Unit 15: Electrostatics — Short Questions

Coulomb's law states that the magnitude of the electrostatic force between two point charges is directly proportional to the product of the magnitudes of the charges and inversely proportional to the square of the distance between them.

Formula F = k(q₁q₂ / r²)
Unit: Force is measured in Newton (N).

An electric field is the region around a charged object where it exerts a force on another charge. The electric field strength decreases with distance from the charge, following an inverse square law for point charges. The electric field is responsible for attraction or repulsion between charges.

Electrical conductors Copper and aluminum are good conductors because they have free electrons that can move easily.
Electrical insulators: Rubber and plastic are insulators because their electrons are tightly bound and cannot move freely.

Electrostatics is used in photocopiers to produce images.
(i) Electrostatic precipitators are used to remove dust from industrial gases.
(ii) Electrostatic spray painting provides uniform coating.
(iii) Lightning rods protect buildings from lightning.

Like charges repel each other when brought close. Unlike charges attract each other when brought near. This behaviour is due to electrostatic force between charges. It is a basic property of electric charge.

Electrical breakdown occurs when a strong electric field passes through a gas and ionizes its atoms. It occurs when the electric field becomes strong enough to free electrons. This results in a sudden flow of electric current. Lightning is a natural example of electrical breakdown.

Dangers of static electricity • Electrical shock: Sudden discharge of accumulated charge can cause painful shocks when touching metal objects or other people.
• Fire hazard: Static electricity can ignite flammable substances, gases, or dust in industrial environments, causing explosions.
• Damage to electronic components: Sensitive electronic devices and microchips can be permanently damaged by static discharge.
• Disruption of manufacturing: Static charges can interfere with precision manufacturing processes, affecting product quality.
• Health risks: In certain environments, accumulated static electricity can pose serious safety hazards to workers.
• Lightning danger: Large accumulation of charges in the atmosphere can lead to lightning strikes, which can be fatal and cause structural damage.

The property of matter due to which it produces attraction or repulsion between objects is called electric charge. Electric charge is an intrinsic property of matter.
Unit: The SI unit of electric charge is coulomb (C).

When two glass rods rubbed with silk are brought close, they repel each other showing like charges repel. When two plastic rods rubbed with wool are brought close, they also repel. When a glass rod and a plastic rod are brought together, they attract, showing unlike charges attract.

Electric charge exists in two types: positive and negative.
(i) Like charges repel each other.
(ii) Unlike charges attract.
(iii) Electric charge can be transferred from one body to another by friction, contact, or induction.

The process in which a conductor gets charged without direct contact with a charged body is called electrostatic induction. A charged body causes redistribution of charges inside the conductor while the total charge remains unchanged.
(OR) In the presence of a charged body an insulator conductor develops positive charge at one end and negative charge at another end is called electrostatic induction.

A charged object can attract a neutral object due to electrostatic induction. The neutral object develops opposite charges on its near side, causing attraction. Therefore, attraction alone does not confirm that both objects are charged.

A charged rod is brought near a neutral conductor causing separation of charges. The conductor is then connected to Earth, allowing unwanted charges to flow away. After removing the earth connection and rod, the conductor remains charged.

Gold Leaf Electroscope A gold leaf electroscope is a device used to detect electric charge. It consists of a brass rod, brass disc, two thin gold leaves, an insulating stopper, and a glass jar.

When a charged object is brought near the disc, charges redistribute by induction. Both gold leaves acquire similar charges and repel each other. The divergence of leaves indicates the presence of electric charge.

An electrostatic precipitator removes solid particles from exhaust gases. Particles are charged using corona discharge and attracted to oppositely charged plates. This device helps in reducing air pollution.

A photocopier works on electrostatic charging and discharging. A charged drum forms an image when light falls on it. Negatively charged toner sticks to charged areas and transfers the image to paper.

Statement The electrostatic force between two point charges is directly proportional to the product of their charges and inversely proportional to the square of the distance between them.
Formula: F = k q₁q₂ / r²

The electrostatic force per unit positive test charge at a point is called electric field intensity.
Formula: E = F / q
Unit: Newton per Coulomb (N C⁻¹)

Electric field lines originate from positive charges and terminate at negative charges. The direction of the electric field is the direction of force on a positive test charge. Electric field lines never intersect each other.

Corona discharge is a visible form of electrical breakdown where a high-voltage electric field ionizes the air surrounding a conductor.
Observation: It produces a faint glow and is commonly observed around sharp points or edges of high-voltage power lines, transformers, or antennas, especially in dark or humid conditions.

Conductors allow electric charge to flow; have free electrons. Example: Copper, Aluminium.
Insulators: resist the flow of charge; electrons are tightly bound. Example: Rubber, Plastic.

Electrical breakdown occurs when a strong electric field ionizes a gas, making it conductive.
Examples: Corona discharge, Lichtenberg figures.

Some materials gain electrons while others lose electrons when rubbed together because of differences in their atomic structure and how tightly bound their electrons are.

Reason Different materials have different numbers of valence electrons (outermost electrons) and varying degrees of binding strength. When two materials are rubbed together, the material with electrons that are more loosely bound will transfer its electrons to the material with more tightly bound electrons. The property that determines this behavior is the electron-binding property of the material. Materials like plastic tend to gain electrons easily (become negatively charged), while materials like wool or silk tend to lose electrons easily (become positively charged). This difference in electron-binding properties is due to the different atomic structures and the varying number of valence electrons in each material.

The paper pieces fall off because they become charged by the comb through electrostatic induction.

Reason Initially, the charged comb attracts the neutral paper pieces by inducing opposite charges on them (the side of the paper closest to the comb becomes oppositely charged). However, once the paper touches the comb, electrons transfer to (or from) the paper, giving it the same type of charge as the comb. Since like charges repel, the comb now repels the paper pieces, and they fall off.

Dust particles get attracted to screens because the screens develop an electric charge when operating.

Reason When television or computer monitors are turned on, they generate electric charges on their surfaces due to the flow of electrons. Dust particles, particularly those suspended in the air, become charged through electrostatic induction. Since the screen has an electric charge and the dust particles develop opposite charges, the electrostatic force of attraction pulls the dust particles toward the screen. This is why we often see a layer of dust accumulation on screens.

The bending of water toward the balloon reveals that water molecules are polar molecules.

Reason When the balloon is rubbed on dry hair, it becomes negatively charged. When this charged balloon is placed near a stream of water, the water bends toward the balloon, indicating attraction. This happens because water molecules have a polar structure — they have a slightly positive charge on one end and a slightly negative charge on the other end. Even though the water stream itself is neutral overall, the water molecules can rotate and align themselves in the electric field created by the charged balloon. The slightly negative charges in the water molecules are repelled to the far side, while the slightly positive charges orient toward the negatively charged balloon, resulting in a net attraction.

The conductor allows free electrons to move, causing charge redistribution, while the insulator does not.

Reason In a conductor, free electrons move easily. When placed in an electric field, electrons redistribute accumulating on one side and leaving the opposite side positively charged (electrostatic induction). In an insulator, electrons are tightly bound and cannot move freely, so no significant charge redistribution occurs; it remains practically unaffected by the external field.