Electrostatics Flashcards

Electrostatics

There are four fundamental interactions in the universe:
- Gravity
- Strong Nuclear Force
- Weak Force
- Electromagnetic Force
All forces in the world can be attributed to these four interactions.

Strong Nuclear Force:
- Holds the particles of the nucleus of an atom together.
- Short-range attractive force.
- Much larger in magnitude than gravitational or electromagnetic forces.
Weak Force:
- Involved in the transmutation of particles within the nucleus.
- Observed only in radioactive decay.
- Stronger only than the gravitational force.
Gravitational Force:
- Attractive force that exists between all objects.
- The gravitational force between the Earth and the Moon keeps the Moon in orbit.
- The weakest of all the forces but most evident.
Electromagnetic Force:
- Charged particles at rest or in motion exert electric forces on each other.
- Gives materials their strength and ability to bend, squeeze, stretch, or shatter.
- When charged particles are in motion, they produce magnetic forces on each other.
- Electric and magnetic forces are both considered aspects of this single force.

Study of properties and results of electric charges at rest.
Atom is neutrally charged:
- Positive charge on the nucleus is exactly balanced by the negative charge of the electrons.
An electron can be removed from an atom to create a positive ion.
A freed electron can be:
- Unattached and Free – Creating a negative charged particle
- Attached to an atom – Creating a negative Ion

Two kinds of electric charges:
- Positive (protons - p+)
- Negative (electrons - e-)
Charges exert a force on other charges over a distance.
Like charges repel.
Unlike charges attract.
Electroscope: Instrument to determine charge.

A proton, electron, or an excess of protons or electrons.
Positively charged body: More positive charges (protons) than negative charges (electrons).
Negatively charged body: More negative charges (electrons) than positive charges (protons).
Fundamental property of matter.
Measured in a unit called Coulomb, abbreviated as C.
The Coulomb was named after French Physicist Charles Augustin de Coulomb (1736-1806).
There are 15 C of charge in a typical lightning bolt.
Charge is denoted by the letter q. (e.g., q = 15 C for a typical lightning bolt).
Charge is analogous to mass in a mechanical system.
Charge force law: Like charges repel, unlike charges attract.

Insulators: Materials that inhibit the flow of free charged particles.
- Examples: wood, air & rubber
Conductors: Materials that allow the flow of free charged particles.
- Examples: metal & water
Semiconductors: Intermediate class, conduction between an insulator and conductor.
- Examples: germanium, silicon, circuits, CPU’s, televisions, mobile phones

Charging by Friction:
- When you rub one material against another, they are charged by friction.
- The material losing electrons becomes positively charged, and the material gaining electrons becomes negatively charged.
- The amount of gained and lost electrons is equal to each other.
- The charges of the system are conserved.
Charging by Conduction:
- Charging a neutral body by touching it with a charged body.
Charging by Induction:
- Charging a neutral object by bringing a charged body close to but not touching the object.

Charles Coulomb (1738-1806) was a French physicist and military engineer.
Because of his expertise with simple machines, he was able to build an apparatus to measure the electrical force between two charged objects.
He derived Coulomb’s Law, which gives the relationship between charges, their separation, and the electrical force.

Coulomb’s Law states that the size of the electric force between two charged particles depends on the size of the charges and the distance between them.
q is a unit of charge measured in coulombs (C).
d or r is the distance between the charged objects.
K is a constant = 9.0 \times 10^9 Nm^2/C^2
The charge on one electron or one proton is called an elementary charge.
- e^- = -1.60 \times 10^{-19} C
- p^+ = +1.60 \times 10^{-19} C
One coulomb of charge has 6.25 \times 10^{18} electrons or protons
One lightning bolt may have 10 C of charge.
Example Calculations:
- -5.0 C = 3.1 \times 10^{19} e^-
- +1.0 C = 6.25 \times 10^{18} p

Robert Millikan was an American physicist who determined the charge on an electron using charged oil drop experiments in 1909 (1868-1953).

F_e = electrical force on the charged oil drop
W = weight of oil drop
F_e = qE
W = mg
F_e = W when the charged oil drop was suspended by the electric field.
qE = mg \implies q = \frac{mg}{E}
Millikan put a charge on a tiny drop of oil and measured how strong an applied electric field had to be in order to stop the oil drop from falling.
He worked out the mass of the oil drop and calculated the force of gravity on one drop, then determined the electric charge that the drop must have.
By varying the charge on different drops, he noticed that the charge was always a multiple of -1.6 \times 10^{-19} C, the charge on a single electron.
This meant that electrons were carrying this unit charge.
Millikan's oil-drop experiment was far superior to previous determinations of the charge of an electron because Millikan used single drops, first of water and then of oil.
Millikan emphasized that his data refuted conclusively the minority of scientists who still held that electrons (and perhaps atoms too) were not necessarily fundamental, discrete particles.
He provided a value for the electronic charge which, when inserted in Niels Bohr's theoretical formula for the hydrogen spectrum, accurately gave the Rydberg constant—the first and most convincing proof of Bohr's quantum theory of the atom.