• Coulomb’s Law and the SI units.
• Coulomb’s Law: Suppose that 2 point charges, q and q’ are distance r apart in vacuum. If q and q’ have the same sign (+ and + or - and -) the 2 charges will repel each other. If q and q’ have opposite sign (+ and - or - and +) then they will attract each other. The Force experienced by one charge due to the other is given by Coulomb’s Law, F=k(qq’ ÷ r˛ ) where k is positive constant.
• SI units: In the SI the unit of charge is coulomb (C). to avoid the small # in practical work the 1µC=10^-6C and 1nC=10^-9C
- The fundamental smallest charge found in nature is denoted e. It value is 1.60219x10^-19 C
- The electron has charge – e, while the proton charge is +e
- The coulomb constant k = 8.988 x 10^9 N.m2/C2 = 9 x 10^9 N.m^2/C^2 or k= 1/(4*3.14*Eo) where Eo=8.85 x 10-12 C^2/N.m^2
• Electric fields at the point in space.
• The electric field E at the point is the vector force experienced by the unit positive test charge placed at the point. The units of electric field E is N/C. To find the electric field E due to the point charge q, we must use Coulomb’s Law. If a point charge q’ is placed at the distance r from the charge q it will experience a force F =1/(4*3,14*Eo) x (qq’ ÷ r˛)
*Summary of Electric Potential:
• The electric potential is the electric potential energy at the point P = minus the work done by the electric forces or the plus the work done against these force in taking an object (in this case the charge q) from given reference point to point P.
• If the electric field is due to a single charge q1 and the reference point for potential energy is r=infinite, we have F=qE=[(kqq1)/r12] r1. The r1 is the point from q to q1
• The unit of electric potential is volt, where 1V=1J/C
*****************************
Here is good article talking about The History of Electricity when we were visited the website at: http://historia.et.tudelft.nl/wggesch/geschiedenis/electricity/
Electricity is a form of energy, a phenomenon that is a result of the existence of electrical charge. The theory of electricity and its inseparable effect, magnetism, is probably the most accurate and complete of all scientific theories. The understanding of electricity has led to the invention of motors, generators, telephones, radio and television, X-ray devices, computers, and nuclear energy systems. Electricity is a necessity to modern civilization.
Electric Charge
Amber is a yellowish, translucent mineral. As early as 600 BC the Greeks were aware of its peculiar property: when rubbed with a piece of fur, amber develops the ability to attract small pieces of material such as feathers. For centuries this strange, inexplicable property was thought to be unique to amber.
Two thousand years later, in the 16th century, William Gilbert proved that many other substances are electric (from the Greek word for amber, elektron) and that they have two electrical effects. When rubbed with fur, amber acquires resinous electricity; glass, however, when rubbed with silk, acquires vitreous electricity. Electricity repels the same kind and attracts the opposite kind of electricity. Scientists thought that the friction actually created the electricity (their word for charge). They did not realize that an equal amount of opposite electricity remained on the fur or silk.
In 1747, Benjamin Franklin in America and William Watson (1715-87) in England independently reached the same conclusion: all materials possess a single kind of electrical "fluid" that can penetrate matter freely but that can be neither created nor destroyed. The action of rubbing merely transfers the fluid from one body to another, electrifying both. Franklin and Watson originated the principle of conservation of charge: the total quantity of electricity in an insulated system is constant.
Franklin defined the fluid, which corresponded to vitreous electricity, as positive and the lack of fluid as negative. Therefore, according to Franklin, the direction of flow was from positive to negative--the opposite of what is now known to be true. A subsequent two-fluid theory was developed, according to which samples of the same type attract, whereas those of opposite types repel.
Benjamin Franklin (1706-90) was an American printer, author, philosopher, diplomat, scientist, and inventor. (The Bettmann Archive)
*****************************
Exercise
1. A static electric field E gives rise to a conservative force: When a charge of 1C moves through a potential difference of +1 volt (V), It loses exactly 1J of kinetic energy. Show that the units of E may be stated as V/m.
Since 1V=1j/C and 1J = 1N.m
| N | = | j /m | = | j / C | = | V |
| C | | C | | m | | m |
2. Tow metal spheres are located 100km apart ( about 60mil) in space. One sphere carries as a surface charge all the electrons, and the other all the positive nuclei, in one cubic centimeter of copper. What is the force of attraction between the spheres? Copper has atomic weight 63.54, atomic number 29, density 8.96*10^3 kg/m^3, Avogadro’s number is 6.022*10^23 atoms/mol.
The number of atoms in one cubic centimeter is:
(8.96/63.54)(6.022*10^23) = 8.49 *10^22
The total negative charge of electrons is then
(8.49*10^22)(29)(1.6 *10^-19) = 3.94 * 10^5 C
which, of course, is also the total positive charge of the nuclei. Hence, the
force of attraction is
F = (9*10^9)((3.94*10^5)^2) /((10^5)^2 = 1.4*10^11 N or about 16
Million tons.
************************
"SOME IDEAS ABOUT ELECTRIC CHARGE:
We are studying about electric charge and electric
field. Everyday we touch on the electric charge around
us, but we forget to explain how does it work. We know
everything has a charge. Even if it is considered a
neutral charge, it still has a charge value of zero.
When you examine an atom, the nucleus holds the
positive charge. That's because the nucleus holds the
protons for the atom, protons have a positive charge (
this is the answer for question #1 in practice midterm
last week) The electrons which circle the nucleus
balance out that positive charge. Each electron holds
onto one negative charge. The charged objects in
electricity are either too many or too few electrons
for one atom. If there are not enough, that means
there are more protons and a positive charge develops.
If there are too many electrons, the atom has a
negative charge. When an atom has a charge, we call
"ion".
Some problems in our Physic 4B text book talk
about static charge as we rub fur on glass rods. When
we do that rubbing, we are actually rubbing electrons
off of one object and onto another. It's called static
electricity because it's just sitting there. No
current is flowing. There is an excess of positive on
one side, and an excess of negative of the other side.
If we connect the two, the charges can flow from one
object to the other. When that flow happens you make a
small current. So we can mention that, the charge
cause of the current between two objects.
Finally, the charge is very important for human
life, because we need electricity and charge is the
first step of electricity. "
************************
Answer and explanation to some of the Practice exam 1
1- Proton carry positive charges.
2-If we glass rod and silk together . then the rod lose some electron and become positive and the silk gain electron. Now it have excess electron and become negative.
3- If we have a negative charge object and put it close to metal with no charge .t he positive charge come closer and negative charge on metal run away as far as they can.
5- Internal electric field is zero because the value acting o n each become zero.
6- Number of electron and proton always equal in a atom.
7-electron will never attract each other, in fact the repel each other.
8- F=k*q1*q2/r^2 there for r=square root of k*q1*q2/F
9*10^9*15*10^-3*10.3*10^-6/0.40 =59 m
9- positive charge will radiate outward.
11-A good conductor will have same potienitial because it will carry the flow easily.
12- THE NET WORK?
Delta U=Uf-Ui= 50-(-50)= 100 v
NET WORK=e* total U = - 1.6*10^-18
14- Q1+Q2+Q3 =11 Because the abs value.
V= K*Q Total/r =1.0*10^5 V
15-SAME FOR AS 14
16-
V= U/Q=10/2= 5 J/C
20-ELECTRIC DIPOLE HAS SAME MAGNITUDE BUT DIFFERENT SIGN CHARGES.
************************
If you have any question please send us an E-mail
Last Updated: Sunday, February 10, 2002 - 2:06 p.m. Pacific Time
