physics.edu/becker/physics51
C 2009 J. Becker
Electric charge: Chapter 21
•Protons have positive charge
•Electrons have negative charge
•Opposite signs attract
•Similar signs repel
•Electric field – used to calculate force
C 2009 J. Becker
PROTONS are massive and are held
inside the nucleus. They do not move
from place to place in an object.
ELECTRONS are not as massive and
generally can move from one object to
another. This is the way electric
charge is transferred from one object
to another: one object loses electrons
and the other gains electrons
C 2009 J. Becker
LITHIUM (Li) ELEMENT
Atom: electrically neutral 3 protons and 3 elec.
Positive ion: missing one electron so net
charge is positive
Negative ion: has added electron so net
charge is negative
Protons carry a +e charge
Electrons carry a -e charge
The fundamental unit of electric charge is
e = 1.60 (10)-19 COULOMB
C 2009 J. Becker
CONDUCTORS: materials that have
freely moving electrons that respond
to an electric field.
INSULATORS: materials that have
fixed, immobile electrons that are not
easy to move.
C 2009 J. Becker
Two positive charges or two negative charges
repel each other. A positive charge and a
negative charge attract each other.
Metal ball is charged negatively as shown in A
B
A
C
Copper is a good conductor of electricity;
Glass and nylon are good insulators
CHARGING METAL SPHERE BY INDUCTION
Charges are free to move in a conductor but
are tightly bound in an insulator. The earth
(ground) is a large conductor having many free
charges.
CHARGED COMB ATTRACTS
PAPER
In an insulator the charges
can move slightly (called
polarization of the insulator).
A piece of paper is attracted
to a charged comb because
the positive charges are
closer to the negatively
charged comb (in the upper
figure).
FORCE
between two charges
is given by
Coulomb’s Law:
| F | = k | Q qo | / r 2
We can use our notion of the
gravitational field to form the concept of an
ELECTRIC FIELD (E)
Recall force between two masses: F = m g
g is the gravitational field (9.8 m/sec2)
| F | = G | M m | / r2
The force between two charges Q and qo is
given by: F = qo E
| F | = k | Q qo | / r2
C 2009 J. F. Becker
Coulomb’s Law:
| F | = k | Q qo | / r 2
Rearranged:
| F | = | qo [k Q/r2] |
Gives us:
F = qo E
where the electric field
E is:
| E | = | k Q / r2 |
ELECTRIC FIELD LINES START AND END
AT ELECTRIC CHARGES
An electric charge is surrounded by an electric
field just as a mass is surrounded by a
gravitational field.
ELECTRIC FIELD LINES PERPENDICULAR TO
EQUIPOTENTIAL LINES
In Lab #2 you
will use a
voltmeter to
measure the
equipotential lines
(in Volts) in order
to determine the
magnitude and
direction of the
electric field
lines.
Electric field and equipotential lines
for a charge near a conductor
Forces on electron beam in a TV tube (CRT)
F = Q E and F = m g (vector equations)
TV tube with electron-deflecting charged
plates (orange)
F=QE
Review
See www.physics.edu/becker/physics51
C 2009 J. F. Becker