Chapter 11
  Electricity
  By Keiji Oenoki and Hector Judez [physics@amersol.edu.pe]

We often experience the power of nature. One of the most fearsome natural phenomena is lightning. Some people might be curious why and how it occurs. Today, we know that lightning is a flow of electricity. In this chapter we are going to study about characteristics of electricity.

 
  1. Net Electrical Charge
  2. Conductors and Insulators
  3. Charged Objects
    1. Method of Charging
  4. Unit of Charge: The Coulomb
  5. Coulomb's Law
  6. Chapter Quiz

 

  Section
Section 1. Net Electrical Charge
  Electrons around an atom Matters are made of atoms. An atom is basically composed of three different components -- electrons, protons, and neutrons. An electron can be removed easily from an atom.
  Normally, an atom is electrically neutral, which means that there are equal numbers of protons and electrons. Positive charge of protons is balanced by negative charge of electrons. It has no net electrical charge.

When atoms gain or lose electrons, they are called "ions."

  • A positive ion is a cation that misses electron(s).
  • A negative ion is an anion that gains extra electron(s).

 

  Important!
repell and attract What is charge? Objects that exert electric forces are said to have charge. Charge is the source of electrical force. There are two kinds of electrical charges, positive and negative. Same charges (+ and +, or - and -) repel and opposite charges (+ and -) attract each other.

 

  The law of consevation of charge says that charge is neither created nor destroyed. This state ment brings us new question. What do you think about this statement?

 

  Section
Section 2. Conductors and Insulators



  Important!
Substances can be classified into three types -- insulators, conductors, and semiconductors.

Insulators are materials which allow very little electrical charges and heat energy to flow. Plastics, glass, dry air and wood are examples of insulators.

Conductors are materials which electrical charges and heat energy can be transmitted very easily. Almost all metals such as gold, silver, copper, iron, and lead are good conductors.

Semiconductors are materials which allow the electrical charges to flow better than insulators, but less than conductors. Examples are silicon and germanium.

 

 
Example Problem 1.

Which one of these is a conductor?
(a) dry air (b) lead (c) silicon (d) glass

(e.g. "a" )

 

  Section
Section 3. Charged Objects
  Net charges
  When two objects are rubbed together, some electrons from one object move to another object. For example, when a plastic bar is rubbed with fur, electrons will move from the fur to the plastic stick. Therefore, plastic bar will be negatively charged and the fur will be positively charged.

 

  a. Methods of Charging
  Charging by Induction
(Electroscope)		 When you bring a negatively charged object close to another object, electrons in the second object will be repelled from the first object. Therefore, that end will have a negative charge. This process is called charging by induction.
  When a negatively charged object touches a neutral body, electrons will spread on both objects and make both objects negatively charged. This process is called charging by conduction. The other case, positively charged object touching the neutral body, is just the same in principle. Charge by Conduction
(Electroscope)

 

  How does lightning start and end?

 

  Section
Section 4. Unit of Electrical Charge: The Coulomb " C "
  Important!
The symbol for electric charge is written q, -q or Q. The unit of electric charge is coulomb "C". The charge of one electron is equal to the charge of one proton, which is 1.6 * 10-19C. This number is given a symbol "e".

 

 
Example Problem 2.

How many electrons are there in 1 C of charge?
(e.g. "1.00*10^10")

 

  Section
Section 5. Coulomb's Law




  Important!
The magnitude of force that a particle exerts on another particle is directly proportional to the product of their charges and inversely proportional to the square of the distance between them.
Direction of the force between particles

where:

F is the force between the two particles,
q1 is the net charge on particle A,
q2 is the net charge on particle B,
d is the distance between the particles,
k is a proportionality constant which is 9.0 * 109 Nm2/C2.

The direction of the force is on the line from one particle to the other.

 

 
Example problem 3.

Object X has a positive charge of 5.0 * 10-6 C. Object Y, carrying a positive charge of 2.5 * 10-6 C, is 0.03 m away.

A. Calculate the force on X.
(e.g. "+1.0*10^10 N", or "-1.0*10^10 N")

B. What would the force be if the charge on Y were negative.

Solution

 

 
Example problem 4.

An object, A, with +8.25 * 10-6 C charge, has two other charges nearby. Object B, -3.5 * 10-6 C, is 0.030 m to the right. Object C, +2.50 * 10-6 C, is 0.050 m below. What is the net force and the angle on A?
(e.g. "1.0*10^10 N, 10 deg.")

The electric force is vector quantity. Try to solve the problem graphically.

Solution

 

  Section
Section 6. Chapter Quiz

Try Chapter 11 Quiz and see how much you learned.

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Colegio Franklin Delano Roosevelt
July 1997

Comments or Questions to physics@amersol.edu.pe