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What is Electric Charge?Exertion of Electrical Force Identifies Charge as Measured by the Unit of Coulomb
Jun 20, 2010 © Harry P. Schlanger
Objects that exert electric forces are said to have charge. Charge is the source of electrical force and there are two kinds of electrical charges, positive and negative.
Electrical charges of the same kind are given a like sign, either positive or negative.
Net Electrical Charge
Matter is made of atoms. An atom is basically composed of three different components - electrons, protons,
and neutrons. An electron can be removed easily from an atom.
Normally, an atom is electrically neutral, which means that there are equal numbers of protons and
electrons. Positive charges of protons are balanced by negative charges of electrons. The atom has zero
net electrical charge.
When atoms gain or lose electrons, the balance is upset and they acquire net charge. Such atoms are then
called "ions".
- A positive ion is short of one or more electrons
- A negative ion has one or more extra electrons
Charging Objects
When two objects are rubbed together, some electrons from one object move to another object. For example,
when a plastic bar is rubbed with fur, electrons will move from the fur to the plastic stick. Therefore,
the plastic bar will be negatively charged and correspondingly, the fur will be positively charged.
Unit of Electrical Charge: The Coulomb " C "
The unit for electric charge is the Coulomb "C", named after the French physicist,
Charles-Augustin de Coulomb
(1736 – 1806).
Charge is written as q, -q or Q. The charge of one electron is equal to the charge of one proton, which
is 1.6 * 10^-19C. This number is given a symbol "e" and is originally derived from amount of charge that
passes through a 1 amp current over 1 second.
As a corollary, how many electrons are there in 1 C of charge? Answer is, the reciprocal of e, or
6.25 * 10^18 electrons.
Coulomb's Law
Coulomb's Law states that the magnitude of force that a particle exerts on another particle is directly
proportional to the product of their charges and inversely proportional to the square of the distance
between them.
F is the force between the two particles,
q1, q2 are the charges on particle A and B, respectively,
d is the distance between the particles, and
k is a proportionality constant, which is 9.0 x 10^9 Nm2/C2.
The direction of the force is on the line from one particle to the other.
Example:
An object has +16.5 x 10^-6 C of charge, while another object charged object nearby has -7 x 10^-6 C,
Their separation is 0.030 m. What is the net force due to these charges?
Solution:
Substituting the above values into the Coulomb's Law equation, one gets:
F = (9.0 * 10^9) x (+16.5 x 10^-6) x (-7 x 10^-6) / 0.0302
= -1.16 kN
Since the sign is negative, the force is one of attraction.
This article discussed the origin of charge and the force exerted between two "point" charges. Essentially,
the charges were not moving, a study belonging to the realm of electrostatics. It will be seen in a
future article that when charges do move, we then have what constitutes an electrical current.
References:
- Carmel Fry et al. Heinemann Physics 11. Harcourt Education. Melbourne Australia. 2nd Edition. 2010.
The copyright of the article What is Electric Charge?: Exertion of Electrical Force Identifies Charge as Measured by the Unit of Coulomb is owned by Harry P. Schlanger. Permission to republish in print or online must be granted by the author in writing.
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