Electric fields and charges 1

Question 1

Static means

Answer 1

anything
that does not move or change with time.

Question 2

Electrostatics

Answer 2

Electrostatics deals with
the study of forces, fields and potentials arising from
static charges.

Question 3

Thales of Miletus, Greece,

Answer 3

• Historically the credit of discovery of the fact that amber rubbed with
  wool or silk cloth attracts light objects goes to Thales of Miletus, Greece,
  around 600 BC.
• The name electricity is coined from the Greek word elektron meaning amber.

Question 4

polarity of charge.

Answer 4

The bodies like
glass or plastic rods, silk, fur and pith balls are electrified. They acquire
an electric charge on rubbing. There are two kinds of electrification and
we find that (i) like charges repel and (ii) unlike charges attract each
other. The property which differentiates the two kinds of charges is called
the polarity of charge.

Question 5

the charges acquired after rubbing are lost when the charged
bodies are brought in contact. Justify.

Answer 5

• When a glass rod is rubbed with silk, the rod acquires one kind of
  charge and the silk acquires the second kind of charge. This is true for
  any pair of objects that are rubbed to be electrified. Now if the electrified
  glass rod is brought in contact with silk, with which it was rubbed, they
  no longer attract each other. They also do not attract or repel other light
  objects as they did on being electrified.
• ? It just tells us that unlike charges acquired by the objects
  neutralise or nullify each other’s effect. Therefore, the charges were named
  as positive and negative by the American scientist Benjamin Franklin.
• By convention, the charge on glass rod or cat’s fur is called positive and
  that on plastic rod or silk is termed negative. If an object possesses an
  electric charge, it is said to be electrified or charged. When it has no charge
  it is said to be electrically neutral.

Question 6

electroscope

Answer 6

• A simple apparatus to detect charge on a body is the gold-leaf
  electroscope.
• . It consists of a vertical metal rod housed in a
  box, with two thin gold leaves attached to its bottom end.
• When a charged
  object touches the metal knob at the top of the rod, charge flows on to
  the leaves and they diverge.
• The degree of divergance is an indicator of
  the amount of charge.

Question 7

the electric force is all pervasive and
it encompasses almost each and every field associated with our life. It is
therefore essential that we learn more about such a force. Justify

Answer 7

• all matter is made up of atoms and/or molecules.
• Although normally the
  materials are electrically neutral, they do contain charges; but their charges
  are exactly balanced.
• Forces that hold the molecules together, forces that
  hold atoms together in a solid, the adhesive force of glue, forces associated
  with surface tension, all are basically electrical in nature, arising from the
  forces between charged particles.

Question 8

When we say that a body is charged, we always refer to excess charge or deficit of charge. Justify

Answer 8

• In solids, some of the electrons, being
  less tightly bound in the atom, are the charges which are transferred
  from one body to the other. A body can thus be charged positively by
  losing some of its electrons. Similarly, a body can be charged negatively
  by gaining electrons.
• When we rub a glass rod with silk, some of the
  electrons from the rod are transferred to the silk cloth. Thus the rod gets
  positively charged and the silk gets negatively charged. No new charge is
  created in the process of rubbing. Also the number of electrons, that are
  transferred, is a very small fraction of the total number of electrons in the
  material body.

Question 9

Conductors

Answer 9

• Substances which allow electricity to pass through them easily are
  called conductors. They have electric charges (electrons) that are
  comparatively free to move inside the material. Metals, human and animal
  bodies and earth are conductors.
• When some charge is transferred to a conductor, it readily gets
  distributed over the entire surface of the conductor.

Question 10

Insulators

Answer 10

• Most of the non-metals like glass,
  porcelain, plastic, nylon, wood offer high resistance to the passage of
  electricity through them. They are called insulators.
• if some
  charge is put on an insulator, it stays at the same place.

Question 11

Semi - conductors

Answer 11

semiconductors, offer resistance to the
movement of charges which is intermediate between the conductors and insulators.