Electrostatics

Electrostatics

~branch of science that deals with the phenomena arising from

stationary or slow-moving electric charges.

Electric Charge

~Charge is the fundamental property of a matter that

exhibit electrostatic attraction or repulsion over other matter.

Two types of charges:•Positive [Cations]•Negative[Anions]

Electric Charge

Law of conservation of Charges:

Charge cannot be created nor destroyed; it can only

be transferred

Electric Charge

Law of conservation of Charges:

Q(t2)= Q(t1) + Qin-Qout

Electric Charge

Charged objects exert electric forces on each other:

Opposite charges attract;Like charges repel

Electric Charge

Units for Electric Charge

~The symbol for electric charge is usually a “q”.

~The elementary charge unit (e) is equal to the charge of a single proton or electron:

qproton = +1e

qelectron = –1e

~The elementary charge unit is not the standard SI unit.

~The SI system uses the Coulomb (C) as the standard unit of charge.

Units for Electric Charge

A Coulomb is defined as the number of electrons passing through a current of

1 Amp each second:C = A·s.

The conversion between C and e was determined by Robert A. Millikan:

1 e = 1.60×10–19 C.

Methods of Charging

•Friction •Polarization•Conduction•Induction

Polarization

When a charge (either + or –) is brought near a neutral object, the electrons and protons are pushed in opposite directions

Polarization

The two edges of the object acquire opposite charges. The object is

polarized.

Polarization

Friction

• Charging by Friction occurs when two different materials rub against each other, causing a transfer of electrons.

• The material with the greater affinity for electrons becomes negative, and the other material becomes positive.

Conduction

Charging by Conduction occurs when a neutral object is placed in contact with an already-charged

object.

Induction

Coulomb’s Law

q1q2

F= ke

r2 

The Electroscope

• The presence of excess electric charge can be detected by an electroscope. The term electroscope is given to instruments which serve two primary purposes:

1. to determine if a body is electrified

2. to determine the nature of the  electrification

The Electroscope

The Electroscope

• When the electroscope is uncharged, the leaves hang straight down.

The Electroscope

• When the knob is charged, electric charge travels along the rod and into the leaves. Then, the leaves repel each other because they have the same charge.

The Electroscope

Conductors and Insulators

• relative mobility of electrons within a material is known as

electric  conductivity• Conductivity is determined by the

types of atoms in a material and how the atoms are linked together

with one another.

Conductors and Insulators

• The electrons of different types of atoms have different degrees of freedom to move around.

• With some types of materials, such as metals, the outermost electrons in the atoms are so loosely bound that they chaotically move in the space between the atoms of that material by nothing more than the influence of room-temperature heat energy.

• Because these virtually unbound electrons are free to leave their respective atoms and float around in the space between adjacent atoms, they are often called free electrons.

Conductors and Insulators

• Materials with high electron mobility (many free electrons) are

called conductors• materials with low electron

mobility (few or no free electrons) are called insulators