《物理双语教学课件》Chapter 17 Electric Potential 电势

Chapter 17 Electric Potential

17.1 Electric Potential Energy

1. Newton ’s law for the gravitational force and Coulomb ’s law for the electrostatic force are mathematically identical . Thus the general feature we have discussed for the gravitational force should apply to the electrostatic force.

2. In particular, we can infer that the electrostatic force is a conservative force . Thus when that force acts between two or more charged particles within a system of particles, we can assign an electric potential energy U to the system.

3. Moreover, If the system changes its configuration from an initial state i to a different final state f, the electrostatic force does work W on the particles. The resulting change

U ∆ in the potential energy of the system is W U U U i f -=-=∆. As with other conservative forces, the work done by the electrostatic force is path independent.

4. For convenience, we usually take the reference configuration of a system of charged particles to be that in which the particles are all infinitely separated from each other. And we usually set the corresponding reference potential energy to be zero .

17.2 Electric Potential

1. The potential energy of a charged particle in an electric field depends on the magnitude of the charge. However, the potential energy per unit charge has a unique value at any point in the electric field. Thus the potential energy per unit charge, which can be symbolized as U/q, is independent of the charge q of the particle and is characteristic only as the electric field we are investigating. The potential energy per unit charge at a point in an electric field is called the electric potential V (or simply the potential) at that point . Thus q U V

=. Electric potential is a scalar, not a vector.

2. The electric potential difference V ∆ between any two points i and f in an electric field is equal to the difference in potential energy per unit charge between the two point : q W q U V V V i f -=∆=-=∆. The potential difference between two point is thus the negative of the work done by the electrostatic force per unit charge that move from one point to the other .

3. The SI unit for electric potential is the joule per coulomb. This combination occurs so often that a special unit, the volt (abbreviated V) is used to represent it.

4. One electron-volt (eV) is the energy equal to the work

required to move a single elementary charge e through a potential difference of exactly one volt, so J

1-

=.

⨯

.1

60

eV19

10

17.3 Equi-potential Surfaces

1.Adjacent points that have the same electric potential form an

equipotential surface, which can be either an imaginary surface or a real, physical surface. No net work W is done on a

charged particle by an

electric field when the

particle moves between

two points i and f on the

same equipotential surface. See the Figure.

2.Figure shows the electric field lines and cross sections of

equipotential surface for several cases. We can find that equipotential surfaces are always perpendicular to electric field lines and thus to E which is always tangent to these lines.