22 April 2012

Electric Charge & Electric Field


Electric Charge :

Charge is the fundamental property of forms of matter that exhibit electrostatic attraction or repulsion in the presence of other matter. The SI unit of electric charge is the coulomb (C).

Twentieth-century experiments demonstrated that electric charge is quantized; that is, it comes in multiples of individual small units called the elementary charge, e, approximately equal to1.602×10−19 coulombs (except for particles called quarks, which have charges that are multiples of e). The proton has a charge of e, and the electron has a charge of −e

The coulomb is defined as the quantity of charge that has passed through the cross section of an electrical conductor carrying one ampere within one second. The symbol Q is often used to denote a quantity of electricity or charge. One coulomb charge means deficiency of 6.25×1018 electrons from a neutral body.
The quantity of electric charge can be directly measured with an electrometer, or indirectly measured with a ballistic galvanometer.

Electric field :

The concept of an electric field was introduced by Michael Faraday.

The electric field is a region in which a charged particle experiences a force.

The electric field strength is a vector quantity. Its direction is away from a positive charge and towards the negative charge. 

Its unit is newton per coulomb (N C−1) or volt per metre (V m−1).

The SI base units of the electric field are kg•m•s−3•A−1.

The electric field strength (E) at any point is defined as the force experienced by per unit infinitesimal positive charge when placed at the point.

Questions and Answers:

1. The quantity electric potential is defined as the amount of _____.
a. electric potential energy      b. force acting upon a charge c. potential energy per charge                  d. force per charge

Answer: C [Electric potential is the amount of potential energy per unit of charge]
2. Complete the following statement:
When work is done on a positive test charge by an external force to move it from one location to another, potential energy _________ (increases, decreases) and electric potential _________ (increases, decreases).

Answer: increases and increases.