Alexei Volkov
When two objects rub against each other, friction is created, resulting in the transfer of electrons between them. This phenomenon, known as triboelectricity, causes one object to gain electrons and become negatively charged, while the other loses electrons and becomes positively charged. The process of charging objects by rubbing them together is called charging by friction or electrification by friction. For example, when a plastic rod is rubbed with a wool cloth, the rod becomes negatively charged due to the transfer of electrons, while the cloth becomes positively charged. This principle has been understood since 600 BCE when Thales of Miletus observed that rubbing amber with wool caused the amber to attract bits of straw.
| Characteristics | Values |
|---|---|
| What happens when plastic is rubbed? | Electrons are rubbed off the cloth and onto the plastic by friction. |
| What charge does plastic gain? | Negative charge. |
| What is the process called? | Triboelectricity, electrification by friction, charging by friction, or charging by rubbing. |
| What happens to the other object rubbed against plastic? | It gains a positive charge. |
| What is the unit of electric charge? | Coulomb (C). |
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What You'll Learn
Friction and the transfer of electrons
The process of charging objects by rubbing them together is called "triboelectricity", or the "triboelectric effect". This phenomenon is dependent on the varying affinities that different materials have for electrons. When two materials are in contact, a chemical bond develops between the surface molecules of both materials due to differences in their electrochemical potential. When the materials are separated, the electrons may stay with one of the materials, resulting in a charge.
When two objects rub against each other, friction is created. Due to friction, one object's electrons are transferred to the other. The object that loses electrons becomes positively charged, while the object that gains electrons becomes negatively charged. The charge gained by the negatively charged body equals the charge lost by the positively charged object, so the charge is always conserved.
The triboelectric series ranks materials in order of their tendency to pull electrons from other materials or deposit them onto other materials. For example, when a plastic rod is rubbed with a wool cloth, the wool cloth has a stronger affinity for electrons and pulls them from the plastic rod. This results in the wool cloth becoming negatively charged and the plastic rod becoming positively charged.
The charged objects can then be used to demonstrate electrostatic attraction and repulsion. For example, if a negatively charged plastic rod is placed on a swivel stand, it will be attracted to the wool cloth it was rubbed with. Similarly, if a glass rod is rubbed with a silk cloth, the glass rod will become positively charged and the silk cloth will become negatively charged. These charged rods can then be used to demonstrate electrostatic attraction and repulsion when placed on swivel stands.
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Triboelectricity and material affinities
Triboelectricity, or the triboelectric effect, is the transfer of electric charge between two objects when they come into contact or slide against each other. This phenomenon is not limited to solid materials, as it can also occur between combinations of solids, liquids, and gases. The triboelectric effect is responsible for static electricity, which occurs when the transferred charge remains on one or both objects.
The triboelectric effect is influenced by the varying affinities that different materials have for electrons. When two materials are rubbed together, the material with the stronger affinity for electrons pulls them from the other material. This results in a net negative charge on the material that has gained electrons and a net positive charge on the material that has lost electrons. The amount of charge transferred is directly proportional to the amount of energy used to rub the materials together.
Materials with an affinity near zero, such as cotton, nitrile rubber, polycarbonate, and ABS, do not acquire much charge when rubbed against metals or themselves. On the other hand, materials with a strong affinity for electrons, like wool, can effectively transfer electrons to materials with a weaker affinity, like plastic. This results in the plastic becoming negatively charged while the wool acquires a positive charge.
The triboelectric series is a ranking of materials based on their tendency to gain or lose electrons when touched or slid against another material. A material towards the bottom of the series, when touched to a material near the top, will acquire a more negative charge. This series helps in understanding and predicting the behaviour of different materials when they come into contact or rub against each other.
The triboelectric effect has been known for centuries, with early experiments involving amber and other materials contributing to our understanding of static electricity and electrons. However, the first major scientific analysis is attributed to William Gilbert, who discovered that various materials, including sulphur, wax, and glass, could produce static electricity when rubbed.
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Charging by induction
An example of induction charging is when a ruler is rubbed against hair several times, causing the ruler to become charged. When the ruler is then brought close to small pieces of paper, it attracts the paper through induction charging.
Induction charging can also be demonstrated using an electroscope, an instrument used to detect the presence and magnitude of electric charges on a body. In this demonstration, a charged particle is brought close to, but not touching, the electroscope. The electrons in the electroscope are induced to move due to the presence of the charged particle above the plate of the electroscope. When the electroscope is touched, electrons flow between the electroscope and the ground, giving the electroscope an overall charge.
It is important to note that the amount of charging by the induction method for a piece of metal is greater than that for a piece of plastic. This is because plastic is an insulator, while metal is a conductor. Insulators, like plastic, generally cannot be charged by the induction method because they do not allow electric charge to flow through them.
In summary, charging by induction involves bringing a charged object close to an uncharged object without direct contact, inducing a charge in the uncharged object with opposite polarity. While induction charging can be used to charge some objects, it is less effective for insulating materials like plastic.
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Charging by conduction
When two objects rub against each other, friction is created, and electrons are transferred between them. The object that loses electrons becomes positively charged, and the other becomes negatively charged. This transfer of electrons is known as triboelectricity.
The plastic comb in your hair is a classic example of charging by conduction. When you comb your hair, the friction of combing knocks electrons out of place, and they transfer to the comb from your hair. This is because the comb has a greater affinity for electrons than your hair. Your hair loses electrons and becomes positively charged, while the comb gains electrons and becomes negatively charged.
The same thing happens when you rub a plastic rod with a wool cloth. The wool cloth has a stronger affinity for electrons than the plastic rod, so the electrons are pulled from the plastic rod to the wool cloth. This leaves a net positive charge on the wool cloth and a net negative charge on the plastic rod.
Another example is when you rub a glass rod with a silk cloth. In this case, the glass rod gains electrons and becomes negatively charged, while the silk cloth loses electrons and becomes positively charged.
The process of charging by conduction involves the transfer of electrons between two objects that are in contact with each other. This can happen when two similar conductors come into contact, and the electrons separate as much as possible. The two conductors will then share the original charge. For example, if you have two neutral metal spheres and you bring them into contact, they will both become slightly positive on one side and slightly negative on the other side.
It's important to note that charging by conduction is different from charging by induction, where a charge is induced in a neutral object by bringing a charged object close to it without touching.
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Positive and negative charges
When two objects rub against each other, electrons are transferred between them due to friction. This phenomenon is known as triboelectricity or electrification by friction. One object loses electrons and becomes positively charged, while the other gains electrons and becomes negatively charged. The charges gained and lost are equal, and the total charge is conserved.
In the context of plastic being charged by rubbing, let's consider the example of a plastic rod being rubbed with a wool cloth. Due to the friction between the two materials, electrons are transferred from the wool cloth to the plastic rod. The plastic rod gains electrons and becomes negatively charged. Conversely, the wool cloth loses electrons and acquires a net positive charge.
The specific type of plastic used in this experiment is polyethylene terephthalate, also known as PETG or glycol-modified polyethylene terephthalate. The choice of material is important because different materials have varying affinities for electrons. In this case, the wool cloth has a stronger affinity for electrons, causing it to pull electrons from the plastic rod during the rubbing process.
The positive and negative charges on the plastic rod and wool cloth lead to electrostatic attraction between them. If the charged plastic rod is placed on a swivel stand, the wool cloth can be brought close to it, causing the rod to be attracted to the cloth and move towards it. This demonstrates the electrostatic force at play between the two charged objects.
Additionally, the charged plastic rod can be used to attract neutral objects, such as small pieces of paper. Even though paper is not a conductor, the negative charges on the plastic rod are mobile enough to induce a temporary charge on the nearby paper bits. The parts of the paper closest to the rod acquire a positive charge, creating an attractive force that causes the paper bits to fly up and stick to the rod.
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Frequently asked questions
When two objects rub against each other, friction is created, and electrons are transferred between them. One object gains electrons and becomes negatively charged, while the other loses electrons and becomes positively charged.
The transfer of charge by friction, also known as triboelectricity, depends on the varying affinities that different materials have for electrons. The material with the stronger affinity for electrons will pull them from the other material.
Yes, if you rub a plastic rod with a wool cloth, the rod will become negatively charged due to gaining electrons from the cloth. The wool cloth will become positively charged as it loses electrons to the plastic rod.
