1/11/2008 J.Velkovska , PHYS117B 1

PHYS117B: Lecture 2 3 things to remember

about the electric charge Electric charge is a

fundamental property of matter

Electric charge is quantized: the smallest unit is

qe = 1.6 x 10-19 C The net electric charge is

conserved. Even if you create new particles converting energy to mass (like in the picture from RHIC) you always produce +/- particle pairs.

Au+Au ~ thousand charged particles , but the NET charge is conserved

1/11/2008 J.Velkovska , PHYS117B 2

Last lecture: There are 2 and only 2 types of electric charge. We proved that

by observing that neutral bodies do not interact with neutral bodies, while all other combinations (+/-,+/+,-/-,+/0,-/0) did. Note that this is NOT a trivial statement. Gravity has only 1 type of mass, the strong force that acts between the quarks and gluons inside the protons and the neutrons has color charges: there are red, blue, green and anti-red, anti-blue, anti-green (total of 6 color charges).

We learned how to charge an object ( by rubbing it we break some molecular bonds and sweep electrons from one body to the other) or by touching a neutral conductor with a charged conductor

We learned about polarization (in insulators) and induced charge( in conductors)

1/11/2008 J.Velkovska , PHYS117B 3

Electric Force and Electric Field Now it is time to get quantitative

How big is the force between two charges? How does it compare to other forces in nature ? What if we have more than 2 bodies (charges)

interacting ? How does the force change ? Principle of superposition

How does a charge “feel” another charge at a distance ? Introduce the electric field.

Calculate the electric field for different charge configurations ( we’ll start today and do more on Monday)

1/11/2008 J.Velkovska , PHYS117B 4

Let’s start with the force:

1/11/2008 J.Velkovska , PHYS117B 5

How big is the electric force compared to gravity ? Calculate the ratio of: Fc/Fg for 2 protons

Calculate the ratio of Fc/Fg for 1 proton and the Earth

Calculate Fc/Fg for 2 objects like the Earth Done on the blackboard . See also example 21.1

in the book

1/11/2008 J.Velkovska , PHYS117B 6

The electric force is a vector

1. It has magnitude and direction (have to consider both)

2. When more than 1 force act simultaneously on 1 body, the forces add following the rules for vector addition

3. There are applets on MP : you can play with an applet to get a feeling of how the electric force depends on distance, charge and how vectors add together.

1/11/2008 J.Velkovska , PHYS117B 7

Electric field Every charge creates electric field in every point in

space. This is a VECTOR field

1/11/2008 J.Velkovska , PHYS117B 8

1/11/2008 J.Velkovska , PHYS117B 9

Field of an electric dipole:

Use Coulomb’s law and the principle of superposition to find the electricfield in points a, b and c.