PHYSICS STUDY GUIDE
CHAPTER 12: ELECTRICITY
TOPICS:
• Electrostatics
• Electric Charge
• Electric Force
• Electric Field
• Electric Potential Energy
• Electric Potential
• Change in Electric Potential Energy & Electric Potential difference
1. INTRO TO ELECTROSTATICS
THE ATOM
Protons and Neutrons are found in the nucleus of an atom.
The number of protons in the nucleus of an atom is known as the atomic
number, which determines the chemical element to which the atom belongs
Example:
6 protons = Carbon (C)
16 protons = Sulfur (S)
26 protons = Iron (Fe)
Electrons are found in orbitals which are the physical region where there is a high probability of finding
any electron of an atom in any specific region around the atom's nucleus.
2. ELECTRIC CHARGE
PHYSICAL QUANTITY
Electric charge ( q )
DEFINITION
Fundamental property of matter associated with atomic particles: the electron
and the proton.
UNITS
Coulombs ( C )
•
•
•
•
To differentiate the charge of an electron from the charge of a proton we use the NEGATIVE SIGN
for the CHARGE OF AN ELECTRON.
The charge of one ( 1 ) Electron: q = -1.6x10-19 C
The charge of one ( 1 ) Proton: q = +1.6x10-19 C
One (1) Coulomb is the charge of a rather big number of electrons.
3. ELECTROSTATICS
• Study of electric charges that can be held in one place.
• Opposite charges attract
• Like charges repel.
• A charged object will attract a neutral object.
CHARGING
• Objects can be charged by friction by rubbing another object (fur rubbing rods).
• Objects can be charged by conduction by touching a charged object (rods touching Van de Graaff
generator.
MATERIALS
• Conductors: Materials where negative charges can move freely.
• Insulators: Materials where negative charges canNOT move freely.
INDUCTION
• A charged object brought near a conductor (no touching) will induce the negative charges in the
conductor by either repelling them as far as possible or attracting them as close as possible.
Charges will be either repelled or attracted in a conductor, because negative charges can move
freely in it.
- - +- - -+-- --- + + - -+----+
-- - -- - +
-- - + -- - +
+
- -- - + - -- -+Negatively charged object
Conductor will not be charged
Conductor
- -- -- - +- +
+ - + -+ - ----+
- -+
+
-+
+
- - -- - + - + Positively charged object
A negatively charged object will repel (induce)
the negative charges in the conductor.
A positively charged object will attract (induce)
the negative charges in the conductor.
Conductor will not be charged
Conductor
POLARIZATION
• A charged object brought near an insulator (no touching) will polarize the atoms in the insulators.
Charges in an insulator can not move freely. Therefore, the atoms will be polarized, which means
that the atom will elongate and center of the atom will NOT coincide with the nucleus of the atom.
One side of the atom will be more positive and the other side of the atom will be more negative.
- - + - - - + -- - - -+
-- - +
- - +- -- - - -+-
Negatively charged object
+- + - + - + - + - + +- + - + - + - + - + -
Insulator
A negatively charged object polarizes the
atom. The negative side of the atom will be
repelled by the negatively charged object.
4. ELECTRIC FORCE
PHYSICAL QUANTITY
Electric Force ( FqAonqB )
DEFINITION
Attractive or repulsive interaction between two charges.
MATH MODEL
FqAonqB =
UNITS
Newtons (N)
•
•
•
•
•
(K · qA · qB )
d2
Electric Force is exerted at a distance (field force).
The force increases with the magnitude of the charges
The force decreases with the separation between the charges.
The direction of the electric force depends on the direction of the repulsion or the attraction; it
does NOT depend on the sign of the charge.
According to Newton’s third law of motion, the force that charge A exerts on charge B is the same
force that charge B exerts on charge A (same magnitude, opposite direction).
5. ELECTRIC FIELD
PHYSICAL QUANTITY
Electric Field - created by a charge.
DEFINITION
Area surrounding a charged object, where another charged object can be
detected.
MATH MODEL
E=
UNITS
Newtons per Coulomb (N/C)
•
•
(E)
(K · qA )
d2
The Electric field increases with the magnitude of the charge.
The Electric field decreases with distance.
PHYSICAL QUANTITY
Electric Field - felt by a charge.
MATH MODEL
E=
(E)
Fq
q
ELECTRIC FIELD LINES.
•
•
Graphic representation to analyze and study electric fields.
Electric fields lines do not exist in real life.
Properties of Electric Field Lines:
•
1. Electric field lines “leave” positive charges & Electric Field lines “enter” negative charges.
POSITIVE CHARGE
•
NEGATIVE CHARGE
2. Electric field lines increase with the charge. The bigger the charge, the bigger the number of Electric
Field lines.
SINGLE CHARGE
q
DOUBLED CHARGE
2q
•
3. Electric Field lines do not cross each other.
OPPOSITE CHARGES (ATTRACTION)
•
LIKE CHARGES (REPULSION)
Uniform (constant) electric fields: You must have two opposite charged parallel plates.
6. ELECTRIC POTENTIAL ENERGY
PHYSICAL QUANTITY
Electric Potential energy ( Uq )
DEFINITION
Potential to change the velocity (kinetic energy) of a charged particle
MATH MODEL
Uq =
UNITS
Joules ( J )
•
(K · qA · qB )
d
Charges near each other will interact by either attracting (opposite charges) or repelling (like
charges) each other. If charges move, then charges gain velocity, therefore they change their
kinetic energy. The change in kinetic energy is due to the electric Potential energy.
7. ELECTRIC POTENTIAL
PHYSICAL QUANTITY
Electric Potential ( V )
DEFINITION
Potential to change the energy of a charged particle.
MATH MODEL
V=
UNITS
Volts (v)
(K · qA )
d
•
Charges near each other will interact by either attracting (opposite charges) or repelling (like
charges) each other. If charges move, then charges gain velocity, therefore they change their
kinetic energy. The change in kinetic energy is due to the electric Potential energy.
8. ELECTRIC POTENTIAL DIFFERENCE & CHANGE IN ELECTRIC POTENTIAL ENERGY
•
A bowling ball on the table has the potential to do something. Like falling on someone’s food
causing a lot of pain. The bowling ball will do something ONLY IF the gravitational potential
changes (the bowling ball rolls down, height changes).
•
Same happens to charges, there must be an electric potential difference so the charges are able to
do something.
•
When a charge changes its location, there is a change in the electric potential energy.
PHYSICAL QUANTITY
Electric Potential difference ( ΔV )
DEFINITION
Difference between two different electric potentials.
MATH MODEL
ΔV = E · Δd
UNITS
Volts ( v )
PHYSICAL QUANTITY
Change in Electric Potential Energy ( ΔUq )
DEFINITION
Energy that changes when a charge moves from one place to another place.
MATH MODEL
ΔUq = ΔV · q
UNITS
Joules ( J )