【双语物理】静电场_1_electric field.docx

Electric Charge Benjamin Franklin’s experiments: He rubbed a glass rod with silk and called the charges on the glass rod positive. He rubbed sealing wax with fur and called the charge on the sealing wax negative. Like charges repel and opposite charges attract each other. Unit: Coulomb (C)The smallest unit of “free” charge known in nature is the charge of an electron or proton, which has a magnitude ofe= 1.602×10-19C Charge of any ordinary matter is quantized in integral multiples of e. In a closed system, the total amount of charge is conserved since charge can neither be created nor destroyed. A charge can, however, be transferred from one body to another. Charge DensityVolume ρ Surface ? Line λCoulombs Law Consider a system of two point charges, q1 and q2, separated by a distance r in vacuum. The force exerted by q1 on q2 is given by Coulombs law: where k is the Coulomb constant, and is a unit vector directed from q1 to q2.?0 is known as the permittivity of free space.Electric FieldMichael Faraday invented field theory; drawing “lines of force” or “field lines” was his way of representing the fields. He also used his drawings of the lines of force to gain insight into the stresses that the fields transmit. He was the first to suggest that these fields, which exist continuously in the space between charged objects, transmit the stresses that result in forces between the objects.-Force on a charged particle in an electric field: F=qE-The electric field is defined as: , to qualify the strength of the field.-One charge creates a field which in turn acts on the other charge. -The normal component of the electric field exhibits discontinuity, with, when crossing a boundary with surface charge density σ.Electric Field Lines The field lines must begin on positive charges (or at infinity) and then terminate on negative charges (or at infinity). -Notice that the direction of field lines is radially outward for a positive charge and radially inward for a negative charge