GEN PHYS 2 | WW1 Reviewer

Charges’ Behavior

• There are two charges: one positive and negative.

• Similar charges repel while opposite ones attract.

Ionization

• causes charges to occur

• when total number of protons equal the ones of the electrons, the total charge is zero

• excess negative charge is obtained through changing the number of positive charge through transferring electrons, protons do not move as they are in the nucleus.

• a positively charged atom is the complement of a negatively charged one

Proton

• positive charge

• has a mass of 1.673 × 10^-27

Neutron

• zero charge

• has a mass of 1.675 × 10^-27

Electron

• negative charge

• has a mass of 9.109 × 10^-31

Conservation of Charge

• algebraic sum of all charges in a closed system is constant

• charge is neither destroyed nor created, merely transferred

• no experimental evidence has violated this

Quantization of Charge

• magnitude of an electric charge is an integer multiple of e = 1.6 × 10^-19

• known as elementary charge

Conductors

permit electric charges to move easily from one region of material to another

Insulator

prevent charges from leaving material

Semiconductors

intermediate to properties between conductors and insulators

Ways to Charge Objects

• rubbing/friction

• conduction/contact

• induction

Charging by Friction

When two different materials are rubbed together, electrons are transferred from one material to another. The object that loses electrons becomes positively charged, while the one that gains electrons becomes negatively charged.

Charging by Conduction

A charged object touches a neutral conductor, transferring electrons by direct contact. Both objects end up with the same type of charge.

Charging by Induction

A method of charging an object without direct contact. A charged object brought near a conductor causes charges in the conductor to rearrange, leading to induced charges.

When a charged object is far away from an uncharged one, what happens?

They attract each other.

When a charged object is touching an uncharged one, what happens?

They repel.

Polarization

occurs when charged objects cause slight shifting if charges within the molecules of a neutral object

Coulumb’s Law

Charles Augustin de Coulumb studied the interaction of charged particles i 1784. This law states that the magnitude of the force applied by an electric charge is

• directly proportional to the product of the charges

• inversly proportional to the square of the distance between them

Value of Coulumb’s Constant

k = 9.00 × 10⁹

Number of electrons in 1 Coulumb

1 C = 6.25 × 10¹⁸

Superposition Principle

Net electric force is the vector sum of all forces acting on the charge.

Electric Field

• how charges exert force

• region where a charge experiences electric force

• the closer the test charge to the source, the stronger the electric field

• in N/C (newtons over coulumbs)

Source Point

location of a charge in an electric field, determines the strength of an electric field

Field Point

point where the field is determined to be, commonly denoted as P

Direction of the Electric Field

By convention, it is the direction of the electrical force on a small positive test charge placed at that point.

Positive Source Charge

It leads to an outward electric field.

Negative Source Charge

It leads to an inward electric field.

What happens if the source point is positive and the field point is positive?

The electric field points away from the source point.

What happens if the source point is positive and the field point is negative?

The electric field points away from the source point.

What happens if the source point is negative and the field point is positive?

The electric field points towards the source point.

What happens if the source point is negative and the field point is negative?

The electric field points away from the source point.

Electric Field Representation

• Field lines point away from positive charges and toward negative charges.

• The electric field vector is tangent to the field line passing through that point.

• Field lines are close together where the field is strong, farther apart when it is weaker.

When does a test charge experience an electric field?

A test charge experiences an electric field in all cases except when:

• the source charge has no magnitude

• the distance between source charge and field point is infinitely large

Electric Flux

• the measure of the electric field passing through a given surface area

• proportional to the number of field lines penetrating some surface

• has the units Nm²/C

Gauss’s Law

the total electric flux through a closed surface is proportional to the charge enclosed by the surface

Electric Dipole

• consists of two equal and opposite charges separated by a distance

• The electric field at points far away from the dipole is proportional to the dipole moment (p = qd) and decreases with distance.