Electrostatics

15.1Static charge

Electrostatics is the study of electric charges that are not moving, which is where the name comes from. Have you ever felt your hair stand up after brushing it, or felt a small jolt on touching a doorknob after leaving a car? These everyday effects are caused by the electrostatic force, the hidden power of electric charge (Figure 15.1).

Electric charge is measured in coulombs (symbol C). The unit is named after the French physicist Charles A. de Coulomb (1736–1806), who discovered a relationship between electric force and charge.

Charge inside the atom

Everything around you, from your hair and clothes to the doorknob, is built from tiny building blocks called atoms. Inside an atom there are even smaller particles. Some carry a positive charge (+) and are called protons; they sit in the nucleus. Others carry a negative charge (−) and are called electrons; they form a cloud around the nucleus. Neutrons have no charge and are also found in the nucleus. Figure 15.2 shows the simplest atom, hydrogen, and Table 15.1 lists the charges and masses of the three particles.

There are two kinds of charge

How do you know that charges are of two types? Rub a glass rod vigorously with a silk cloth and hang it up. Bring a second glass rod, rubbed with silk in the same way, close to it. The two glass rods push each other away. The same happens with two ebonite (hard rubber) rods rubbed with fur.

Now hang a glass rod rubbed with silk and bring an ebonite rod rubbed with fur near it. This time the rods are pulled towards each other. The same happens if the ebonite rod is hung and the glass rod is brought near.

This experiment shows that there are two types of charge: like charges repel, while opposite (unlike) charges attract. By Newton's third law of motion, the force of repulsion or attraction on one rod is equal in size and opposite in direction to the force on the other rod, and the two forces act on different objects.

15.1.1Charge is conservedOften examined

Charge cannot be created or destroyed. So how does an object become charged? When objects are charged, electric charge is not actually created; it is simply transferred from one object to another. This transfer of charge is called electrification.

The excess charge is almost always produced by a transfer of electrons, not protons. Protons are bound in the nucleus and, in common situations, stay fixed (their mass in Table 15.1 is also far greater than that of an electron). Electrons in the outer part of the atom are the ones that can move from one body to another.

Charging a glass rod and a rubber rod

When a glass rod is rubbed with a silk cloth, both the rod and the cloth become charged. Electrons move from the glass rod to the silk cloth. The glass rod, which has lost electrons, is positively charged. The silk cloth, which has gained electrons, is negatively charged. This silk cloth will repel a hanging rubber rod that has been rubbed with fur, in the same way as another rubber rod rubbed with fur does.

Similarly, when a rubber rod is rubbed with animal fur, electrons move from the fur to the rubber rod. The rubber rod becomes negative and the fur becomes positive.

When you charge an object, you either add or remove electrons. If you remove electrons the object has a positive charge; if you add electrons it has a negative charge. The number of protons in the object stays the same in both cases. The net charge on a solid is therefore due to an excess or a deficiency of electrons.

15.2Conductors and insulatorsOften examined

Materials are divided into conductors and insulators by their ability to conduct, or carry, electric charge. Metals are efficient conductors of electric charge. Glass, rubber and most plastics are insulators, or poor conductors.

In a conductor the valence electrons, which are the electrons in the outermost atomic orbits, are loosely bound to their atoms. This loose binding lets them be detached easily, so they move freely inside the conductor and can even leave it altogether (Figure 15.4(a)). These mobile electrons are called free electrons. Because of them a conductor transfers electric charge efficiently.

In an insulator even the least tightly bound electrons are firmly attached to their atoms. They cannot move freely, so charge cannot flow through the material. Electric charge cannot easily be redistributed or removed from an insulator, which makes it ineffective at conducting charge (Figure 15.4(b)).

How charge sits on a conductor and on an insulator

When a charge is placed on a spherical conductor it spreads out evenly over its surface. On other shapes, the charges repel one another and move towards the more pointed parts (Figure 15.5(a)). An insulator does not let charges move easily, so the charge stays in the spot where it was put (Figure 15.5(b)).

So if you charge one part of an insulator, for example by rubbing wool on a plastic rod, the charge stays in that area while the rest of the rod remains neutral.

Telling a conductor from an insulator by experiment

Charge a gold-leaf electroscope (section 15.4) so that its leaf stands out. Connect the electroscope cap to the earth through the material under test, for example by touching the cap with one end of the material while the other end is held in the hand or joined to the ground. If the material is a conductor, the charge flows away through it and the leaf collapses. If it is an insulator, the charge cannot escape and the leaf stays raised. The same idea works when a metal rod and a plastic rod are touched in turn to a charged object.

15.3Charging and dischargingOften examined

Sliding across a car seat moves electrons between materials and leaves an accumulation of negative charge on your body. This electrostatic charge stays where it is (it is "static") and is produced by friction between the surfaces. It remains until the electrons can spread out because they repel one another. When you reach for a metal doorknob the electrons find an escape route and you see a spark. A metal doorknob is a good conductor, so electrons can move freely through it. If you touch plastic or wood you feel no shock.

There are three main ways to charge objects: through friction, contact and induction.

15.3.1Charging by friction

Friction is a basic way for charge to be transferred between objects. The direction and the amount of the transfer depend on two properties of atoms: ionization energy and electron affinity.

The nucleus of an atom attracts the mobile electrons in its outer orbitals. The stronger the attraction, the more energy is needed to remove an electron. Removing an electron from an atom ionizes the atom.

Ionization energy

The energy needed to remove the outermost electron from an atom.

Electron affinity describes how strongly an atom attracts an extra electron. When two different materials are rubbed together, electrons pass from the material whose atoms hold them less strongly to the material whose atoms attract them more strongly. This is why the glass rod loses electrons to silk and the fur loses electrons to the rubber rod (section 15.1.1).

15.3.2Charging by contact (conduction)

When a charged object touches a neutral object, the excess or deficit of electrons on the charged object makes charge pass to the neutral object. In Figure 15.6, a negatively charged object gives some of its excess electrons to the neutral object, which therefore becomes negatively charged. A positively charged rod, on the other hand, pulls electrons out of a neutral object and leaves it with an overall positive charge.

15.3.3Charging by inductionOften examined

Imagine a negatively charged rubber strip held near an insulated metal sphere X that touches a second sphere Y (Figure 15.7(a)). The negative charge on the strip repels the electrons in sphere X and pushes them towards the far side of the connected sphere Y. This creates a temporary separation of charge: an excess of negative charge on the far side of Y and a shortage of electrons on the near side of X. Importantly, no electrons pass from the strip to the spheres.

Now separate sphere X from sphere Y while the charged strip is still nearby (Figure 15.7(b)). The separated charges are "frozen" in place. Sphere X, which lost electrons, becomes positively charged. Sphere Y, which holds the repelled electrons, becomes negatively charged. When the strip is taken away, the charge spreads over each sphere (Figure 15.7(c)). This is a way of using electrostatic induction to separate charges and create charged objects.

From these two activities it follows that bodies can be charged by a process called electrostatic induction.

Electrostatic induction

A change in the distribution of electrical charge in an object, caused by the influence of nearby charges.

Induction with earthing (grounding)

One way to charge a conductor by induction is by grounding it. This means connecting a neutral object to the ground for a short time so that extra charges can escape. Suppose a negatively charged object is brought close to a neutral metal sphere. The positive charges in the sphere are drawn towards the charged object and the negative charges are pushed to the side away from it, so the electrons move away from the side that faces the charged object. Earthing (grounding) means giving charge a path along which it can escape into the ground. If the sphere is touched briefly with a finger, the extra electrons flow through the finger into the ground, and the sphere is left with a net positive charge (Figure 15.8).

15.3.4Methods to discharge insulators

Electrical discharging is the process of neutralizing or dissipating electric charge on a material or surface. In conductors this happens when the excess charge is redistributed or flows away through a conducting path, returning the material to a neutral state. Conductors can be discharged by grounding them or by applying an opposite charge. Insulators resist the flow of charge, but they too can be discharged under certain conditions.

Discharging an insulator usually requires changing the environment of the material so that charge can move or be neutralized. Two effective techniques are heating it over a flame and exposing it to damp (humid) conditions.

A. Heating above a flame. When an insulator is heated with a flame, the heat excites the electrons and can make the surrounding air conductive, which helps to neutralize the charge. The heat can also temporarily increase the surface conductivity of the insulator, so the static charges can move and discharge (Figure 15.9(a)).

B. Exposing to humid conditions. Placing an insulator in damp air promotes discharge because water molecules stick to its surface. These polar molecules interact with the charged areas and form a thin conductive layer that lets charge move. In addition, the ions present in humid air help to neutralize the static charge on the surface of the insulator, which makes this an effective discharging method (Figure 15.9(b)).

15.3.5A surface charge can be induced on insulators by polarization

Run a plastic comb or brush through your hair. Some electrons move from your hair to the comb, which becomes negatively charged. Now hold the comb near small pieces of paper. The pieces, which have a balanced charge, jump towards the comb. How is this possible? Why does a neutral object get attracted rather than repelled?

The paper starts with equal numbers of positive and negative charges. The negatively charged comb creates an electric field around it as it approaches the paper. The negative charge on the comb repels the negative charges in the paper but attracts the positive charges. The positive charges therefore move closer to the comb while the negative charges are pushed away. In the end, the attraction between the comb and the positively charged near side of the paper is stronger than the repulsion between the comb and the negative charges farther away. This net attraction pulls the paper towards the comb, like a tug-of-war won by the nearer, attracting side.

The same effect is seen with a plastic (or amber) rod rubbed with fur (Figure 15.10(a) and (b)). A balloon rubbed against hair can also attract a falling stream of water (Figure 15.10(c) and (d)).

Induced polarization

When a charged object is brought near an insulator, the electric field of the charged object redistributes the charges within the insulator, creating a slight separation of positive and negative charges.

15.4ElectroscopeOften examined

The electroscope is an essential device for detecting and analysing the charge present on an object. It works on the principle that like charges repel and unlike charges attract.

A basic electroscope consists of a metal bar with a metal disk at its upper end. A thin, flexible gold leaf (or another metal such as silver or copper) is attached to the lower end of the bar. The lower part of the electroscope is enclosed in an insulated housing (Figure 15.11).

Detecting a charge

To use the electroscope, touch the metal disk with the object to be tested. For example, if you touch the disk with a negatively charged plastic rod, some of the rod's excess electrons pass to the disk. They spread through the metal bar and the gold leaf by electrostatic induction. The leaf and the bar now carry the same kind of charge, so the flexible leaf repels the bar and rises higher. In the same way, if a positively charged rod is brought near the metal disk, it attracts electrons from the electroscope and leaves the disk with a net positive charge. Again the leaf rises.

When a charged rod is brought near an uncharged electroscope, it shows that the rod is charged, but it does not tell us the sign of the charge.

Finding the sign of a charge

The sign can be found if the electroscope is first given a known kind of charge. For instance, give the electroscope a negative charge by transferring electrons to it from a negatively charged object. The electrons repel one another, and some pass to the leaf, which diverges from the bar (Figure 15.12(a)). If a negatively charged rod is now brought close to the electroscope, more electrons are repelled down to the leaf and it diverges even further (Figure 15.12(b)). A positively charged rod, on the other hand, attracts electrons up to the disk and away from the leaf, so the leaf collapses (Figure 15.12(c)).

Charging an electroscope by induction

It is possible to give an electroscope a positive charge using a negatively charged rubber rod, by induction. The electroscope is earthed with a finger so that electrons can escape from the disk to the ground (Figure 15.13). When the negatively charged rod is brought close to the disk, electrons are repelled through the finger and the body to the Earth. If the finger is removed while the charged rod is kept nearby, the electroscope is left with a net positive charge.

15.5Electric fieldOften examined

One charge exerts a force on another charge. This force is present everywhere around the charge and in theory it extends to infinity, but its strength decreases with distance. The region around a charge where it can attract or repel another charge is described by the concept of the electric field.

Electric field

The region around a charge where it exerts a force on other charges.

The electric field strength decreases with distance from the charge, following an inverse square law for point charges. So although the field is theoretically present everywhere, its influence becomes smaller as the distance grows.

For the electric force to act on a charged particle, the charges do not have to touch. The force is there whenever a test charge is brought into the electric field of a charge (usually called the source charge). Consider a test charge $q_0$ placed in the electric field of a source charge $Q$ (Figure 15.14). The source charge exerts an electrostatic force $F_E$ on the test charge.

Electric field intensity

The electrostatic force per test charge (per unit positive charge) when it is brought into the electric field of a source charge.

The SI unit of electric field intensity is $\mathrm{N}{\mathrm{C}}^{-1}$ (newton per coulomb). Electric field intensity (electric field strength) is a vector quantity: it has both magnitude and direction. The direction of the electric field intensity at a point is the direction of the force on a positive test charge placed at that point.

Rearranging Equation 15.2 gives the force on a charge $q$ placed in a field of strength $E$.

A positive charge feels a force in the direction of $E$. A negative charge feels a force of the same size in the opposite direction.

How the field changes with distance

For a point charge the field strength follows an inverse square law, so it falls quickly with distance. If $E_0$ is the strength at distance $r_0$, then at $2r_0$ it is $\frac{1}{4}{E}_0$ and at $3r_0$ it is $\frac{1}{9}{E}_0$.

15.5.1Electric field linesOften examined

To picture the electric field around a source charge, the direction of the force on a positive test charge is drawn as a line. This line gives the direction of the electric field intensity at that point or region, and so these lines are called electric field lines. They were first introduced by Michael Faraday. Field lines are imaginary and have no physical existence.

For a positive charge, the field lines are directed away from the charge, because a positive charge repels a positive test charge (Figure 15.15(a)). For a negative charge, the field lines are directed towards the charge, because it attracts a positive test charge brought into its field (Figure 15.15(b)).

Figure 15.16(a) shows the symmetric field lines of a pair of point charges of equal size and opposite sign. Such a pair forms an electric dipole. The lines start on the positive charge and end on the negative charge. The crowded lines between the charges show a powerful electric field in that region, particularly between the two charges.

Figure 15.16(b) shows two particles with the same kind of charge (both positive). They repel each other. The field lines show the direction and strength of the field around the charges: the lines from the two charges bend away from each other and do not join.

Field of a charged conducting sphere

The electric fields of charged conductors have interesting properties. A charged conducting sphere makes an electric field that spreads out uniformly if the sphere is positive (Figure 15.17), or points inwards if it is negative. Outside the sphere the field behaves like that of a point charge, decreasing with distance according to the inverse square law. Inside the sphere the electric field is zero, because the charges distribute themselves evenly on the surface and produce a balanced field within. A point charge, in contrast, produces a field that spreads out from a single point and decreases with the inverse square of the distance, without any interior region.

Field between two parallel plates

Another important example is two parallel plates that are oppositely charged: one plate is positive and the other negative (Figure 15.18). The two plates get their charge by being joined to a battery. The electric field in the region between them is uniform, which means that both the strength and the direction of the field stay constant throughout the space between the plates. The field lines run in straight parallel paths from the positive plate to the negative plate and show the direction in which a positive charge would move. This uniformity is an important property of the field between parallel plates, because it simplifies calculations and predictions about charged particles in the field.

15.6.1Electrostatic precipitator and dust extraction

Electrostatics has a number of applications in daily life. The first is the electrostatic precipitator, also called a dust extractor, which works on the principle of electrostatics. To reduce air pollution, coal-burning power stations use electrostatic precipitation to remove dust from the smoke in their chimneys before it is released into the environment (Figure 15.19).

Air mixed with dust enters the device across a positively charged mesh. The airborne particles become positively charged when they touch the mesh. They then pass through a second, negatively charged mesh. The electrostatic force of attraction between the positively charged particles in the air and the negatively charged mesh makes the particles precipitate out on the surface of the mesh.

15.6.2Photocopier machine

Most photocopying machines use an electrostatic process called xerography, from the Greek words "xeros" (dry) and "graphos" (writing). The main steps are: charging the photoconducting drum, transferring an image, creating a positive charge duplicate, attracting toner to the charged parts of the drum, and transferring the toner to the paper (Figure 15.20).

The process begins with a drum of aluminium coated with selenium, which is sprayed with a positive charge by a device called a corotron. Selenium is a photoconductor: it behaves as an insulator in the dark and becomes a conductor when it is exposed to light. This property lets the drum hold a static charge in the dark areas while it discharges in the areas that are exposed to light, so a hidden (latent) image is created on the surface of the drum. The toner particles, which are oppositely charged, are then attracted to the charged areas of the drum. Finally the toner is transferred to a piece of paper and fused on with heat and pressure, which gives a clear copy of the original document.

15.7Electrical breakdown

Insulating materials such as rubber, glass and some plastics do not normally conduct electricity. When the electric field is strong enough, however, they can reach a breakdown voltage. Electrons are then pulled out of their atoms and current can flow.

Gases are generally poor conductors too, but a strong electric field can ionize gas atoms, removing electrons and creating free electrons and positively charged ions. The gas can then conduct electricity. This can set off an avalanche effect, in which freed electrons collide with other atoms and cause further ionization.

Visible examples

Corona discharge and Lichtenberg figures show how electrical breakdown happens.

Corona discharge appears as a faint glow around sharp points or edges of conductors at high voltage. It occurs because the air molecules around the conductor become ionized, which causes a localized breakdown. You can see this effect around power lines (Figure 15.21) or during thunderstorms.

Lichtenberg figures are intricate branching patterns formed on insulating surfaces such as acrylic or glass (Figure 15.22) when they are subjected to a high-voltage discharge. They are a visual record of the path taken by the electric current as it travels through the material, leaving behind permanent tree-like patterns. Both phenomena clearly demonstrate electrical breakdown in different situations.

15.7.2Lightning generation

Lightning is a powerful release of electrical energy that occurs in clouds when air, water droplets or volcanic ash collide and cause electrical charges to separate. Positive charges rise to the top of the cloud, while negative charges sink. As the cloud becomes more highly charged, a conducting plasma forms. Excess electrons at the base of the cloud travel towards the ground in a zigzag pattern called a step leader.

The negative charge at the bottom of the cloud induces a positive charge on the surface of the Earth, which is a polarized effect. Nearby air molecules ionize and rise, forming a positive charge known as a streamer. When the streamer meets the step leader, a complete path is established and a lightning strike results, with negative charge moving to the ground at speeds of up to 100\,000\,\mathrm{km\,s^{-1}}.

15.7.3Varieties of atmospheric lightning

Lightning is not limited to the common cloud-to-ground strike. Most lightning occurs entirely within a cloud (intracloud discharges), where it cannot be seen. Visible discharges do take place between clouds (cloud-to-cloud discharges) and between a cloud and the Earth (cloud-to-ground discharges). There are also many fascinating, less understood types of lightning that occur high in the atmosphere, such as:

• Sprites: short-lived, red flashes above thunderstorms.
• Jets: narrow, upward bursts of blue light.
• Elves: expanding rings of faint, white light.
• Others with even more whimsical names, such as trolls, pixies, ghosts and ball lightning. These are all being actively researched by scientists.

15.7.4Uncontrolled charging and lightning rodsOften examined

Uncontrolled electrical charging in nature can lead to serious problems, such as lightning. When clouds accumulate positive and negative electricity, the air can no longer insulate, and a lightning strike can damage property, harm people and start fires.

To stay safe during thunderstorms, take shelter in a sturdy building. If you are outside, crouch with your feet together and stay away from tall objects and water.

Lightning is destructive because it sends very strong currents through poor conductors, which creates extreme heat that can endanger buildings. A lightning rod, invented by Benjamin Franklin in the 1750s, helps to protect structures. It is a pointed metal rod placed on top of a building and connected to a buried rod (Figure 15.23). This system attracts the lightning and gives its current a safe path to the ground, which prevents damage and stops harmful electrical surges.

Formula sheet

Key terms

Electrostatics

The study of electric charges that are not moving.

Coulomb

The SI unit of electric charge, symbol C, named after the French physicist Charles A. de Coulomb.

Proton, electron and neutron

Proton: positive particle in the nucleus (+1.6\times10^{-19}\,\mathrm{C}). Electron: negative particle outside the nucleus (-1.6\times10^{-19}\,\mathrm{C}). Neutron: uncharged particle in the nucleus.

Like and unlike charges

Like charges repel one another; unlike (opposite) charges attract one another.

Electrification

The process by which an object acquires an electric charge, by friction, contact or induction.

Conservation of charge

The total electric charge in an isolated system remains constant: charge cannot be created or destroyed, only transferred between objects.

Quantization of charge

Electric charge exists in discrete units, those of the electron (negative) and the proton (positive), and cannot be divided further.

Conductor

A material that allows electric charge to flow freely through it because it has loosely bound (free) electrons, for example copper and aluminium.

Insulator

A material that resists the flow of electric charge because its electrons are tightly bound, for example rubber, plastic and wood.

Free electrons

The loosely bound outer (valence) electrons of a conductor, which are free to move through the material.

Charging by friction

Rubbing two dissimilar materials together so that electrons pass from one to the other.

Ionization energy

The energy needed to remove the outermost electron from an atom.

Charging by contact

A charged object touches a neutral object and electrons pass between them, so the neutral object gets the same kind of charge, to a lesser degree.

Electrostatic induction

A change in the distribution of electrical charge in an object caused by the influence of nearby charges, without touching.

Earthing (grounding)

Giving charge a path along which it can escape into the ground.

Discharging

The process by which an object loses its electric charge, by conduction, convection or a spark.

Induced polarization

The slight separation of positive and negative charge inside an insulator caused by the electric field of a charged object brought near it.

Electroscope

A simple instrument that detects the presence and the sign of electric charge; its leaf diverges when it is charged.

Electric field

The region around a charge where it exerts a force on other charges.

Electric field intensity

The electrostatic force per unit positive test charge in the field of a source charge: E=F_E/q_0, in \mathrm{N\,C^{-1}}.

Electric field lines

Imaginary lines whose direction at a point is the direction of the force on a positive charge there. They point away from positive charges and towards negative charges, and are closer together where the field is stronger.

Electric dipole

A pair of point charges of equal size and opposite sign.

Uniform electric field

A field with the same strength and direction everywhere, such as that between two oppositely charged parallel plates.

Electrostatic precipitator

A device that uses an electric field to remove dust and other pollutants from air by attracting the charged particles to collecting plates.

Xerography

The electrostatic process used by photocopiers: a charged photoconducting drum, an image, toner attracted to the charged parts, and toner transferred to the paper.

Electrical breakdown

The failure of an insulator, including a gas such as air, when a strong electric field pulls electrons from its atoms and lets current flow.

Corona discharge

A faint glow, or ionization of the air, caused by a strong electric field around a sharp point or edge of a conductor.

Lichtenberg figures

Branching, tree-like patterns left on the surface of an insulator when a high-voltage discharge passes through it.

Lightning

The rapid discharge of electricity within a cloud or between a cloud and the ground, caused by the build-up of charge in the cloud.

Step leader and streamer

The step leader is the zigzag path of electrons that travels from the cloud base towards the ground; the streamer is the positive charge that rises from the ground to meet it.

Van de Graaff generator

An electrostatic generator that uses a moving insulating belt to collect charge on a hollow metal globe at the top of an insulated column, giving very high voltages at low current.

Lightning rod

A pointed metal rod on a building, joined to a buried rod, which gives lightning current a safe path to the ground.

FBISE 2026 questions

Questions from the FBISE SSC-II 2026 (1st Annual) papers that belong to this unit, answered the way an examiner expects.

Exam focus

1. What is an electric field? Define electric field intensity, give its unit and direction, and state the force on a positive and a negative test charge, F=qE. (short)
   Asked as Q.2 (ii) (3 marks, 1+2) in all three 2026 sets: B (effect on a test charge), D (positive test charge) and M (two characteristics). The surest short question of the unit.
   Go to answer
2. Explain how a charged insulator is discharged: by heating over a flame and by moist air. (short)
   Q.2 (vii) OR (3 marks) in all three 2026 sets: moisture in Set B, flame in Set D, heating in Set M. Section 15.3.4 gives both methods.
   Go to answer
3. Which particles are transferred when solids are charged by friction? Say what makes an object positive or negative. (mcq)
   MCQ 3 in all three 2026 sets: electrons (Set B), negative charge (Set D), gains electrons (Set M). Also short question 3 of the book.
   Go to answer
4. Calculate the electric field strength, the force on a charge or the test charge from E=F/q_0 and F=qE. (numerical)
   No numerical from this unit appeared in the 2026 papers, but numerical problems 1 to 6 of the book all use these two relations and the SI unit \mathrm{N\,C^{-1}} is easily asked.
   Go to answer
5. Describe charging by friction, contact and induction, and say whether insulators can be charged by induction. (long)
   The three methods of charging are the core of the unit and the basis of the friction MCQ in all 2026 sets; comprehensive questions 4 and 5 and short question 8 of the book test them. No long question from this unit appeared in 2026.
   Go to answer
6. How do positive and negative charges behave differently in an electric field? (short)
   Short question 5 of the book is the same idea as the test-charge part of Q.2 (ii) in Sets B and D of 2026.
   Go to answer
7. Explain how an electroscope detects a charge. (long)
   Not asked in 2026, but a standard long question of the unit (comprehensive question 7) and the topic of MCQ 1 and MCQ 9 of the book.
   Go to answer
8. Explain how lightning is produced and how a lightning rod protects a building. (long)
   Not asked in 2026, but comprehensive question 10 and short questions 12 and 13 of the book make it a likely long question.
   Go to answer