《Electromagnetism, principles and application》教学课件（PPT讲稿）Chapter 2 Fields of Stationary Electric Charges

enormous magnitude Fn≈9×10 Qaq r while the gravitational force btwn masses ma and mb lamb=6.67 X 10-11mam 2 For instance, two protons(Q=e=1.6x 10-C, mp=1.67x10kg )at a distance of one meter Coulomb force Fo10-28N gravitational force: F N 10-N The Coulomb force is roughly 1036 times greater than the gravitational force But, we have experienced much more influence of grav ity than Coulomb force. Why? The Universe is presumedly neutral--cosmology

2.2 The electric field intensity e Definition: the electric field intensity due to the point charge Qa is ab Q2 ar. Qb4m∈0r Explanation (1)it's the force exerted on a unit test charge; (2)E has a unit volt/meter (3)Ea is a vector field over the whole space; (4 )Ea exists as long as Qa is present, regardless of Qb(important) (5)It' very simple to get the force acting on Q F=QE

2. 3 The principle of superposition If more than one charge Qi,(i= 1, 2 …)are tributed each generates a field e The resultant e is simply the vector sum of individual E Q2 E(r)=2Ei 24丌or-;/2(r-ri)1, where(r-rill is a unit vector pointing from ri to r This is called the principle of superposition Explanation experimentally set up; (2 ) theoretically, the Maxwell theory(equations )is a linear theory; (so electromagnetism is easy to study, in comparison with other fundamental forces