Physics
Chapter 20: Static Electricity
Section 20.1: Electric Charge
• Electrostatics is the study of electric charges that can be collected and held in one
place.
• Have you ever noticed the way that your hair is
attracted to the comb when you comb your hair on a
dry day or the way that your hair stands on end after it
is rubbed with a balloon?
• If so, you will recognize the attraction of the bits of
paper to a plastic ruler shown in the adjacent figure.
• There must be a new, relatively strong force causing
this upward acceleration because it is larger than the
downward acceleration caused by the gravitational
force of Earth.
A Microscopic View of Charge
• Electric charges exist within atoms.
• J.J. Thomson discovered that all materials contain light, negatively charged
particles that he called electrons.
• Ernest Rutherford discovered that the atom has a massive, positively charged
nucleus.
• When the positive charge of the nucleus equals the negative charge of the
surrounding electrons, then the atom is neutral.
• With the addition of energy, the outer electrons can be removed from atoms.
• An atom missing electrons has an overall positive charge, and consequently, any
matter made of these electron-deficient atoms is positively charged.
• The freed electrons can remain unattached or become attached to other atoms,
resulting in negatively charged particles.
• From a microscopic viewpoint, acquiring charge is a process of transferring
electrons.
• If two neutral objects are rubbed together, each can become charged.
• For instance, when rubber and wool are rubbed together, electrons from atoms on
the wool are transferred to the rubber, as shown in the figure below.
• The extra electrons on the rubber result in a net negative charge. The electrons
missing from the wool result in a net positive charge.
• The combined total charge of the two objects remains the same. Charge is
conserved, which is one way of saying that individual charges never are created
or destroyed.
• All that happens is that the positive and negative charges are separated through a
transfer of electrons
.
• Processes inside a thundercloud can cause the cloud bottom to become negatively
charged and the cloud top to become positively charged. In this case, charge is
not created, but separated.
Conductors and Insulators
• A material through which a charge will not move easily is called an electric
insulator
• Glass, dry wood, most plastics, cloth, and dry air are all good insulators.
• A material that allows charges to move about easily is called an electric
conductor.
• Electrons carry, or conduct, electric charge through the XXXXl.
• XXXXls like copper, iron, gold, aluminum etc … are good conductors because at
least one electron on each atom of the XXXXl can be removed easily.
• These electrons move freely
throughout the piece of XXXXl.
• The figure below contrasts how
charges behave when they are
placed on a conductor with
how they behave on an
insulator.
• Copper and aluminum are XXXX
excellent conductors and are
used commercially to carry electricity. Plasma, a highly ionized gas, and graphite
also are good conductors of electric charge.
• Air is an insulator.
• The spark that jumps between your finger and a doorknob after you have rubbed
your feet on a carpet discharges you.
• Similarly, lightning discharges a thundercloud. In XXXX of these cases, air became
a conductor for a brief moment.
• For a spark or lightning to occur, freely moving charged particles must be formed
in the normally neutral air.
• In the case of lightning, excess charges in the cloud and on the ground are great
enough to remove electrons from the molecules in the air.
• The electrons and positively or negatively charged atoms form plasma, which is a
conductor.
• The discharge of Earth and the thundercloud by means of this conductor forms a
luminous arc called lightning.
• In the case of your finger and the doorknob, the discharge is called a spark.
Answer the following questions:
1- What will happen if you rub two identical plastic rulers with a woolen cloth and bring
them together?
A. The rulers will attract each other.
B. The rulers will repel each other.
C. There will neither attract nor repel.
D. The rulers will partially attract and partially repel.
Answer :B
2- Which of the following statements about charges is true?
A. Charges are created by gaining electrons.
B. Charges are destroyed by losing electrons.
C. Charges are separated through a transfer of electrons.
D. Charges cannot be created, destroyed, or separated.
Answer : C
3- A negative charge is generated in a rubber rod by rubbing it with wool. When two
materials A and B are brought near the rubber rod, material A is attracted to the rod,
whereas material B is repelled. What can you say about the charges on the two materials?
A. A is positively charged, while B is negatively charged.
B. XXXX A and B are negatively charged.
C. XXXX A and B are positively charged.
D. B is positively charged, while A is negatively charged.
Answer: A