Electricity | ICSE Class 8 Physics Notes

This note covers electric charge, current, cells and batteries, conductors and insulators, charging by rubbing, conduction and induction, conservation of charge, electroscopes, household electrical energy, domestic wiring, electrical safety and lightning protection.

What is the difference between static electricity and electric current?

Electric charge is the property involved in electrical attraction and repulsion. There are two kinds, called positive charge and negative charge. Attraction means pulling towards one another; repulsion means pushing away from one another.

A charged object has an excess or deficit of charge. An electrically neutral object has balanced positive and negative charges. Neutral does not mean that the material contains no charged particles.

When are charges static?

The charges produced by rubbing are called static charges. They do not move by themselves. A plastic comb rubbed with dry hair acquires a small charge and can attract small pieces of paper. The hair also becomes charged during rubbing.

A used ballpen refill rubbed vigorously with polythene provides another example. It acquires a small charge and can attract small pieces of paper. Avoid touching its rubbed end with a hand or a metallic object while observing its behaviour.

What happens when charges move?

When charges move, they constitute an electric current. The current that makes a bulb glow, or makes a wire hot, is a motion of charges. Static electricity and current electricity therefore involve the same property in different situations.

An electric circuit is a continuous, closed conducting path through which current can flow. A conductor is a material that allows electricity to pass through it easily. A cell or battery supplies the energy needed to maintain current in a circuit.

Definition: Static electricity concerns charges at rest; electric current involves the movement of charges. The distinction is about whether charges move, rather than about two unrelated kinds of electricity.

A bulb uses electrical energy while current flows through it. Electrons, the negatively charged particles that move through metal wires, are not consumed by the bulb. Electrical energy is transferred and used in producing effects such as light and heat.

How do cells, batteries and switches form a working circuit?

An electric cell is a source of electrical energy with two connection points called terminals. One terminal is positive and the other is negative. A battery is a combination of two or more cells connected together.

To make the battery arrangement considered here, connect the positive terminal of one cell to the negative terminal of the next. This end-to-end electrical connection is called a series connection. The important feature is the connection between terminals.

Must cells stand in one straight row?

Cells in a device are not always placed one after another. Sometimes they lie side by side. There is usually a thick wire or metal strip connecting the positive terminal of one cell to the negative terminal of the next.

The battery compartment usually has a plus sign, meaning positive, and a minus sign, meaning negative, to indicate correct placement. Torches, toys and television remote controls are examples of devices using batteries.

The upper drawing shows two cells placed end to end. The lower drawing shows four cells in an end-to-end row. The two arrangements are labelled (a) and (b).

See Fig. 10.2 in your NCERT textbook

How does a switch control the circuit?

A switch is a device used to complete or break a circuit. With the switch in the ON position, the circuit is closed. With the switch in the OFF position, the circuit is open and no current flows through it.

The filament is the thin wire inside a filament bulb that glows when current passes through it. A broken filament interrupts the circuit. Closing the switch cannot repair this break inside the bulb.

How do conductors and insulators affect the movement of charge?

Conductors allow electricity to pass through them easily. Metals, human and animal bodies, and the earth are conductors. In metals, some electrons are comparatively free to move within the material, allowing charge to be transferred.

Insulators offer high resistance to the passage of electricity. Resistance means opposition to the flow of electric current. Most non-metals such as glass, porcelain, plastic, nylon and wood offer high resistance and are called insulators.

Where does transferred charge go?

When charge is transferred to a conductor, it readily spreads over its entire surface. Charge placed on an insulator stays at the same place. This difference helps explain why the way an object is held matters when charging it.

A plastic or nylon comb can become charged when rubbed or passed through dry hair. A metal spoon held directly in the hand does not retain charge in the same way: charge leaks through the body to the ground.

This does not mean that a metal object cannot be charged. A metal rod with a wooden or plastic handle shows signs of charging when rubbed without touching its metal part. The handle interrupts the conducting route through the hand.

Insulation is a covering of insulating material around a conducting wire. It separates the conductor from its surroundings. If wire insulation is damaged, conducting parts may touch and allow an excessive current, creating a dangerous situation.

Note: Do not turn “most non-metals” into “all non-metals”. Also distinguish failure to retain charge while held in the hand from an inability of a material to receive charge.

How does rubbing charge objects without creating new charge?

Charging by rubbing involves the transfer of electrons between materials. Electrons carry negative charge. A body becomes negatively charged by gaining electrons and positively charged by losing some electrons.

When a glass rod is rubbed with silk, some electrons move from the rod to the silk. The glass rod becomes positively charged, while the silk becomes negatively charged. Both objects take part in the transfer.

What is conservation of charge?

Conservation of charge means that the total charge of an isolated system remains unchanged. An isolated system, in this discussion, is a group of objects that does not exchange charge with its surroundings.

Rubbing does not create new charge. What one body gains, the other loses. Charges may be redistributed between the bodies, but the total charge of the isolated group stays the same. This is why charging both the rod and the silk is consistent with conservation.

How do charged objects interact?

Like charges, meaning charges of the same kind, repel. Unlike charges, meaning charges of different kinds, attract. Positive and positive repel; negative and negative repel; positive and negative attract.

Keep the method of charging attached to each example. The observation concerns objects prepared in the stated way. Merely naming a balloon or a refill, without saying how it was charged, leaves out an important part of the activity.

The positive sign assigned to a glass rod rubbed with silk is a convention, meaning an agreed naming rule. A charged plastic straw rubbed with polythene is negatively charged and is attracted by the positively charged glass rod.

How do conduction, induction and earthing differ?

Charging by conduction means transferring charge through direct electrical contact. A charged refill touching a metal paper clip can transfer charge through the clip to aluminium foil strips attached to it. Metals provide the conducting route.

Electrostatic induction is the redistribution of charge in a conductor under the influence of a nearby charged object, without direct contact with that object. The conductor's mobile electrons rearrange in response to electrical forces.

Does redistribution mean new charge has appeared?

A nearby charge can cause positive and negative charge to be distributed differently across an initially neutral conductor. If no charge enters or leaves the conductor, its total charge remains unchanged. Separation within the object is different from a gain of net charge.

The word net means the overall result after accounting for positive and negative charges together. Thus a body can contain both kinds of charge and still have zero net charge. Induction must be understood alongside conservation of charge.

What happens during earthing?

Earthing is the transfer of charge from a charged object to the earth. In a simple electroscope, touching the metal paper clip with a hand lets charge pass from the foil strips through the body to the earth.

The foil strips then lose their charge and collapse. They are said to be discharged. The charge has been transferred to the earth; it has not been destroyed. The earth provides the destination for this transfer.

In each process, identify the objects involved and whether there is a conducting connection between them. This makes it easier to distinguish rearrangement within an object from transfer between separate objects.

How does an electroscope detect electric charge?

An electroscope is a device used to detect whether an object carries charge. Its operation depends on charge transfer through a conductor and repulsion between parts that acquire similar charges.

How is a simple electroscope made?

1. Take an empty jam bottle and a piece of cardboard slightly larger than its mouth.
2. Make a hole in the cardboard and insert an opened-out metal paper clip.
3. Hang two aluminium foil strips, each about 4 centimetres long and 1 centimetre wide, from the lower part of the clip. A centimetre, written cm, is a unit of length.
4. Fit the cardboard over the bottle so that the paper clip passes through the lid and the foil strips hang inside.

The drawing shows a hand bringing a refill to a bent metal paper clip above a cardboard lid. The clip passes through the lid of a glass bottle, with two foil strips hanging apart inside.

See Fig. 12.4 in your NCERT textbook

Why do the leaves move apart?

Touch the upper end of the clip with a charged refill. Charge reaches the aluminium strips through the metal clip. Both strips receive similar charge, repel one another and spread apart. This spreading is called divergence.

Touching the clip gently with a hand allows charge to pass to the earth through the body. The strips return to their original state. Charging and then discharging the strips demonstrates both transfer of charge and earthing.

What is a gold-leaf electroscope?

A gold-leaf electroscope has a vertical metal rod housed in a box and two thin gold leaves attached to its lower end. A charged object touching the metal knob at the top transfers charge to the leaves, which diverge.

The drawing labels a metal knob, metal rod, rubber, glass window and gold leaves. The rod enters through the top of a box, and the leaves hang from its lower end inside.

See Fig. 1.2a in your NCERT textbook

The degree of divergence indicates the amount of charge. The aluminium-foil device and the gold-leaf instrument use the same basic idea: similar charges on two light leaves cause visible repulsion.

How are current, charge and potential difference measured?

Current measures the charge passing through a cross-section of a conductor per unit time. For a steady current, $I=\frac{Q}{t}$, where $I$ is current in amperes, $Q$ is charge in coulombs and $t$ is time in seconds.

The SI unit of electric charge is the coulomb, written C. The SI unit of electric current is the ampere, written A. One ampere means that one coulomb of charge passes through a cross-section each second.

The SI unit of potential difference is the volt, written V. One volt means that one joule of work is done in moving one coulomb of charge between two points.

Potential difference is $V=\frac{W}{Q}$, where $W$ is work done in joules and $Q$ is charge in coulombs. Rearranging gives $W=VQ$. Here the italic symbol $W$ represents work; the upright unit symbol W represents watts.

Worked example 1. Find the work done in moving a charge of 2 C between points with a potential difference of 12 V.

Formula: $W=VQ$.

Substitute: $W=12\,\mathrm{V}\times2\,\mathrm{C}=24\,\mathrm{J}$.

Answer: The work done is 24 J.

How are electrical power and household energy consumption calculated?

Electrical energy is the energy supplied to electrical devices and used in effects such as heating, lighting or motion. Electric power is the rate at which electrical energy is consumed or transferred.

Let E represent electrical energy consumed, P represent power and t represent operating time. For an appliance operating at constant power, energy is the product of its power and the time for which it operates.

$E=Pt$

Derivation: Electrical power from charge flow

Consider charge $Q$ passing through a potential difference $V$ in time $t$, with steady current $I$. Use coulombs, volts, seconds and amperes respectively; energy $E$ is in joules and power $P$ is in watts.

1. Moving the charge requires work, so the energy supplied is $E=VQ$.
2. Power is energy supplied per unit time: $P=\frac{E}{t}=\frac{VQ}{t}$.
3. Since $I=\frac{Q}{t}$, substitute current for charge divided by time to obtain $P=VI$.

Result: $P=VI$. For constant power, the energy supplied in time $t$ is $E=Pt=VIt$.

Which units belong in the calculation?

The International System of Units, abbreviated SI, provides standard measurement units. The SI unit of power is the watt, written W. The joule, written J, measures energy, and the second, written s, measures time. One watt means one joule per second.

A kilowatt, written kW, equals 1000 watts. A watt hour, written Wh, is the energy consumed when one watt of power is used for one hour. The hour is written h in unit symbols.

A kilowatt hour, written kWh, is the energy consumed when one kilowatt of power is used for one hour. It is the commercial unit of electrical energy, commonly called one unit. One kilowatt hour equals 3.6 × 10⁶ joules.

Derivation: Converting a kilowatt hour to joules

A kilowatt hour is the energy used by a device operating at one kilowatt for one hour. Convert both factors before multiplying them.

1. Convert power: $1\,\mathrm{kW}=1000\,\mathrm{W}$.
2. Convert time: $1\,\mathrm{h}=3600\,\mathrm{s}$.
3. Multiply power by time: $1\,\mathrm{kWh}=1000\,\mathrm{W}\times3600\,\mathrm{s}=3.6\times10^6\,\mathrm{J}$, because one watt second is one joule.

Result: One kilowatt hour equals 3.6 million joules. The SI unit of electrical energy is the joule, written J; a kilowatt hour is the commercial unit used in electricity bills.

How do we calculate an electricity charge?

Worked example 2. A refrigerator rated 400 W operates for 8 hours per day for 30 days. Calculate its energy consumption and cost at ₹3.00 per kWh. Here ₹ denotes rupees.

Formula: $E=Pt$ and $C=Er$, where $C$ is the cost in rupees and $r$ is the price per kWh. Use power in kW and time in hours to obtain energy in kWh.

Substitute: $P=\frac{400}{1000}\,\mathrm{kW}=0.4\,\mathrm{kW}$ and $t=8\times30\,\mathrm{h}=240\,\mathrm{h}$.

Then $E=0.4\times240\,\mathrm{kWh}=96\,\mathrm{kWh}$. Multiplying 96 by ₹3.00 gives ₹288.00.

Answer: Energy consumed is 96 kWh and the cost is ₹288.00.

How do voltage and current determine appliance power?

Worked example 3. An electric bulb connected to a 220 V generator carries a current of 0.50 A. Find its power.

Formula: $P=VI$.

Substitute: $P=220\,\mathrm{V}\times0.50\,\mathrm{A}=110\,\mathrm{W}$.

Answer: The bulb's power is 110 W.

Worked example 4. An electric motor takes 5 A from a 220 V line. Find its power and the energy consumed in 2 hours.

Formula: $P=VI$ and $E=Pt$. Use watts and seconds for energy in joules.

Substitute: $P=220\times5\,\mathrm{W}=1100\,\mathrm{W}$. The operating time is $t=2\times3600\,\mathrm{s}=7200\,\mathrm{s}$.

Then $E=1100\times7200\,\mathrm{J}=7920000\,\mathrm{J}$. Alternatively, $E=1.1\,\mathrm{kW}\times2\,\mathrm{h}=2.2\,\mathrm{kWh}$.

Answer: Power is 1100 W and energy consumed is 7920000 J, or 2.2 kWh.

How do we compare energy use and calculate heating?

Worked example 5. Which uses more energy: a 250 W television operating for 1 hour or a 1200 W toaster operating for 10 minutes?

Formula: $E=Pt$. Convert the operating times to seconds: one hour is 3600 s, while 10 minutes is 600 s.

Substitute for the television: $E=250\times3600\,\mathrm{J}=900000\,\mathrm{J}$.

Substitute for the toaster: $E=1200\times600\,\mathrm{J}=720000\,\mathrm{J}$.

Answer: The television uses 900000 J and the toaster uses 720000 J. The television uses 180000 J more, despite its lower power rating.

Worked example 6. Calculate the heat generated when 96000 C of charge passes through a potential difference of 50 V in one hour.

Formula: $H=VQ$, where $H$ is heat in joules when the supplied electrical energy is dissipated as heat. This follows from $H=VIt$ and $Q=It$.

Substitute: $H=50\,\mathrm{V}\times96000\,\mathrm{C}=4800000\,\mathrm{J}$. The total charge is already given, so it is unnecessary to calculate the current first.

Answer: The heat generated is 4800000 J.

A power rating states the rate of energy use for the stated operating conditions. It is not itself an amount of energy. The refrigerator calculation needs both the power and the total operating time, followed by the price for each unit of energy.

Note: A kilowatt measures power; a kilowatt hour measures energy. Multiplying power by time gives energy, so do not label an energy answer simply kW.

What roles do live, neutral and earth wires have in a home?

The mains is the main electricity supply received by a home through overhead wires or underground cables. The live wire and neutral wire are the supply wires across which household appliances are connected.

Potential difference means the work done in moving a unit charge between two points. It is also called voltage and is measured in volts, written V. This difference supplies the driving condition for current in a complete circuit.

How are appliances connected?

The supply wires pass through the meter-board arrangement and main switch to separate circuits in the house. An electricity meter records the electrical energy supplied. Each appliance has a separate switch controlling current through it.

Appliances are connected in parallel, meaning that each is connected across the same pair of supply wires on its own branch. This gives each appliance the same potential difference. A branch is a separate conducting path within the circuit.

This differs from connecting cells in series to make a battery. In the battery, the positive terminal of one cell connects to the negative terminal of the next. In domestic wiring, appliances have separate paths across the live and neutral wires.

Why is an earth wire added?

The earth wire is a safety connection. It is usually connected to a metal plate deep in the earth near the house. The metallic body of an appliance is connected to this wire.

If current leaks to the metal body, the earth connection provides a path of low resistance. Low resistance means little opposition to current. This helps keep the appliance body at the earth's potential, and the user may not get a severe electric shock.

Examples of appliances with metallic bodies include an electric press, toaster, table fan and refrigerator. Earthing is a protective arrangement, not a reason to touch exposed wires or investigate a mains circuit.

How do fuses and circuit breakers protect electrical circuits?

The heating effect of current is the production of heat when current passes through a wire. A wire carrying a large current may become hot enough to melt and break. This effect is used in an electric fuse.

An electric fuse is a safety device containing a wire chosen to melt when current becomes excessive. Melting breaks the circuit and stops the current. This protects the circuit and appliances from possible damage and fire.

What causes excessive current?

A short circuit occurs when live and neutral wires come into direct contact, for example because insulation is damaged or an appliance develops a fault. The current then increases abruptly.

Overloading means drawing an excessive current beyond the circuit's safe limit. Connecting too many appliances to one socket can cause it. An accidental rise in supply voltage can also cause overloading.

1. A fault or excessive load allows current to exceed the safe limit of a circuit.
2. The excessive current produces heating in the fuse wire.
3. The fuse wire melts and breaks, interrupting the conducting path.
4. The open circuit stops the flow of current through the protected path.

How is a circuit breaker different?

A miniature circuit breaker, abbreviated MCB, is a switch that automatically turns off when current exceeds the safe limit. Turning it on again completes the circuit. Its operation can be explained qualitatively without calculating the internal mechanism.

Use the proper fuse specified for the application. Never substitute just any wire or metal strip. Do not investigate a mains-connected fuse yourself. For simple circuit activities, use electric cells rather than mains, generator or inverter supplies.

How does lightning occur, and how does a lightning conductor help?

Lightning is an electric spark on a huge scale. It results from the accumulation of charges in clouds. An electric discharge is the movement of accumulated charge that produces the bright streaks and sound associated with lightning.

How do charges separate in a thunderstorm?

During the development of a thunderstorm, a storm accompanied by thunder and lightning, air currents move upward while water droplets move downward. These vigorous movements cause separation of charges.

By a process not yet completely understood, positive charges collect near the upper edges of clouds and negative charges accumulate near their lower edges. Positive charges also accumulate near the ground.

Air is normally a poor conductor of electricity. When the accumulated charges become very large, air is no longer able to resist their flow. Positive and negative charges meet, producing streaks of bright light and sound.

The discharge can take place between two or more clouds, or between clouds and the earth. Thus lightning is not restricted to a cloud-to-ground event. A lightning strike could destroy life and property.

How is a building protected?

A lightning conductor is a device used to protect a building from the effects of lightning. It consists of a metallic rod taller than the building, installed in its walls during construction.

One end remains above the building in the air, while the other is buried deep in the ground. The metal provides an easy route for transferring electric charge to the earth.

The drawing shows a conductor projecting above a building and continuing down to the ground. The upper conductor and the buried copper plate are labelled.

See Fig. 12.7 in your NCERT textbook

The metal columns, electrical wires and water pipes in a building also protect to an extent. Do not touch them during a thunderstorm. A lightning conductor gives charge a route to the earth; its presence does not remove the need for personal precautions.

What precautions reduce danger during thunderstorms?

During lightning and a thunderstorm, no open place is safe. Hearing thunder, the sound accompanying lightning, is an alert to move to a safer place. After the last thunder, wait for some time before leaving that place.

Where should shelter be sought?

A house or building provides shelter. A person travelling in a car or bus is safe inside with its windows and doors shut. Open vehicles, such as motorbikes, tractors and open cars, do not provide this protection.

Open fields, tall trees, park shelters and elevated places do not protect against lightning strokes. Carrying an umbrella is not a good idea during a thunderstorm. Avoid poles and other metal objects.

If in a forest, take shelter under shorter trees. If no shelter is available in an open field, stay far from trees. Do not lie on the ground. Squat low, place the hands on the knees and keep the head between the hands.

What precautions apply indoors?

Lightning can strike telephone cords, electrical wires and metal pipes. Avoid contact with them during a thunderstorm. Mobile and cordless phones are safer to use, but avoid calling someone who is receiving the call on a wired phone.

Avoid bathing during a thunderstorm because it involves contact with running water. Electrical appliances such as computers and televisions should be unplugged. Keep the different hazards in mind: being indoors and avoiding conducting routes are connected precautions.

Note: Hearing thunder signals the need to move to a safer place. Remain there for some time after the last thunder instead of leaving as soon as the sound stops.

Thunderstorm precautions and domestic circuit protection serve related but different purposes. A fuse interrupts excessive current in a circuit. An earth connection provides a safety path from an appliance body. A lightning conductor gives lightning charge an easy route into the ground.

Glossary

• Electric charge — The electrical property responsible for attraction and repulsion, occurring in positive and negative kinds.
• Static electricity — Electricity associated with charges at rest, such as charges produced on objects by rubbing.
• Electric current — The movement of electric charges through a conducting path.
• Conductor — A material that allows electricity to pass through it easily.
• Insulator — A material offering high resistance to the passage of electricity through it.
• Battery — A combination of two or more cells connected together to supply electrical energy.
• Conservation of charge — The principle that the total charge of an isolated system remains unchanged.
• Electrostatic induction — Redistribution of charge within a conductor caused by a nearby charged object without contact.
• Electroscope — A device used to detect electric charge through the behaviour of its light conducting leaves.
• Earthing — The process of transferring charge from a charged object to the earth.
• Electric power — The rate at which electrical energy is consumed or transferred by a device.
• Kilowatt hour — The energy consumed when one kilowatt of power is used for one hour.
• Electric fuse — A safety device whose wire melts under excessive current and breaks the circuit.
• Short circuit — Direct contact between live and neutral wires causing an abrupt increase in current.
• Lightning conductor — A metallic device that protects a building by providing an easy path for lightning charge to earth.

Common errors and misconceptions

• Misconception: A neutral object contains no charges. Correct: Its positive and negative charges are balanced, so its net charge is zero.
• Misconception: Rubbing creates electric charge. Correct: Electrons transfer between the rubbed objects; the total charge of the isolated pair remains unchanged.
• Misconception: Only opposite charges repel. Correct: Like charges repel, while unlike charges attract one another.
• Misconception: A metal object cannot be charged. Correct: Charge can leak through the hand; a suitable insulating handle allows a metal rod to retain charge.
• Misconception: Earthing destroys charge. Correct: Charge passes to the earth through a conducting route, so it has been transferred.
• Misconception: Kilowatts and kilowatt hours measure the same quantity. Correct: Kilowatts measure power, while kilowatt hours measure energy consumed over time.
• Misconception: Any wire can replace a fuse wire. Correct: Use the proper fuse specified for the application, because its response to excessive current provides protection.
• Misconception: Lightning occurs only between a cloud and the earth. Correct: Electric discharge can also occur between two or more clouds.

Exam-style questions with model answers

Q1. Distinguish static electricity from electric current. [2 marks]

1. Static electricity concerns charges at rest, such as the charges produced on a plastic comb by rubbing it with dry hair.
2. Electric current involves moving charges, as in the conducting circuit that makes an electric bulb glow.

Q2. An initially neutral glass rod and silk cloth are rubbed together, with no charge exchanged with their surroundings. Electrons pass from the rod to the silk. Explain the charges produced and conservation of charge. [3 marks]

1. The glass rod loses negatively charged electrons, so it becomes positively charged. Its positive charge represents a deficit of electrons rather than the creation of new positive charge.
2. The silk gains the electrons lost by the glass rod and becomes negatively charged. Both objects therefore become charged during the rubbing process.
3. The total charge of the isolated rod-and-silk pair remains unchanged because charge is transferred between them, not created or destroyed.

Q3. A simple electroscope has two aluminium foil strips hanging from a metal paper clip through a cardboard bottle lid. A charged refill touches the clip, and later a hand touches it. Explain the observations. [4 marks]

1. When the charged refill touches the metal clip, charge transfers through this conductor to the aluminium foil strips suspended inside the bottle.
2. Both strips receive similar charge, and like charges repel, causing the strips to move apart or diverge.
3. The divergence shows that the object touching the clip is carrying electric charge; the apparatus acts as an electroscope.
4. Touching the clip with a hand allows charge to pass through the body to the earth. The strips discharge and collapse.

Q4. A refrigerator uses 400 W for 8 hours each day over 30 days. Energy costs ₹3.00 per kWh. Given 1000 W = 1 kW, calculate the energy consumed and its cost, showing four stages. [4 marks]

1. Convert the operating power into kilowatts: 400 W divided by 1000 W per kW gives 0.4 kW.
2. Find the total operating time: 8 hours per day multiplied by 30 days gives 240 hours of use.
3. Multiply power by time to obtain electrical energy: 0.4 kW × 240 h = 96 kWh.
4. Multiply the energy by the stated price per unit: 96 kWh × ₹3.00 per kWh = ₹288.00.

Q5. Explain five distinct features of domestic electrical protection: earthing, a fuse, a miniature circuit breaker, prevention of socket overloading and safe use of fuse wire. [5 marks]

1. Earthing connects an appliance's metallic body to the earth through a low-resistance conducting path. It helps protect the user if current leaks to the body.
2. A fuse contains a wire that melts when current becomes excessive. The resulting break interrupts the circuit and stops the flow of current.
3. A miniature circuit breaker is a switch that automatically turns off when current exceeds the safe limit. Turning it on again completes the circuit.
4. Avoid connecting too many appliances to one socket. Such a connection can cause overloading, with excessive current and heating in the circuit.
5. Use the proper fuse specified for the application. Do not replace it with just any wire or metal strip, or investigate a mains-connected fuse yourself.

Q6. Explain the development of lightning and the protection provided by a lightning conductor in six points. [6 marks]

1. During a developing thunderstorm, upward air currents and downward water droplets move vigorously. These movements bring about the separation of electric charges.
2. By a process not yet completely understood, positive charges collect near upper cloud edges and negative charges near lower edges. Positive charges accumulate near the ground too.
3. Air is normally a poor conductor. When accumulated charges become very large, air can no longer resist their flow, and a discharge occurs.
4. The meeting of positive and negative charges produces bright streaks and sound. Discharge can occur between clouds or between clouds and the earth.
5. A lightning conductor is a metallic rod installed in a building, extending above it, with its lower end buried deep in the ground.
6. The rod provides an easy route for electric charge to reach the ground, protecting the building from the effects of lightning.

Key takeaways

• Static charges are at rest, whereas electric current involves the motion of charges through a conducting path.
• A battery joins cells with the positive terminal of one connected to the negative terminal of the next.
• Rubbing transfers electrons between objects, while the total charge of an isolated system remains unchanged.
• Like charges repel and unlike charges attract; repulsion between similarly charged leaves makes an electroscope work.
• Earthing transfers charge to the earth and provides a protective conducting path from metallic appliance bodies.
• Electrical energy equals power multiplied by operating time; kilowatt hours measure energy and kilowatts measure power.
• Fuses and miniature circuit breakers interrupt excessive current, while overloading and short circuits can create dangerous heating.
• Lightning can occur between clouds or between clouds and earth; lightning conductors provide a route for charge to ground.

Test yourself

Why is an electrically neutral object not necessarily free of charged particles?

It contains balanced positive and negative charges, giving zero net charge rather than an absence of charge.

How are neighbouring cells connected in a battery?

The positive terminal of one cell is connected to the negative terminal of the next cell.

Why can a metal rod with an insulating handle retain charge?

The handle interrupts the conducting route through the hand, limiting leakage of charge from the rod to the ground.

What makes similarly charged electroscope leaves spread apart?

Both leaves receive the same kind of charge, and like charges repel one another.

What does one kilowatt hour mean?

It is the energy consumed when one kilowatt of power is used for one hour.

What is the difference between a fuse acting and an MCB acting?

A fuse wire melts and breaks the circuit; an MCB automatically switches the circuit off when current exceeds the safe limit.

Does a lightning conductor prevent charge from reaching the ground?

No. It provides an easy conducting route that transfers electric charge to the ground.

Why is it incorrect to call air a good conductor without qualification?

Air is normally a poor conductor. Very large accumulated charges can overcome its resistance to their flow during lightning.