Electric Charges and Fields Test 1

Insulators are those materials in which electron are tightly bound and cannot move freely at room temperature.

• The law of conservation of charge states that electric charge can neither be created nor destroyed.
• In a closed system, the amount of charge remains the same. When something changes its charge, it doesn't create a charge but transfers it.

► Current = Charge / Time

► Therefore Charge = Current. Time (unit of current = ampere, unit of time = second)

So, Charge = Ampere.Second

• The -ve charges of the glass rod get transferred to the silk cloth and hence itself becomes positively charged as there is the deficiency of -ve charge(electrons) on the glass rod due to their transfer.
• But the total charge of the system of rod and cloth remains zero.

Electric charge between two bodies can be produced by rubbing with each other. The body having lower work function loses electron and becomes positive and the other body gains electrons become negative.
Example: When glass is rubbed with silk, glass becomes positive while silk becomes negative.

This can be possible by induction, if a negative charge rod is placed near the sphere (at a distance not in touch) than it attracts positive charge inside the sphere towards its side and the negative charges collected on other side and when the surface of sphere connected to earth than the negative charge goes to earth and the sphere remain positively charged.

• If an object is positively charged it loses some of its electrons.
• The mass of an electron is 9.11*10^-31 kg.
• So, if a positively charged body loses ‘n’ number of electrons, it mass decrease by the amount n*9.11e^-31kg.

• The theoretical reason for making the conclusion that the total charge of the universe is constant is the Law of Conservation of Charge.
• Because of certain symmetries in the structure of the universe, the total electric charge of an isolated system is always conserved. This means that the total charge of an isolated system is the same at all points in time.

To make an uncharged object have a negative charge we must increase charge on particle and this can be done only by adding electrons to particle.

A substance is said to be a conductor if it conducts electricity. In the case of a conductor, the electrons are move in the opposite direction of the current, so it is able to conduct.

After earthing a positively charged conductor electrons flow from earth to conductor and if a negatively charged conductor is earthed then electrons flows from conductor to earth. 

Ions are formed when atoms gain or lose electrons. Gaining electrons changes an atom into a negative ion. Losing electrons changes an atom into a positive ion.

• On rubbing an rubber rod with wool, the electrons from the fur get transferred to the rubber rod.
• The wool now has a deficiency of electrons and so is positively-charged.
• On the other hand, the rubber rod has an excess of electrons and hence is negatively-charged.
• In the above example, the wool undergoes positive electrification.
• It is to be noted that it is the electrons and not the protons that get transferred during positive (and negative) electrification.
• Here, the transfer of electrons and mass transfer respectively takes place from wool to the rubber rod.

Here, n = 30 
► e = -1.6 x 10^-19 C
So, q = n ∗ e = 30 * (1.6 x 10^-19 C)
q = - 4.8 ∗ 10^-18 C

• He atom has 2 electrons.
• So, 1 mole of He has 2*N(N is Avogadro's no.) electrons.
• Then total -ve charge in 1 mole He gas is =  2 * N * charge of 1 electron
  = 2 * 6.022 * 10²³ * 1.6 * 10^-19 = 1.92 * 10⁵ C 

• When glass rod is rubbed with silk, glass happens to lose electrons easily and silk grabs them away from the glass atoms. So after rubbing, the glass becomes positively charged and the silk becomes negatively charged.
• Thus equal but opposite charge are produced on both. Hence, option D is correct.

According to Quantisation of Charge we know that,
Q = ne
Therefore, 1 coulomb of Charge = n * 1.6 * 10^-19 
⇒ 1/ 1.6 × 10^-19 = n
⇒ n = 6.25 × 10¹⁸

If we have two bodies with charge q₁ and q₂ on them and we are given q₁ + q₂ = 0. It means the net charge on the system is zero.

This is possible only if q₁ and q₂ are equal and opposite.

That is, q₁ = +q and q₂ = -q or vice versa. 

Such that, q₁ + q₂ = +q + (-q) = 0

During rubbing, glass rod acquires + ve charges and silk cloth acquires - ve charges.
∴ the charge on the glass rod is + 1.6×10^{- 19}C

• A body consists of atoms that have electrons, protons and neutrons. The electrons have a negative charge. The protons are positively charged and the neutrons are neutral.
• The atom is neutral because the positive and negative charges cancel out each other.
• On removing electrons, an excess of protons are present and thus the body is said to be positively charged.