普物甲下

1、Lectures:每週三、五, 第3、4節(jié) (10:20~12:10), Feb.18 ~ Jun.19, 2009April 3 (Friday) 溫書假, May 29 (Friday) 彈性放假Exams:期中考：April 15/17 (10:20~12:10), 2009期末考：June 17/19 (10:20~12:10), 2009,普通物理學甲下 (202 101A2)General Physics(A

2、)(2)臺大物理　吳俊輝,Electricity I,Coulomb’s Law,Static electricity is analogous to gravity.It was found that “charges” can attract or repel each other according to a force law – Coulomb’s Law.SI unit of charge is coulomb (C)

3、 e0 is the permittivity constant.,,Some properties of electric charges,Charge is quantized in the units of e, the electric charge ( ). Charge is conserved (in a reaction)In P

4、ET, we saw the annihilation reaction There is also a reaction called the pair production in which a photon is converted into an electron-positron pair,,The concept of a field,In static electricity, it is useful to fi

5、gure out the electric effect of a charge distribution, so the concept of an electric field is introduced.Basically, an electric field is defined to be the force acting on a small + test charge when the test charg

6、e is placed near another charge distribution.,,Electric field of a point charge,In the case of a point charge, we know from Coulomb’s Law thatAnd the magnitude of the electric field is Directions?,Electric field ex

7、amples – a pairs of + charges and a pair of + and - charge,Electric field example – large charged conducting plane,The electric field generated by a large + charged conducting plate is perpendicular and directed away fro

8、m the conducting plane.,Electric field and ink jet printer,Operational principles of an ink jet printer:1. Generator G shoot out ink drops.2. Charging unit C charges the ink drops to different levels.3. The electri

9、c field of the deflecting plates then direct the ink drops to a position on the paper that depends on the amount of charge on the ink drops.,Electric dipole –a very important example,Consider a pair of + and – charges s

10、eparated by a distance d.,Electric dipole moment,So we can define is the electric dipole moment with magnitude given byand it is a vector pointing from – to + charge.,,,Molecular dipole,An electric dipole is a

11、very important model because it is the “simplest” way to describe charge distribution in a system.For example, in a water molecule, there is a net electric dipole moment pointing away from the oxygen atom toward the hyd

12、rogen atoms.When we get to quantum mechanics, we will see that the interaction of a molecule with photons require the use of electric dipole.,Electric dipole,,,,,,,,,,,,,,,,,,,,,,Net electric dipole,Suppose we have the

13、given charge distribution. Although it looks complicated, we know that the sum of all electric dipoles will give rise to a net electric dipole that points somewhat towards the right.So the electric dipole is an easy and

14、 convenient way for us to understand the electric properties of a charge distribution.,Electric dipole in an external E field,The torque on the dipole:Basically the torque will rotate the electric dipole until it is

15、 aligned with the E field such that,,x,,Associated with the torque is a Potential Energy (U).Define U at a given q is the external work (Wext) required to rotate the electric dipole from 90 degrees to q (which equals