01.Define Charge density, Electric field.
Charge density:
In electromagnetism, charge density is the amount of electric charge per unit length, surface area, or volume. Volume charge density (symbolized by the Greek letter ρ) is the quantity of charge per unit volume, measured in the SI system in coulombs per cubic meter (C⋅m−3), at any point in a volume.
Electric field:
An electric field(বৈদ্যুতিক ক্ষেত্র) surrounds an electric charge, and exerts force (বল প্রয়োগ)on other charges in the field, attracting or repelling them.Electric fields are created by electric charges, or by time-varying magnetic fields. Electric fields and magnetic fields are both manifestations of the electromagnetic force, one of the four fundamental forces (or interactions=পারস্পরিক ক্রিয়ার) of nature.
Electric fields are important in many areas of physics, and are exploited practically in electrical technology. On an atomic scale, the electric field is responsible for the attractive force between the atomic nucleus and electrons that holds atoms together, and the forces between atoms that cause chemical bonding.
The electric field is defined mathematically as a vector field that associates to each point in space the (electrostatic or Coulomb) force per unit of charge exerted on an infinitesimal positive test charge at rest at that point. The SI unit for electric field strength is volt per meter (V/m), exactly equivalent to newton per coulomb (N/C) in the SI system.
02.Define electric dipole moment.
An electric dipole is defined by the first-order term of the multipole expansion(সম্প্রসারণ); it consists of two equal and opposite charges that are infinitesimally(অসীম কাছাকাছি) close together, although real dipoles have separated charge. However, when making measurements at a distance much larger than the charge separation, the dipole gives a good approximation(পড়তা) of the actual electric field. The dipole is represented by a vector from the negative charge towards the positive charge.The electric dipole moment is a measure of the separation of positive and negative electrical charges within a system, that is, a measure of the system's overall polarity. The electric dipole moment for a pair of opposite charges of magnitude q is defined as the magnitude of the charge times the distance between them and the defined direction is toward the positive charge. It is a useful concept in atoms and molecules where the effects of charge separation are measurable, but the distances between the charges are too small to be easily measurable. It is also a useful concept in dielectrics and other applications in solid and liquid materials.