Static Electricity and Charge Concepts

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Questions and Answers

What does static electricity indicate about the charge of a body?

Static electricity indicates an excess or deficit of electrons compared to protons.

What is the SI unit of charge and how is it defined?

The SI unit of charge is the Coulomb (C), defined as the charge passing a point when 1 Ampere flows for 1 second.

How many electrons are equivalent to one Coulomb of charge?

One Coulomb is approximately equivalent to 6.25 × 10¹⁸ electrons.

What happens to the charge of a body when it loses or gains electrons?

A body becomes positively charged by losing electrons and negatively charged by gaining electrons. Signup and view all the answers

How do different substances behave when rubbed together in terms of electron attraction?

Different substances attract electrons with varying strength; the substance with weaker attraction loses electrons. Signup and view all the answers

What occurs when polythene and Perspex rods are rubbed with cotton?

Polythene gains electrons (becoming negative), while Perspex loses electrons (becoming positive). Signup and view all the answers

What observation is made when two like-charged rods, such as polythene or Perspex, are brought close together?

They repel each other. Signup and view all the answers

What is the main reason for dust accumulation on CRT screens compared to LCD or plasma screens?

Dust accumulates on CRT screens due to the static charge created by emitted electron beams. Signup and view all the answers

How does grounding an earthed metal sphere affect its charge retention after removing a negatively charged rod?

Grounding allows negative charges to escape, creating an imbalance that retains a positive charge after the rod is removed. Signup and view all the answers

What characterizes a conductor and why are metals considered good conductors?

A conductor allows electric charge to flow freely, and metals have a lattice structure with freely moving electrons. Signup and view all the answers

What is the significance of using conductive floor tiles in hospital operating theatres?

They prevent static build-up, minimizing explosion risks associated with volatile anesthetics. Signup and view all the answers

What is the first step in the procedure for charging a metal sphere by induction using a polythene rod?

Charge the polythene rod by rubbing it with cloth. Signup and view all the answers

What occurs to the charge distribution in a conductor when a negatively charged rod is brought close to it?

The rod induces charge separation, leading to a positive charge near the rod and a negative charge on the far side. Signup and view all the answers

How does rubbing a polythene rod with cloth charge it, and what happens when it touches a sphere?

Rubbing the polythene rod with cloth gives it a negative charge, and when it touches a sphere, it induces a negative charge on the sphere as well. Signup and view all the answers

What is the significance of charge distribution in an insulated conductor?

All static charges reside on the outer surface of a conductor to maximize separation, and they tend to accumulate at pointed areas. Signup and view all the answers

Describe point discharge and its impact on the surrounding area.

Point discharge is when ions in the air are attracted to or repelled from a charged conductor's tip, creating a strong electric field that can ionize the surrounding air. Signup and view all the answers

What are the main components of a Van de Graaff generator and its primary purpose?

The main components include a smooth dome, a rubber belt, and two metal combs; its purpose is to demonstrate electrostatic effects by generating high voltage. Signup and view all the answers

How does the operation of the Van de Graaff generator lead to charge accumulation on its dome?

The lower pulley positively charges via friction, inducing negative charge on the belt, which then transfers to the upper comb and neutralizes charge while building negativity on the dome. Signup and view all the answers

What factors influence the performance of a Van de Graaff generator?

The performance is affected by the size of the dome, the quality of insulating pillars, and the voltage, which can reach up to tens of thousands of volts. Signup and view all the answers

Why does charge accumulate on the exterior of the Van de Graaff generator's dome?

Charge accumulates on the exterior because like charges repel, maximizing their distance from each other. Signup and view all the answers

What observable effect occurs when a static spinner is attached to the Van de Graaff generator and activated?

The arms of the static spinner rotate when the generator is turned on. Signup and view all the answers

How does charge distribution differ at sharp points on a conductor compared to smooth areas?

Static charges tend to accumulate at sharp points on a conductor, creating a stronger electric field compared to smooth areas. Signup and view all the answers

What did Benjamin Franklin demonstrate about lightning?

He demonstrated that lightning is a large electric spark. Signup and view all the answers

How do lightning conductors protect buildings?

Lightning conductors, usually copper rods, run from the roof to the ground and facilitate point discharge, reducing the likelihood of direct strikes. Signup and view all the answers

Why is it unsafe to play golf or shelter under tall trees during a storm?

It is unsafe because pointed conductors can attract lightning, increasing the risk of being struck. Signup and view all the answers

What is the main purpose of a gold leaf electroscope?

The main purpose is to study electric charge. Signup and view all the answers

What are the main components of the gold leaf electroscope?

It consists of a metal disc, a metal rod, an earthed container, and a gold leaf near the base of the rod. Signup and view all the answers

Why is the container of a gold leaf electroscope earthed?

The container is earthed for safety and to influence the deflection of the leaves based on potential difference. Signup and view all the answers

How does the gold leaf electroscope detect charge?

A charged object attracts opposite charge to the cap, causing like charges to repel down the rod, making the gold leaf diverge. Signup and view all the answers

What happens when the gold leaf electroscope is earthed?

Touching the cap allows charge to flow to the earth, causing the gold leaf to collapse back to its original position. Signup and view all the answers

What occurs after removing the earth and the charged insulator from the electroscope?

The electroscope ends up with an excess of the opposite type of charge compared to the charged insulator. Signup and view all the answers

How can you distinguish between an insulator and a conductor using the electroscope?

If the leaf collapses when touching the cap with the object, it's a conductor; if it doesn't collapse, the object is an insulator. Signup and view all the answers

What apparatus is used to demonstrate the concept that charge resides on the outside of a hollow conductor?

The apparatus includes a Van de Graaff generator, a metal can, a gold leaf electroscope, and a proof plane. Signup and view all the answers

What conclusion can be drawn from the observation of the gold leaf electroscope deflecting only when testing the outside of the metal can?

The conclusion is that all the charge resides on the outside of a hollow conductor. Signup and view all the answers

Explain the relationship between the electrostatic force and the distance between two point charges according to Coulomb's Law.

The electrostatic force is inversely proportional to the square of the distance between the charges. Signup and view all the answers

What happens to the electrostatic force when the distance between two charges doubles?

The force becomes one-fourth as strong when the distance doubles. Signup and view all the answers

What is the primary difference between gravitational force and electrostatic force?

Gravitational force is always attractive and much weaker, while electrostatic force can be attractive or repulsive and is significantly stronger. Signup and view all the answers

How does permittivity of a medium affect the electrostatic force?

A lower permittivity results in a higher electrostatic force, with maximum force occurring in a vacuum. Signup and view all the answers

Describe the interaction between collinear charges according to Coulomb's Law.

The magnitude of force is the same on each charge, with direction along the line joining them. Signup and view all the answers

Calculate the total electrostatic force on charge B if it has forces of 1.4 N to the right and 3.4 N to the left acting on it.

The resultant force on B is a 2 N force to the left (towards A). Signup and view all the answers

What is the relationship between protons and electrons in static electricity?

In static electricity, a body has an excess or deficit of electrons compared to protons, affecting its overall charge. Signup and view all the answers

How does the process of rubbing influence the charge of materials like polythene and Perspex?

Rubbing causes one material to lose electrons and become positively charged, while the other gains electrons and becomes negatively charged. Signup and view all the answers

Explain how a neutral atom maintains charge balance.

A neutral atom maintains charge balance by having an equal number of electrons and protons. Signup and view all the answers

What happens to charge distribution when different substances are rubbed together?

When different substances are rubbed together, one substance loses electrons and becomes positively charged, while the other gains electrons and becomes negatively charged. Signup and view all the answers

Describe how charge can be transferred through induction.

Charge can be transferred through induction by bringing a charged object close to a neutral conductor, causing a redistribution of charge within the conductor. Signup and view all the answers

What happens when a positively charged rod comes close to a negatively charged conductor?

It induces charge separation in the conductor, attracting the positive charge towards the rod. Signup and view all the answers

How do charged rods affect dust accumulation on surfaces?

Charged rods can create a static electric field that attracts airborne particles, leading to increased dust accumulation on surfaces like CRT screens. Signup and view all the answers

What conclusion can be drawn about the nature of charges when two polythene rods repel each other?

The conclusion is that like charges repel each other, indicating both rods possess the same type of charge. Signup and view all the answers

What safety measures are taken in industries to minimize risks associated with static electricity?

Industries implement grounding techniques and use conductive materials to prevent static charge build-up, reducing the risk of sparks. Signup and view all the answers

What role does earthing play in the process of charging by induction?

Earthing allows excess charges to escape from the conductor, resulting in a permanent charge imbalance when the charged rod is removed. Signup and view all the answers

Study Notes

Static Electricity Overview

Static electricity arises from an excess or deficit of electrons compared to protons within a body.
The SI unit of charge is the Coulomb (C), defined as the charge passing a point when 1 Ampere (A) flows for 1 second.
One Coulomb (C) is approximately equal to 6.25 × 10¹⁸ electrons (6.25 quintillion).
The charge of a single electron is 1.6 × 10−19C.

Atomic Structure and Charge

Atoms are composed of protons, neutrons, and electrons.
Protons and neutrons form the nucleus of an atom, while electrons orbit the nucleus.
Like charges repel each other, unlike charges attract each other.

Charge Transfer and Static Electricity

A body becomes positively charged by losing electrons and negatively charged by gaining electrons.
Protons remain unchanged during these processes.
Different substances attract electrons with varying strength; the material with weaker attraction loses electrons when rubbed together.
For example, rubbing a polythene rod with cotton causes the polythene to gain electrons (becoming negatively charged), while Perspex loses electrons (becoming positively charged).

Demonstrating Forces Between Charges

The experiment uses Perspex and polythene rods, a retort stand, a clamp, string, and a sling.
Two like-charged rods (polythene or Perspex) repel each other.
Polythene and Perspex rods attract each other.
This demonstrates the principle that like charges repel, while unlike charges attract.

Everyday Examples of Static Charge

Dust accumulates more on CRT screens than LCD or plasma screens because CRTs emit a beam of electrons, creating a static charge that attracts airborne particles.
Clothes can build up static charge from friction while being worn, causing them to cling together. Sparks may occur when undressing in the dark as the clothes discharge.
In industries like oil refineries, flour mills, and chemical plants, precautions are essential to prevent explosions caused by static electricity sparks, especially near fine dust or flammable vapors.
In hospital operating theatres, conductive floor tiles are used to prevent static build-up because some anesthetics can form explosive mixtures with air.
Passengers disembarking from charged airplanes may experience shocks when touching handrails.
To mitigate this, aircraft use electrically conductive rubber in tires to safely discharge accumulated charge during refueling. Similarly, Formula 1 racing cars are earthed before refueling for safety.

Conductors

A conductor is any substance through which electric charge can flow.
Metals are good conductors because they have a lattice structure where atomic nuclei are fixed, and some electrons orbit while others move freely, forming a "sea of electrons."
This free movement of electrons allows for charge to flow efficiently.
Electrolytes conduct electricity because ions (charged atoms or groups of atoms) can move within them, carrying charge.
Charge can also move through low-pressure gases due to the movement of electrons and ions.

Insulators

An insulator is any substance through which electric charge cannot flow.
In insulators like glass or plastic, the electrons are not free to move because they are tightly bound to the molecules.

Electrification by Induction

When a negatively charged insulator (e.g., a polythene rod) is brought near a conductor (e.g., a metal sphere), the electric field from the rod induces charge separation in the conductor.
Electrons in the metal sphere are repelled, moving to the far side, resulting in an excess of positive charge (+) on the side near the rod and a negative charge (-) on the opposite side.
When the negatively charged rod is removed, the charge distribution in the sphere returns to uniform, and the sphere is no longer charged.
However, if one side of the sphere is earthed (by touching it with your finger) while it is in this charged state, the sphere will retain a charge after the rod is removed. This occurs because the grounding allows some negative charges to escape, creating an imbalance between the number of positive and negative charges.

Charging by Induction Procedure

The procedure uses a hollow sphere on an insulated stand, polythene and Perspex rods, and cloth.
Charge the polythene rod by rubbing it with cloth.
Bring the charged rod close to the metal sphere.
Earth the sphere by touching it.
Remove the earth, then take away the charged rod.
Repeat the procedure with the Perspex rod.
A gold leaf electroscope demonstrates that the polythene rod induces a negative charge on the sphere, while the Perspex rod induces a positive charge.

Distribution of Charge

When an insulated conductor is charged, the charges repel each other and arrange themselves to maximize their distance apart.
All static charges reside on the outer surface of a conductor because like charges repel.
Static charge tends to accumulate at points where the surface is most pointed (e.g., sharp edges), creating a strong electric field in the surrounding area that can ionize air.

Point Discharge

Point discharge is the phenomenon where ions in the air are strongly attracted to or repelled from the tip of a charged conductor and move towards or away from it.
Ions are attracted to or repelled from the point, causing movement that can generate additional ions through collisions with air molecules.
Oppositely charged ions are drawn to the point, while like-charged ions are repelled, resulting in an "electric wind."
As ions land on the point, they neutralize some of the charge, leading to a loss of charge known as point discharge or the point effect.

Van de Graaff Generator

The Van de Graaff generator is an instrument used to demonstrate various electrostatic effects. It can be charged to a very high voltage, producing large amounts of static electricity on its dome.
It consists of:
- A smooth dome insulated from the earth by tall support pillars.
- A rubber belt that moves on a motor-driven pulley.
- Two metal combs:
  - One at the lower pulley connected to the earth.
  - One at the upper pulley connected to the dome.
This configuration allows for efficient charge transfer and accumulation on the dome.

Van de Graaff Generator Operation

The lower pulley (made of Perspex) rotates via a motor, gaining a positive charge through friction with the rubber belt.
This induces point discharge at the lower comb, spraying negative charge onto the belt.
The belt carries the negative charge to the upper comb, where it sprays positive charge back onto the belt.
This positive charge neutralizes the negative charge on the belt, leading to an increasing negative charge buildup on the dome.
The now uncharged belt returns to the lower comb, and the process repeats.

Factors Affecting the Van de Graaff Generator Performance

The size of the dome influences the amount of charge it can hold.
The quality of the insulating pillars that support the dome affects charge retention.
The voltage can reach tens of thousands of volts on a typical generator.

Key Features of the Van de Graaff Generator

All charge resides on the outside of the dome because like charges repel, maximizing distance apart on the exterior.
The dome is spherical and smooth to retain charge effectively because charge tends to leak from sharp edges due to the point effect.

Demonstration of Point Discharge

The demonstration uses a Van de Graaff generator and a static spinner.
The static spinner is attached to the dome of the generator.
When the Van de Graaff generator is turned on, the arms of the static spinner rotate.
This indicates that an electric wind is blown away from the points of the arms due to point discharge.

Lightning Conductors

Benjamin Franklin demonstrated that lightning is a large electric spark.
Buildings are protected by lightning conductors, usually copper rods, which run from the roof to the ground.
These conductors are pointed to facilitate point discharge, reducing direct lightning strikes.
As lightning approaches, ions near the point accelerate, ionizing the air and making it conductive.
This allows current to flow safely from the ground to the cloud through the conductor, enabling a gradual discharge rather than a sudden lightning bolt.
If a bolt occurs, it will likely strike the conductor due to the opposite charge, providing a safe path.
During a storm, it's unsafe to play golf, use an umbrella, or shelter under tall trees due to the risk of being struck by lightning.

The Gold Leaf Electroscope

It is used to study electric charge.
Components:
- A metal disc attached to a metal rod.
- The rod passes through an insulated plug into an earthed container with a glass front.
- A gold leaf is attached near the base of the metal rod.
The container is earthed for safety and because the potential difference between the leaves and the frame affects the degree of deflection of the leaves.

How the Gold Leaf Electroscope Detects Charge

When a charged object is brought near the electroscope, opposite charge is attracted to the cap.
Like charges are repelled down to the base of the metal rod and the gold leaf.
This repulsion causes the gold leaf to diverge.
This phenomenon occurs regardless of whether the charge on the object is positive or negative.

Charging the Electroscope by Induction

Bring a charged insulator close to the metal cap, inducing a charge on the cap.
Earth the cap by touching it, allowing charge to flow to the earth instead of the base and gold leaf, causing the leaf to collapse back to its original position.
Remove the earth and then the charged insulator. The electroscope now has an excess of the opposite type of charge compared to the charged insulator.

Using the Gold Leaf Electroscope to Detect Small Charges and Estimate Charge Size

The leaf diverges when a charged body is brought close to the cap of the electroscope, indicating the presence of a charge.
The degree of divergence of the leaf is related to the size of the charge. To compare charges on different conductors, each must be placed the same distance from the metal cap.

Distinguishing Insulators from Conductors

Charge the electroscope and touch the cap with the object.
If the leaf collapses (indicating the electroscope is earthed), the object is a conductor.
If it does not collapse, the object is an insulator.

Using the Electroscope to Identify Charge on an Object

Charge the electroscope (positive or negative) and bring the object to be tested close to the metal cap.
Observe the divergence of the leaf. It may be necessary to earth the electroscope and induce the opposite charge, then repeat the process and note the effect.

Charge on Electroscope	Effect on Leaf	Implies That the Object Being Tested is...
Negative	Increase	Negative
Negative	Decrease	Positive or Uncharged
Positive	Increase	Positive
Positive	Decrease	Negative or Uncharged

Charge Resides on the Outside of a Hollow Conductor

The experiment uses a Van de Graff generator, a metal can, a gold leaf electroscope, and a proof plane.
When testing the proof plane against the inside of the metal can, the gold leaf electroscope does not deflect.
When testing the proof plane against the outside of the metal can, the gold leaf electroscope deflects.
This demonstrates that all the charge resides on the outside of a hollow conductor.

Coulomb's Law

Electrostatic force between two point charges is proportional to the product of their charges and inversely proportional to the square of the distance between them.
The force is attractive between unlike charges and repulsive between like charges.
Coulomb's Law:
- F = electrostatic force
- 𝜀 = permittivity of the medium
- 𝑄1 = charge on body 1
- 𝑄2 = charge on body 2
- 𝑑 = distance between the charges

Coulomb's Law vs Newton's law of Gravitation

Both laws share the same mathematical form.
Gravitational force is always attractive, while electrostatic force can be attractive or repulsive.
Gravitational force is significantly weaker than electrostatic force.

Permittivity

The permittivity of a medium affects electrostatic force.
Lower permittivity leads to a higher electrostatic force.
Maximum force occurs in a vacuum (ε₀ = 8.9 × 10⁻¹² F/m).
All media have greater permittivity than vacuum.
Relative permittivity is expressed as multiples of the permittivity of free space.

Forces Between Collinear Charges

The magnitude of the force is the same on each charge, with direction along the line joining them.
Unlike charges attract each other, while like charges repel each other.
Coulomb's Law applies accurately when the distance between charges is large compared to their size, using the center-to-center distance for spherical charges.

Electric Fields

An electric field is a region of space where a static electric charge experiences a force (excluding the force of gravity).
Electric fields are represented by lines called electric field lines, which show the path a positive charge would take in the field.
The density of the lines indicates field strength: closer lines mean a stronger field, while lines that are farther apart indicate a weaker field.

Demonstration of an Electric Field Pattern

The demonstration uses oil, semolina, a large container, a high voltage DC power supply, metal plates, and leads.
Sprinkle semolina on the oil between the plates and turn on the high voltage DC supply.
The semolina will form parallel lines between the plates, illustrating the electric field pattern.
Different electrode shapes or charges can produce various field patterns.

Electric Field Strength

Electric field strength (E) is defined as the force per unit positive charge at a point in an electric field. It's also known as electric field intensity.
E = electric field strength
F = electrostatic force
Q = the charge on the body placed in the electric field
The magnitude of E at a point in an electric field indicates the force on a +1 C charge placed there, with E's direction matching the force's direction.
When calculating electric field strength around a sphere, distance (d) refers to the distance from the sphere's center to the point of interest.

Industrial and Practical Applications

Electrostatic precipitation removes tiny particles from industrial smoke using a -50 kV wire grid to negatively charge dust particles and collect them on positively charged plates in the chimney.
Photocopiers use a high-voltage charged selenium drum, where bright light causes charge loss in white areas, while black areas retain charge. Toner adheres to charged areas, and positively charged paper transfers the image.
Integrated circuits are protected from static charges by grounding personnel and storing sensitive equipment in earthed metal containers (Faraday cages).

Past Paper Questions Summary

Static Electricity Hazards:
- Electric shock
- Fire
Distribution of Charge on a Pear-Shaped Conductor:
- Charge concentrates at pointed ends.
Force on an Electron in an Electric Field:
- F = qE (where q is the charge of the electron and E is the electric field strength).
Coulomb's Law:
- The force between two point charges is directly proportional to the product of the charges and inversely proportional to the square of the distance between them.
Charging an Electroscope by Induction:
- A negatively charged rod is brought near the electroscope, repelling electrons to the leaves.
- The electroscope is earthed, allowing electrons to escape.
- The rod is removed, leaving the electroscope positively charged.
Point Discharge:
- Occurs when a high concentration of charge at a sharp point creates a strong electric field, leading to the ionization of air molecules and a discharge of charge.
Electric Field Strength and Unit:
- Electric field strength is defined as the force per unit positive charge at a point in an electric field.
- Its SI unit is Newton per Coulomb (N C⁻¹).
Electric Field Pattern Demonstration:
- Two metal plates are placed in a container of oil, and semolina is sprinkled on the oil.
- A high voltage DC supply is connected to the plates, creating an electric field.
- The semolina particles align themselves along the electric field lines, revealing the pattern.
Charge on the Surface of a Conductor:
- Charge resides on the outside surface of a conductor due to the repulsion of like charges.
Experiment to Demonstrate Charge on the Outside of a Conductor:
- A metal can is charged using a Van de Graaff generator.
- A proof plane is used to test the charge inside and outside the can.
- The electroscope shows that charge resides on the outer surface.