Tuesday, January 19, 1999
Announcements:  Homework Schedule pushed back by one week due to the snow day. Lab Schedule will remain the same.

Lecture notes:

• All objects tend to contain equal amount of negative and positive charges and stay electrically neutral
• It is possible for an object to pick up an excess of negative charges ( electrons) and become negatively charged. It is also possible for an object to shed some of its electrons and become positively charged
• The smallest quantity of charge to be transferred is that contained in an electron (-e = 1.6 x 10^-19Coulomb)
• The amount of charge to be transferred comes in "quantized" units of that in an electron
• q=ne
• n= +1, +2, +3 ....
• The number of electrons picked up or shed by an object can be very large ( n= 10⁹ to 10¹⁰) but it is still very small compared to the total number of electrons and protons in an object.
• example: a penny contains about 8.6 x 10²³ electrons and protons
• The amount of charge contained in a proton is positive and has a magnitude equal to that of an electron.
• In SI units, charges are measured in units of Coulomb (C)
• 1 coulomb is the amount of charge that flows through the cross section of a wire in 1 second when there is a current of 1 ampere [to be defined in a later chapter]
• The amount of charge carried by an electron is -1.6 x 10^-19C
• 1 Coulomb = the amount of charge in 6.25 x 10¹⁸ electrons
• In some materials, electrons move (or flow) rather easily, these are called conductors
• e.g.  metals
• In other materials ( glasses and plastics) electrons cannot move easily. These are insulators.
• Semiconductors ( silicone and germanium) are intermediate between conductors and insulators. The motion of electrons in semiconductors can be manipulated much more intricately. These are therefore important for modern electronics.

• Let us consider a case when the charged objects are very small and can be considered as particles.
• particle 1 has a charge q₁
• particle 2 has a charge q₂
• It was found...

• F₁₂= the force of 2 on 1
• How do you find the magnitude of this force?
• F= F₁₂= F₂₁= k q1q2
                                        r²
• q1 is the charge on the first particle
• q2 is the charge of the second particle
• r is the distance separating the two
• k is a constant equal to 8.99 x 10⁹ Nm²/C²
• Coulomb's law is also written F = 1       x   q1q2

• e_o is the permittivity constant which is equal to 8.85 x 10^-12 C²/Nm²

• The first form of Coulomb's Law is "identical" to the force due to gravitation between two masses
• F= G m1m2
                    r²
• Gravitational force is always attractive whereas electrostatic force can be either attractive or repulsive
• Charges must be added vectorially; they must be broken up into x and y components to be added.
• If the charged object is a sphere and if the charges on the sphere is uniformly distributed, then, the force exerted by q₁ on q₂ ( and vice-versa) can be calculated and it is the same as if the charges in q₁ is concentrated at the center of the sphere.

• If the sphere is a conductor, then all the charges will be uniformly distributed at the external surface.
• Why?
• If the charges are all the same, they will repel each other and want to get as far away from each other as is possible. The farthest away they can get is the outside edge of the sphere.