Electric fields and charges 2

Question 1

BASIC PROPERTIES OF ELECTRIC
CHARGE

Answer 1

• There are two types of charges, namely
  positive and negative and their effects tend to cancel each
  other.
• If the sizes of charged bodies are very small as
  compared to the distances between them, we treat them
  as point charges.
• All the charge content of the body is
  assumed to be concentrated at one point in space.

Question 2

Conservation of charges

Answer 2

• When
  we rub two bodies, what one body gains in charge the other body loses.
  Within an isolated system consisting of many charged bodies, due to
  interactions among the bodies, charges may get redistributed but it is
  found that the total charge of the isolated system is always conserved.
  Conservation of charge has been established experimentally.
• It is not possible to create or destroy net charge carried by any isolated
  system although the charge carrying particles may be created or destroyed in a process. Sometimes nature creates charged particles: a neutron turns
  into a proton and an electron. The proton and electron thus created have
  equal and opposite charges and the total charge is zero before and after
  the creation.

Question 3

Quantisation of charge

Answer 3

• Experimentally it is established that all free charges are integral multiples
  of a basic unit of charge denoted by e. Thus charge q on a body is always
  given by
  q = ne
• where n is any integer, positive or negative. This basic unit of charge is
  the charge that an electron or proton carries.
• The fact that electric charge is always an integral multiple of e is termed
  as quantisation of charge.
• The quantisation of charge
  was first suggested by the experimental laws of electrolysis discovered by
  English experimentalist Faraday. It was experimentally demonstrated by
  Millikan in 1912.

Question 4

unit of charge

Answer 4

• C- Coloumb
• one coulomb is the charge
  flowing through a wire in 1 s if the current is 1 A (ampere).
• e = 1.602192 × 10^(–19) C

Question 5

s the charge on any
body is always an integral multiple of e and can be increased or
decreased also in steps of e. Justify

Answer 5

• If the protons and electrons are the only basic charges in the
  universe, all the observable charges have to be integral multiples of e.
  Thus, if a body contains n1
  electrons and n2
  protons, the total amount
  of charge on the body is
  n2× e + n1× (–e) = (n2– n1) e. Since n1
  and n2
  are integers, their difference is also an integer.
• Thus the charge on any
  body is always an integral multiple of e and can be increased or
  decreased also in steps of e.

Question 6

If 109
electrons move out of a body to another body
every second, how much time is required to get a total charge of 1 C
on the other body?

Answer 6

• In one second 109 electrons move out of the body. Therefore
  the charge given out in one second is 1.6 × 10–19 × 109 C = 1.6 × 10–10 C.
• The time required to accumulate a charge of 1 C can then be estimated
  to be 1 C ÷ (1.6 × 10–10 C/s) = 6.25 × 109
  s = 6.25 × 109 ÷ (365 × 24 ×
  3600) years = 198 years.
• Thus to collect a charge of one coulomb,
  from a body from which 109
  electrons move out every second, we will
  need approximately 200 years. One coulomb is, therefore, a very large
  unit for many practical purposes
• It is, however, also important to know what is roughly the number of
  electrons contained in a piece of one cubic centimetre of a material.
  A cubic piece of copper of side 1 cm contains about 2.5 × 1024
  electrons.

Question 7

How much positive and negative charge is there in a
cup of water?

Answer 7

• Let us assume that the mass of one cup of water is
  250 g. The molecular mass of water is 18g. Thus, one mole
  (= 6.02 × 1023 molecules) of water is 18 g. Therefore the number of
  molecules in one cup of water is (250/18) × 6.02 × 1023
  .
• Each molecule of water contains two hydrogen atoms and one oxygen
  atom, i.e., 10 electrons and 10 protons. Hence the total positive and
  total negative charge has the same magnitude. It is equal to
  (250/18) × 6.02 × 1023 × 10 × 1.6 × 10–19 C = 1.34 × 107
  C.