Download Laws of Electric Charges

Laws of Electric Charges
Electric Charge – electric
charges exert forces on each
other even when they are not
in direct contact
Law of Electric Charges
• Opposite charges attract
• Similar charges repel
Every charged object creates an
electric field in the space
around it
•We use field lines to show an
electrical field and the direction
of magnetic force
•In an electric field, every
charged object will experience a
force of electrical attraction or
repulsion
•The direction of a field line
in the electric field is the
direction of the force on a
small POSITIVE charge. If
negative charge, direction of
the force will be the
opposite
Draw your own electric fields
Draw the following charges in positions of your
choosing (unless otherwise indicated) and create
the electric field that would be present:
a) Four + charges at the four points of a square
b) Two + charges and a – charge at the three
points of a triangle
c) Three negative charges and one positive charge
Coulomb’s Law
•related the magnitude of the
force between two charged
objects to the charge of each
object and the distance
between them
Coulomb’s Law
kQ1Q2
F
2
d
• Q1,2 – charge on each object
• d – distance between the charged
objects
• K – constant (9.0x 109 Nm2/C2)
• F – electric force between two
charged objects
Robert Millikan’s Experiment
Determined the charge of one electron, used to
determine the charge on an object
Q = Ne
• Q – Total charge (C)
1 C = 6.24x1018 electrons
• N – number of electrons (in excess or in
deficit)
• e – elementary charge (charge on one
electron) e = 1.60 x 10-19 C
•An electron has a negative
elementary charge (-e) while a
proton has a positive
elementary charge (e)
Practice Problem
• What is the charge in coulombs
on an object that has an excess of
6.25x1019 electrons?
• Ans: Q = -10C
Q = Ne
= (6.25x1019)( -1.60x10-19 C)
= -10 C
• A substance that has an excess of
6.25x1019 electrons has a charge
of -10 C.
Homework
1. What is the charge on an object that has an
excess of 3.02 x 109 protons ?
2. Draw the electric field lines present when you
have two fixed positive charges and one fixed
negative charges in a region of space (draw
your own positions for each charge).
3. How many electrons are in excess or deficit if
the charge on an object is -13.2C