What is 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:
Charging by friction
Charging by conduction
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.
Conclusion
The induction charging method is the most efficient and is used in the majority of applications. The charging by induction process in commercial products is governed by induction coils. In smartphones, both the phone and the charging dock contain iron induction coils wrapped in copper wire. When we place the phone on the charging dock, we create an electromagnetic field between the induction coils. Once the electromagnetic field is created, electricity can flow between the two induction coils and charge the phone wirelessly.
FAQs on Charging by Rubbing
1. What is meant by charging by rubbing and how does it occur between different materials?
Charging by rubbing is the process where two objects are rubbed together, resulting in the transfer of electrons from one material to another. The material that loses electrons becomes positively charged, while the material that gains electrons becomes negatively charged. This phenomenon occurs because different materials have varying abilities to attract and hold onto electrons.
2. Why are insulators used instead of conductors in charging by rubbing experiments?
Insulators are used in charging by rubbing because their electrons are tightly bound and remain on the object's surface after transfer. In contrast, conductors allow electrons to move freely, so any gained charge quickly disperses through the material or into the ground, making the effect temporary.
3. Can you list real-life examples of charging by rubbing observed in daily activities?
Common real-life examples of charging by rubbing include:
- Combing dry hair, which may cause hair strands to stand up or the comb to attract small paper bits
- Rubbing a balloon on hair or wool, allowing the balloon to stick to walls
- Shuffling feet on a carpet, often resulting in a static spark when touching metal
- Rubbing a plastic or rubber rod with fur or wool, where the rod gains negative charge
4. How does the conservation of charge apply during charging by friction?
Charge is always conserved during charging by friction. The total amount of charge before and after the process remains constant. When one object gains electrons (negative charge), the other loses the same number of electrons (becoming positive). No charge is created or destroyed—it's only transferred.
5. What is the role of work function in determining which object becomes positively or negatively charged during rubbing?
The work function is the minimum energy needed to remove an electron from a material. During rubbing, the material with the lower work function loses electrons more easily and becomes positively charged. The material with a higher work function gains those electrons and becomes negatively charged.
6. Why does the mass of an object change minutely after being charged by rubbing?
When an object gains electrons during charging by rubbing, its mass increases very slightly, and if it loses electrons, its mass decreases as well. This change is extremely small because the mass of an electron is only about 9.1 × 10−31 kg, making the difference negligible in practical terms.
7. How does charging by rubbing differ from charging by conduction and induction?
Charging by rubbing transfers electrons through direct friction between two insulators. Charging by conduction requires physical contact and transfer of charge from a charged object to a neutral one. Charging by induction involves bringing a charged object near a neutral conductor so charges rearrange without direct contact. Each method results in different ways of distributing electrical charge.
8. Which materials are most effective for demonstrating charging by rubbing, and what factors influence the process?
The most effective materials are insulators such as plastics, rubber, wool, and glass. Factors that influence charging by rubbing include the electron affinity of the materials, their surface roughness, and environmental conditions such as humidity, which can cause charges to dissipate quickly.
9. Is charging by rubbing effective in humid weather? Explain why or why not.
Charging by rubbing is less effective in humid conditions because water molecules in the air provide a pathway for charges to leak away. This prevents significant build-up of static electricity on object surfaces.
10. What is a common misconception about charging by rubbing?
A common misconception is that charging by rubbing creates new charge. In reality, charge is only transferred—it is neither created nor destroyed. The principle of conservation of charge always applies.
11. What is the SI unit of electric charge, and how is it defined?
The SI unit of electric charge is the coulomb (C). One coulomb is defined as the quantity of electricity transported by a current of one ampere in one second.
12. How does the concept of electricity in the human body differ from charging by rubbing?
In the human body, electricity is generated and used via the movement of ions (such as sodium, potassium, calcium, magnesium) through cell membranes. This is different from charging by rubbing, which involves the transfer of electrons between objects.
13. What potential risks can arise if electronic devices accumulate static charge from charging by rubbing?
If electronic devices accumulate static charge, it can result in electrostatic discharge (ESD), which may damage sensitive circuits, cause malfunctions, or result in data loss. Grounding and antistatic measures are essential to prevent such issues.
14. Why does rubbing a plastic rod with fur result in the rod gaining a negative charge?
When a plastic rod is rubbed with fur, electrons transfer from the fur (which has a lower electron affinity) to the plastic. As a result, the rod gains excess electrons and becomes negatively charged, while the fur becomes positively charged.
15. What determines whether an object becomes positively or negatively charged after rubbing?
The outcome depends on the relative electron affinities and work functions of the two materials. The object that more readily loses electrons becomes positively charged; the one that gains electrons becomes negatively charged.
