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This lecture covers the concept of static electricity, Coulomb's Law, properties of electric charges, electric fields, electric dipoles, and their interactions with external electric fields. The lecture also discusses the practical applications of electric fields in ink jet printers.
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普通物理學甲下(202 101A2) General Physics(A)(2) 台大物理 吳俊輝 • 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
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) • e0is 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 • 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. Net electric dipole
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 q is the external work (Wext) required to rotate the electric dipole from 90 degrees to q (which equals the negative of the work WE done by the E field):
