Electric Charge, Charging Methods and Electrostatics for SS 1

Electric Charge

Performance Objectives

By the end of the lesson, students should be able to:

1. define electric charge and state the electrostatic law;
2. demonstrate how a body is charged by friction, induction and contact;
3. generate and store charges;
4. detect and identify types of charges on bodies;
5. illustrate the concept of earthing and construct a typical metal spike.

Introduction to Electric Charge

Electric charge is a fundamental property of matter that causes it to experience a force when placed in an electromagnetic field. It is responsible for all electrical phenomena, from lightning in the sky to the operation of electronic devices.

Many everyday observations demonstrate the presence of electric charge. For example, when a plastic comb is rubbed through dry hair, it can attract small pieces of paper. Similarly, rubbing a plastic pen on a woollen cloth allows it to pick up tiny dust particles. These effects are due to the accumulation of electric charges on the surfaces of the objects.

Types of Electric Charges

There are two main types of electric charges:

1. Positive Charge: This type of charge is typically found on protons within the nucleus of an atom.
2. Negative Charge: This type of charge is carried by electrons, which orbit the nucleus of an atom.

Most objects in their normal state are electrically neutral, meaning they have an equal number of positive and negative charges. When an object gains or loses electrons, it becomes electrically charged.

The Law of Electrostatics

The interaction between electric charges is governed by a fundamental principle known as the Law of Electrostatics. This law describes how charged objects exert forces on each other.

The Law of Electrostatics states that:

1. Like charges repel each other (e.g., positive repels positive, negative repels negative).
2. Unlike charges attract each other (e.g., positive attracts negative).

This law explains why a charged comb can attract uncharged paper (by inducing opposite charges) and why two similarly charged balloons push each other away.

Methods of Charging a Body

An electrically neutral body can be given an electric charge through several methods. These methods involve the transfer or redistribution of electrons.

1. Charging by Friction (Triboelectric Effect)

Charging by friction occurs when two different materials are rubbed against each other. During rubbing, electrons are transferred from one material to the other, causing both materials to become oppositely charged.

The procedure for charging by friction involves the following steps:

1. Take a glass rod and a piece of silk cloth. Both are initially neutral.
2. Rub the glass rod vigorously with the silk cloth.
3. Electrons are transferred from the glass rod to the silk cloth.
4. The glass rod becomes positively charged (loses electrons), and the silk cloth becomes negatively charged (gains electrons).

Other examples include rubbing a plastic rod or a biro with fur or wool. The plastic rod or biro gains electrons and becomes negatively charged, while the fur or wool loses electrons and becomes positively charged.

2. Charging by Contact (Conduction)

Charging by contact, also known as conduction, involves touching a charged object to an uncharged object. When contact is made, some of the charge from the charged object transfers to the uncharged object, causing both to have the same type of charge.

The procedure for charging by contact involves the following steps:

1. Take a positively charged glass rod and an uncharged metal sphere.
2. Touch the charged glass rod to the surface of the metal sphere.
3. Electrons from the metal sphere are attracted to the positively charged rod and move onto the rod.
4. When the rod is removed, the metal sphere is left with a net positive charge.

If a negatively charged object touches an uncharged object, electrons will move from the negatively charged object to the uncharged object, making the uncharged object negatively charged.

3. Charging by Induction

Charging by induction is a method of charging an object without direct contact with a charged body. It involves redistributing charges within an object using an external charged object and then grounding the object.

The procedure for charging by induction involves the following steps:

1. Take an uncharged metal sphere, supported by an insulating stand. The charges within the sphere are evenly distributed.
2. Bring a negatively charged rod close to the sphere, but do not touch it. The free electrons in the sphere are repelled by the rod and move to the far side of the sphere, leaving the near side positively charged.
3. While the charged rod is still held near the sphere, connect the sphere to the Earth using a conducting wire (earthing). Electrons from the sphere flow into the Earth because they are repelled by the negative rod.
4. Remove the earthing connection first, while the charged rod is still in place. The sphere is now left with a net positive charge.
5. Finally, remove the charged rod. The positive charges redistribute themselves evenly over the surface of the sphere.

If a positively charged rod is used, the sphere will become negatively charged after induction.

Detection and Identification of Electric Charges

The presence and type of electric charge on a body can be detected using a device called an electroscope.

The Gold Leaf Electroscope

The gold leaf electroscope is a sensitive instrument used to detect the presence of electric charge and to determine its type.

Construction: A typical gold leaf electroscope consists of:

1. Metal Cap (or Plate): A flat metal disc or sphere at the top, which collects charge.
2. Brass Rod: A conducting rod connected to the metal cap, extending downwards.
3. Gold Leaves: Two thin, lightweight gold leaves (or aluminium foil strips) attached to the bottom of the brass rod.
4. Insulating Stopper: A rubber or ebonite stopper that insulates the brass rod from the metal case, preventing charge leakage.
5. Metal Case: A protective metal box, often with glass windows, to shield the leaves from air currents and external disturbances. The case is usually earthed.

Principle of Operation: The electroscope works on the principle of electrostatic repulsion between like charges. When a charged object is brought near or touches the metal cap, charge is transferred to the gold leaves, causing them to acquire the same type of charge. Since like charges repel, the gold leaves spread apart.

Uses of the Gold Leaf Electroscope:

1. Detecting Charge: If a charged object is brought near the cap of an uncharged electroscope, the leaves will diverge, indicating the presence of charge.
2. Identifying the Type of Charge:
   1. First, charge the electroscope with a known type of charge (e.g., by touching it with a positively charged rod, making the leaves diverge with positive charge).
   2. Then, bring the object with unknown charge near the cap.
   3. If the leaves diverge further, the unknown charge is the same type as the electroscope’s charge.
   4. If the leaves converge (fall closer together), the unknown charge is opposite to the electroscope’s charge.
3. Estimating the Magnitude of Charge: The degree of divergence of the leaves gives a rough indication of the amount of charge present. Greater divergence means a larger charge.

Generating and Storing Charges: The Electrophorus

The electrophorus is a simple electrostatic generator used to produce and store static electric charges by induction.

Construction: An electrophorus typically consists of:

1. Resin Plate (or Ebonite/Plastic Plate): A flat insulating plate that can be easily charged by friction.
2. Metal Plate (or Disc): A conducting plate with an insulating handle.

Principle of Operation: The electrophorus works on the principle of electrostatic induction and charge separation.

The procedure for generating and storing charges using an electrophorus involves the following steps:

1. Charge the Resin Plate: Rub the resin plate vigorously with a piece of fur or wool. This charges the resin plate negatively by friction.
2. Induce Charge on the Metal Plate: Place the metal plate (holding it by its insulating handle) onto the charged resin plate. The negative charges on the resin plate repel the free electrons in the metal plate to the top surface of the metal plate, leaving the bottom surface positively charged.
3. Earth the Metal Plate: While the metal plate is still on the resin plate, briefly touch the top of the metal plate with a finger. This provides an earthing path, allowing the repelled electrons from the metal plate to flow into the Earth. The metal plate is now left with a net positive charge.
4. Remove Earthing and Lift the Metal Plate: Remove the finger first, then lift the metal plate by its insulating handle. The metal plate is now positively charged and can be used to transfer this charge to other objects.

This process can be repeated multiple times to generate and transfer charges, as the resin plate retains its charge for a long time.

Discharging a Charged Body

A charged body can be discharged, meaning its excess charge is removed, making it electrically neutral again. Common methods of discharging include:

1. Earthing (Grounding): Connecting the charged body to the Earth using a conductor. The Earth acts as an infinite reservoir of charge, either supplying electrons to neutralise a positively charged body or accepting excess electrons from a negatively charged body.
2. Touching with a Conductor: Touching a charged body with a conductor (like a metal object or a person’s hand) provides a path for the excess charge to flow away, neutralising the body.
3. Exposure to Ionised Air: Passing a charged body through a flame or exposing it to ionised air can neutralise it, as the ions in the air combine with the excess charge.

Earthing and Metal Spikes (Lightning Conductors)

Earthing is a critical safety measure in electrical systems and for protecting structures from lightning.

Meaning of Earthing

Earthing, also known as grounding, is the process of connecting an electrical conductor or a charged object directly to the Earth. The Earth is considered a vast reservoir of electric charge, capable of absorbing or supplying a large amount of charge without significantly changing its own electrical potential.

When a charged object is earthed, any excess charge on it quickly flows to the Earth, or electrons flow from the Earth to neutralise a positive charge, thereby making the object electrically neutral.

Advantages of Earthing

Earthing provides several important advantages:

1. Electrical Safety: It protects people from electric shocks by providing a safe path for fault currents to flow directly into the Earth, rather than through a person.
2. Protection of Appliances: It prevents damage to electrical appliances and equipment by diverting excessive currents or surges safely to the ground.
3. Lightning Protection: It is a fundamental component of lightning protection systems for buildings and structures.

Metal Spikes (Lightning Conductors)

Metal spikes, commonly known as lightning conductors or lightning rods, are devices installed on tall buildings and structures to protect them from the damaging effects of lightning strikes.

Construction of a Typical Metal Spike System: A lightning conductor system typically consists of three main parts:

1. Air Terminal (Metal Spikes): These are sharp, pointed metal rods (often copper or aluminium) installed at the highest points of a building. The sharp points are crucial because electric charges tend to concentrate at sharp points, facilitating discharge.
2. Down Conductor: A thick, continuous copper or aluminium strip or cable that runs down the side of the building, connecting the air terminal to the earth electrode. This conductor provides a low-resistance path for lightning current.
3. Earth Electrode (Ground Rod/Plate): A large copper plate or several long metal rods buried deep into the ground, ensuring good electrical contact with the Earth. This component safely dissipates the lightning current into the ground.

How Metal Spikes Work:

1. Charge Concentration at Sharp Points: Electric charges, whether from the ground or from approaching thunderclouds, tend to concentrate at sharp points. The metal spikes on the lightning conductor provide many such points.
2. Silent Discharge: During a thunderstorm, the electric field between the cloud and the ground becomes very strong. The sharp points of the lightning conductor facilitate a continuous, silent discharge of charge from the building into the air, or from the air into the building. This process helps to neutralise the charge in the immediate vicinity of the building, reducing the likelihood of a direct lightning strike.
3. Safe Path for Lightning: If a direct lightning strike does occur, the lightning conductor provides a low-resistance path for the massive electric current to flow safely from the top of the building, through the down conductor, and into the Earth via the earth electrode. This prevents the lightning current from passing through the building’s structure, which could cause fire, structural damage, or electrical system failure.

The effectiveness of metal spikes relies on the principle that charge density is highest at points of high curvature (sharp points), leading to a strong electric field that encourages ionisation of the air and a gradual discharge.