Charging by Rubbing

The frictional charging process involves the transfer of electrons between two rubbing objects. Rubber attracts electrons far more strongly than animal fur. As a result, the atoms of rubber pull electrons from the atoms of animal fur, resulting in a charge imbalance in both objects.

This is a well-known electric phenomenon in which charged particles are transferred from one body to another. When two objects are rubbed together, an attractive force develops between them, especially if the objects are insulators and the surrounding air is dry.

• When an object loses electrons, it becomes positively charged.

• The other object that gains electrons will become negatively charged.

• The force is simply the attraction of opposite-sign charges.
  
  

Uncharged Cloth Got Charged By Rubbing With A Charged Plastic Rod

Charging By Rubbing Examples

• Electrons are transferred from fur to the other material when a piece of amber, plastic, polythene, or hard rubber is rubbed with fur. Fur gains a net positive charge because it contains fewer electrons than protons. Similarly, because of excess electrons, amber, plastic, and hard rubber acquire a net negative charge.

• Hair combing charges the comb in the same way.

• Glass (perspex) gains a net positive charge after being rubbed with silk.

Methods of Charging A Body

We can charge an uncharged body using one of three methods:

1. Charging by friction

2. Charging by conduction

3. Charging by induction

Charging by Friction

Charging by friction is the oldest known method of charging. Friction occurs when two objects in contact move across each other. When two objects rub against each other, electrons from one object transfer from one to the other. Friction is created when two surfaces rub together. As a result of friction, electrons are transferred between two objects. When an object loses electrons, it gains a positive charge and becomes positively charged.

When an object gains electrons, it gains a negative charge and becomes negatively charged. The charge gained by the negatively charged body equals the charge lost by the positively charged object; in other words, the charge is always conserved. As a result, the objects become charged. The friction charge is the charge gained by the objects. This charging method is also known as electrification by friction.

Charging by Conduction

A neutral body has the same number of electrons and protons as a charged body, which has an unequal number of positive and negative charges. When a charged body comes into contact with an uncharged conductor, the charges from the charged body are transferred to the conductor. Charging by conduction is the name given to this method of charging.

Charging by Induction

Charging by induction refers to the process of charging an uncharged body by bringing a charged body close to it. Both bodies are brought close enough to touch but not physically. When a charged body is brought close to an uncharged body, it induces a charge over it, so that the polarity of the charges on the two bodies is opposite. An example of induction charging:

When a ruler is rubbed several times on the hair, it becomes charged. When it is brought close to small pieces of paper, it attracts the bits through induction charging.

Important Questions

1. What is the SI unit of charge?

Coulomb is the unit of electric charge in the metre-kilogram-second-ampere system, the basis of the SI system of physical units. It is abbreviated as C. The coulomb is defined as the quantity of electricity transported in one second by a current of one ampere. Named for the 18th–19th-century French physicist Charles-Augustin de Coulomb, it is approximately equivalent to 6.24 × 1018 electrons, with the charge of one electron, the elementary charge, being defined as $1.602176634 \times 10^{−19}\,C$.

2. How does the human body use electricity?

The elements in our bodies, like sodium, potassium, calcium, and magnesium, have a specific electrical charge. Almost all of our cells can use these charged elements, called ions, to generate electricity. The contents of the cell are protected from the outside environment by a cell membrane. This cell membrane is made up of lipids that create a barrier that only certain substances can cross to reach the cell interior.

Not only does the cell membrane function as a barrier to molecules, it also acts as a way for the cell to generate electrical currents. Resting cells are negatively charged on the inside, while the outside environment is more positively charged. This is due to a slight imbalance between positive and negative ions inside and outside the cell. Cells can achieve this charge separation by allowing charged ions to flow in and out through the membrane. The flow of charges across the cell membrane is what generates electrical currents.

Practice Questions (Multiple Choice Questions)

1. If a body is charged by rubbing, then its weight:

(A) Remains precisely constant

(B) Increases slightly

(C) Decreases slightly

(D) May increase or decrease slightly

Answer: (d)

Step By Step Solution:

• We know, in static electricity, an object gets charged by rubbing. This happens due to an imbalance of charges on the surface of the material.

• We can say an object is charged depending upon the number of electrons gained or lost. Due to rubbing, the number of electrons present on the surface increases, we say that the body is negatively charged and when after rubbing the number of electrons on the surface decreases, we say that the body is negatively charged, thus this gain or loss of electrons indicated the charging of the body.

• We know that the mass of an electron is $9.1\times 10^{−31}\,Kg$. Due to this, the mass of the body after rubbing will differ from the initial mass of the body. This change in mass causes a change in weight. Therefore, when electrons are gained, the mass of the object increases and when electrons are lost, we say the weight slightly decreases.

• Hence, option (D) is correct.

2. During charging by friction

(A) Charge is created

(B) Charge is destroyed

(C) Charge is Conserved

(D) None of these

Answer: (C)

Step by Step Solution:

• When two objects rub each other, then due to friction, one object's electrons get transferred to the other one.

• The one which loses electrons becomes positively charged and the other, negative.

• Thus charging by friction is only due to the transfer of electrons. No charge gets created or destroyed in this process.

• Hence, correct option is (C).

3. Assertion: During charging by rubbing, the insulating material with lower work function becomes positively charged.

Reason: Electrons are negatively charged.

(a) Both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

(b) Both Assertion and Reason are true, but Reason is not the correct explanation of the Assertion.

(c) Assertion is true, but the Reason is false.

(d) both Assertion and Reason are false.

Answer: (b)

Step by Step Solution:

• Insulating material with lower work function loses electrons more easily.

• Therefore, it becomes positively charged.

• Reason is correct, but it is not a true explanation of the assertion.

• Hence, the correct answer is Option B.