 Lectures:  每週三、五, 第 3 、 4 節 (10:20~12:10), Feb.18 ~ Jun.19, 2009  April 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 ( A2) General Physics(A)(2) 台大物理 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)     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 PET, 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 figure 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 charge 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 that  And the magnitude of the electric field is  Directions?

Electric field examples – 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 from 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 electric 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 separated by a distance d.

Electric dipole moment  So we can define  is the electric dipole moment with magnitude given by and it is a vector pointing from – to + charge.

Molecular dipole  An electric dipole is a 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 hydrogen 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 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 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 aligned with the E field such that x

 Associated with the torque is a Potential Energy (U).  Define  U at a given  is the external work (W ext ) required to rotate the electric dipole from 90 degrees to  which equals the negative of the work W E done by the E field) 