Test: Electric Charge

Insulators are materials that impede the free flow of electrons from atom to atom and molecule to molecule. Examples of insulators include plastics, paper, rubber, glass and dry air.

In contrast to insulators, Conductors are materials that permit electrons to flow freely from particle to particle. Examples of conductors include metals, aqueous solutions of salts (i.e., ionic compounds dissolved in water), graphite, and the human body.

• 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.

Therefore, the correct answer is the algebraic sum of charges in any closed system is constant. 

We know that Current = Charge / Time

Also, 

Current = Ampere

Charge = Coulomb

Time = Second

1 ampere = 1 coulomb/ 1 sec

So, one coulomb is ampere times second.

When the glass rod is rubbed with silk cloth, glass quickly loses electrons, and silk takes electrons out of the glass rod. So after rubbing, the glass rod becomes positively charged and the silk gets a negative charge.

Silk cloths: Negatively charged

Glass rod: Positively charged

• Electric charge between two bodies can be produced by rubbing with each other. This is known as triboelectric effect.
• 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) then it attracts positive charge inside the sphere towards its side and the negative charges collected on the other side and when the surface of the sphere connected to the earth than the negative charge goes to earth and the sphere remain positively charged.

Since rubbing the metal ball with fur and a piece of silk cloth involves touching, they can not be the correct answer. 

Therefore, the correct answer is C. 

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

According to the principle of conservation of charge, the net amount of positive charge minus the net amount of negative charge in the universe is always constant.

Thus, the correct answer is the total charge of the universe is constant.

To make an uncharged object have a negative charge we must increase negative charge particles in it and this can be done only by adding electrons to particles as electrons have negative charge.

So, the correct answer is to add some electrons. 

• A substance is said to be a conductor if it conducts electricity.
• When placed in an electric field, the electrons move in the opposite direction of the current, so it is able to conduct.

Explanation: 

• A charged conductor is placed in an external electric field.
• The charge is distributed on the surface of the conductor.
• The free electrons move opposite the direction of the external electric field.
• Thus more charges are induced on the surface of the conductor due to this movement of free electrons.
• The free electrons move until the force due to the external field is negated completely due to the force by induced charges.
• At this point, the net force on a free electron inside the conductor is zero.
• Hence, making the net electric field inside the conductor is zero.

• When a negatively charged body is grounded electrons from the body flow to the earth and the body becomes neutral.
• On the other hand, when a positively charged body is grounded electrons coming from the earth neutralize the body.

Additional Information: Earth is an electric conductor. Earthing is defined as “the process in which the instantaneous discharge of the electrical energy takes place by transferring charges directly to the earth through low resistance wire.” When a charged body is connected to the earth, it becomes electrically neutral. 

• 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.

Therefore, the correct answer is C. 

• On rubbing a rubber rod with wool, the electrons from the wool 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
We know that q = n ∗ e

Therefore, q = 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 

• Physical quantity whose S.I. unit is JC^-1 is Electric potential.
• It is a Scalar quantity.

Additional Information:

• The SI base unit of electric flux is voltmeters (V m) which is also equal to newton-meters squared per coulomb (N m² C^-1). Also, electric flux is a scalar quantity. 
• Electric potential is expressed in units of joules per coulomb (i.e., volts). It is also a scalar quantity. 

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 glass rod will have postive charge equal in value to the negative charge lost by the silk.
∴ the charge on the glass rod is + 1.6×10^{- 19}C

Additional Information: When a 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 charges are produced on both. 

• 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.