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 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
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2. Last lecture:
There are 2 and only 2 types of electric charge. We proved that by observing the 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)
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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)
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4. Let’s start with the force:
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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
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6. The electric force is a vector
It has magnitude and direction (have to consider both)
When more than 1 force act simultaneously on 1 body, the forces add following the rules for vector addition
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.
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7. Electric field
Every charge creates electric field in every point in space. This is a VECTOR field
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8. 1/12/2007

9. Field of an electric dipole:
Do symbolic calculation on the board.
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