Chapter 3: Electric Forces and Fields

It deals with electric charges and fields associated with it.

Electric Charge

1. In an isolated system, the charge is always conserved.

2. Protons and electrons have a quality called electric charge.

3. The charge is invariant in nature.

4. The charge is quantized.

• (Q = n e)

  • e = 1.6 * 10^-19 C

  • n = no. of electrons

  • Q = charge

Ionisation

It involves addition or removal of electrons.

Coulomb’s Law

The electric force between two particles with charges q1 and q2 separated by distance r has a magnitude by the equation:

F = Kq1q2/r^2

• F = force

• K = coulomb’s constant

• q1 and q2 = charges

• r = distance between the charges

Addition of forces

Consider three point charges: q1, q2, and q3. The total electric force acting on, say, is simply the sum of F1-on-2, the electric force on q2 due to q1, and F3-on-2, the electric force on q2 due to q3:

Electric Field

The space is surrounded by a charge in which another charged particle experiences the force.

***E = F on q/ q***

It describes the electric field vector from the force vector on a positive charge.

Electric field due to a point charge

The electric field surrounding the point charge is:

E = 1/4πε0 * Q/r^2

• E = electric field

• Q = charge

• r = distance between charges

• ε0 = permittivity of free space

Three types of electric field

• Radial field

• It is generated by a collection of point charges.

• An infinite sheet of charge.

electric field lines

• The electric fields follow the same addition properties as the electric force.

• The electric field lines never cross.

The uniform electric field

• A lot of problems deal with the uniform electric field.

• The field may be taken as uniform at least in the middle.

• The uniform field just signifies the constant force.

Conductors

Materials which allow the flow of excess charge without resisting it.

Insulators

Materials that resist the flow of electrons.

Charging by friction

It involves rubbing the insulator against another material, thereby stripping electrons from one to another material.

Charging through conduction

When we connect two conductors charge flows from one to another until the potential of both the conductors becomes the same.

Charging through induction

The process of charging by induction may be used to redistribute charges among a pair of neutrally charged spheres.

If the sphere is an insulator made up of glass

• There aren’t any free electrons.

• The atoms make up the sphere will become polarised.

charge of proton

positive

charge of electron

negative

law of charges

the directions of the electric forces on the charges of mutual interaction; like charges repel, opposite charges attract.

net charge

an object with an excess of positive or negative charges

electrostatic charging

accomplished by Friction, Contact, Induction, or Polarization

Charging by Polarization

Charging by Polarization

How do objects become charged?

By gaining/losing electrons

Electric charge is always _______.

Conserved

What is the numerical value of one charge?

1 e = 1.6 x 10^-19 Coulombs

The SI unit of a charge is in ______.

coulombs

What happens when an insulator is charged?

Only the small spot which was directly contacted with a charge remains charged.

What is the name of materials that contain properties somewhere between conductors and insulators?

semiconductors

A dipole consists of:

two equal and opposite charged

In what direction to field lines go?

From positive to negative charges ALWAYS

What indicated field strength?

The density of field lines

What do few field lines between charges indicate?

a weak field

Which one of the following rules, laws, or principles describes how the net electric charge of an isolated system undergoing any process remains constant?

law of conservation of electric charge