2. REVIEW: TRUE OR FALSE
1. Two positive charges or two
negative charges repel each other.
A positive charge and a negative
charge attract each other.
3. REVIEW: TRUE OR FALSE
2. Electric Charge is Conserved. The
algebraic sum of all electric charges in
any closed system is constant. Charge
is not created nor destroyed; it is
merely transferred from one body to
another.
4. REVIEW: TRUE OR FALSE
3. “The magnitude of charge of the
electron or proton is a natural unit of
charge.” Charge can be divided into
amounts smaller than the charge of
one electron or proton.
5. REVIEW: TRUE OR FALSE
4. Conduction is the transfer of
electrons from a charged object to
another object by direct contact. In the
conduction process, a body with one
type of charge produces the same type
of charge on a conductor.
6. REVIEW: TRUE OR FALSE
5. Induction is the movement of
electrons to one part of an object by
the electric field of another object. In
the induction process, the same
type of charge is produced.
8. Charles Agustin de Coulomb (1736-1806),
a French physicist, discovered the nature of
these electrical forces exerted by bodies
upon one another.
9. Coulombs’s Law
“The magnitude of the electric force between two point charges is
directly proportional to the product of the charges and inversely
proportional to the square of the distance between them.”
10.
11. SAMPLE PROBLEM
Two charges are on the x-axis. Charge q1 = +25 nC is at x = 4.00 cm
while charge q2 = -75nC is at x = 7.00 cm. Find the magnitude and
direction of
a) the electric force that q1 exerts on q2; and
b) the electric force the q2 exerts on q1.
13. SOLVE
Three charges and their specific
locations are given as follows:
q1 = - 20 nC; (0, 2.00 cm)
q2 = + 30 nC; (-1.00 cm, 4.00 cm)
q3 = + 10 nC; (-3.00 cm, 0 cm)
Determine the magnitude and
direction of the net force on q3 using
component method.
Editor's Notes
T
T
F
T
F - opposite
Matter is made up of atoms containing electrically charged particles. Many properties of matter result from the electrical forces of attraction and repulsion among the particles. Among these properties are the formation of molecules, cohesion, and adhesion.
torsion balance, device used to measure the gravitational acceleration at the Earth's surface
He used a torsion balance similar to the one used 13 years later by Cavendish to study the much weaker gravitational interaction.
For point charges, charged bodies that are very small in comparison with the distance r between them, Coulomb found that the electric force is proportional to 1/r2. That is, when the distance is doubled, the force decreases to ¼ of its initial value; when the distance is halved, the force increases to four times its initial value.
The electric force between two point charges also depends on the quantity of charge on each body, which we will denote by q or Q. In his experiment, Coulomb found that the forces that two point charges q1 and q2 exert on each other are proportional to each charge and therefore are proportional to the product q1q2 of the two charges. Thus, Coulomb established what we now call the Coulomb’s Law which states that:
In mathematical terms, the magnitude F of the force that each of two point charges q1 and q2 a distance r apart exerts on the other can be expressed as
where k (electrostatic force constant) is a proportionality constant whose numerical value depends on the system of units used. The approximate value of k is 9.0 x 109 N∙m2/C2. The constant Ɛ0 has a value of 8.854 x 10-12 C2/N∙m2. The absolute value bars are used in Eqn. 2.1 because the charges q1 and q2 can either be positive or negative, while the force magnitude F is always positive.
The direction of the forces the two charges exert on each other are always along the line joining them. When the charges have the same sign, either both positive or both negative, the forces are repulsive; when the charges have opposite signs, the forces are attractive. The two forces obey Newton’s third law; they are always equal in magnitude and opposite in direction, even when the charges are not equal in magnitude.
