phy 電一問

Do not blindly remeber formulae (as this is a drawback of HK students), you need to understand the physical concept behind each equation.

The equation, E = Q/(4πεr^2) describes the electric surrounding a point charge, or a pherical charge. The electric field is not uniform, as the field lines extend radially outward from the charge. The separation between adjacent field lines increases with the increase of distance from the charge, hence a decrease of field intensity.

The equation, E = p/ε (I presume that p here stands for the surface charge density) descibes the electric field between two parallel plates, each carries equal magnitude of charges but with opposite polarity. The plates are with surface charge density p. Under such situation, the field is uniform and only depends on the maount of charge on each plate.

That is to say, the two equations are separately describing different electric fields. Both of them are right. The only thing is that you need to apply the right equation in the right situation.