CBSE Class 7 Science Advanced Physics Electricity Notes Set 01

Electricity

Introduction

Life today is unthinkable without electricity. Bulbs, fans, air conditioners, refrigerators and computers all run on it, which makes it the most widely used form of energy. Why is it so convenient, and what actually happens inside a bulb when we press the switch? This chapter looks at these questions step by step.

Some materials, such as glass and plastic, pick up the power to attract light objects after they are rubbed with a suitable cloth. A plastic pen kept near scraps of paper does nothing. Rub its tip on silk for about a minute and bring it close again, and the scraps jump towards it. Any body that shows this attraction is said to be charged, and the pen in this activity is a charged body.

Nature of Charges

A Charged Body Attracts an Uncharged Body

A glass rod rubbed with a woollen cloth becomes charged. If it is brought near a light, uncharged straw hanging from a thread, the straw is drawn towards the rod.

Like Charges Repel

Charge two glass rods by rubbing both with silk. Hang one from a thread and bring the other close to it. The hanging rod swings away, which shows that the two rods repel each other.

Unlike Charges Attract

Rub an ebonite rod with wool and a glass rod with silk. When the two are brought near each other, they pull towards each other.

These activities show that two charged bodies may either attract or repel. Repeating them with many materials leads to the conclusion that nature has only two kinds of charge, called positive charge and negative charge. A charged body carries one or the other. In terms of this idea, like charges repel and unlike charges attract.

Charging a Body by Friction

When a glass rod is rubbed with silk, the rod becomes charged. The silk also picks up small bits of paper afterwards, so the cloth has been charged as well. Whenever two bodies are rubbed together, both become charged. This way of charging is called charging by friction. The two bodies get charges that are equal in size but opposite in kind. Glass rubbed with silk becomes positively charged, and the silk becomes negatively charged.

Charging a Body by Conduction

Bits of paper jump to a charged glass rod, but they do not stay stuck. After touching the rod they are thrown off. This repulsion tells us that the paper now has the same kind of charge as the rod, and it must have received that charge during the contact.

A neutral body gets charged when it is touched by a charged body, and this method is called charging by conduction. The body that is charged this way gets the same kind of charge as the body that charged it.

The reason lies in the structure of matter. Matter is made of molecules, and molecules are made of atoms. Every atom contains tiny particles carrying positive charge and others carrying negative charge, in equal numbers, so an atom as a whole is neutral. The negatively charged particles can move, and if enough energy is supplied they can even leave the atom. The negatively charged particle of an atom is the electron.

When glass is rubbed with silk, some electrons pass from the glass to the silk. The glass is then left with more positive charge than negative charge, and the silk has more negative charge than positive charge.

Electrical Induction

A neutral body has equal amounts of positive and negative charge. Suppose a positively charged body is brought close to it. The negative charges in the neutral body are pulled towards the charged body and the positive charges are pushed away. One end of the neutral body is therefore left with an excess of negative charge, and the other end with an excess of positive charge.

This shifting of charge inside an uncharged body, caused by a nearby charged body, is called electrical induction. It is the reason a charged body can attract an uncharged one.

The Electroscope

How can we tell whether a body is charged, and what kind of charge it carries? An electroscope is an instrument for detecting a charge and finding its nature.

The gold leaf electroscope has a metal rod passing through a cork or seal into a glass jar. The top end of the rod carries a metal disc or sphere (the brass cap). The bottom end, inside the jar, holds two thin metal strips (a brass strip and a gold leaf). Watching how the strips move apart or come together shows whether a charge is present and what kind it is.

Conductors and Insulators

Some materials let electric charge pass through them and others do not. Materials that do not let charge pass are insulators; wood, plastic and air are examples. Materials that let charge flow are conductors, and all metals are good conductors.

Lightning

How Lightning Occurs

Benjamin Franklin showed that lightning is a natural event caused by electric charge. In a cloud, the small water droplets collect positive charge and the heavier ones collect negative charge. The lighter positive droplets drift to the upper part of the cloud and the heavier negative droplets settle in the lower part. A small patch of positive charge also forms near the bottom of the cloud.

The opposite charges are kept apart by air, which is a very good insulator. When a very large charge builds up across a thin layer of air, the air can no longer insulate and it suddenly lets the charge through. This breakdown is called discharge, and it comes with a spark and a crackling sound. Lightning is the discharge of the huge charge stored in clouds through the air. The discharge can also travel to the ground through tall trees or buildings, and radio towers, trees and tall buildings can be badly damaged when lightning hits them.

Lightning Conductor

A lightning conductor protects buildings and other tall structures. It is a thick metal strip with a pointed spike at the top, fixed above the highest part of the building. The strip runs down the outer wall to the ground, where its lower end is joined to a metal plate buried deep in the earth. The charge therefore flows safely into the earth instead of through the building.

Electric Current

So far we have looked at charges that stay in one place. The study of charges at rest is called electrostatics. Most practical uses of electricity, however, depend on charges in motion.

When charges move from one end of a conductor to the other, we say that an electric current flows in it. The current through the filament of a bulb produces heat and light. The current through an electric motor turns the blades of a fan. Current also drives machines in factories and makes the powerful electromagnets used in industry.

An appliance works only when it is joined to a source of electric current by conducting wires, with a switch in the path. The switch has two positions. In the ON position the current from the source passes through the appliance, and in the OFF position no current flows.

Electric Circuit

The path along which an electric current flows is an electric circuit. A drawing that shows this path with standard symbols is a circuit diagram.

To keep diagrams simple, each common component has its own symbol.

A cell is a source of electric current and is drawn as two parallel lines of different length. The longer line is the positive terminal and the shorter, thicker line is the negative terminal. The signs + and − are also marked on real cells.

Wires are shown as straight lines. To get a stronger current, two or more cells are often joined together, always connecting the positive terminal of one cell to the negative terminal of the next. Such a combination is a battery.

A switch opens or closes a circuit and can be placed anywhere in it. With the switch ON, an unbroken path runs from the positive terminal of the battery to its negative terminal. The circuit is then closed and current flows through every part at once. With the switch OFF, the path is broken. The circuit is open and no current flows anywhere in it.

Fig. 6.7 shows a closed circuit.

Fig. 6.8 shows an open circuit.

Electric Cells

A cell is a source of electrical energy: it changes chemical energy into electrical energy. The common types are described below.

Voltaic Cell

A glass vessel contains dilute sulphuric acid, the electrolyte. A copper plate and a zinc plate are dipped in it. When the plates are joined through a bulb, the bulb glows. The copper plate is the positive pole and the zinc plate is the negative pole, and in the outside circuit the current flows from copper to zinc. The e.m.f. of a Voltaic cell is about one volt.

The Voltaic cell has two defects.

1. Local action. Zinc plates contain impurities such as carbon and iron. These grains react with the acid and the zinc and set up many tiny cells and local circuits on the plate, which wastes the zinc. Coating the zinc plate with mercury keeps the impurities away from the acid and prevents local action.

2. Polarization. While the cell delivers current, zinc dissolves in the acid and hydrogen bubbles are formed. The bubbles collect on the copper plate and resist the flow of current. To reduce polarization a depolariser is used. A depolariser is an oxidising agent that combines with the hydrogen and turns it into water. Potassium dichromate, copper sulphate and manganese dioxide are depolarisers.

Leclanché Cell

This cell has a zinc rod and a porous pot standing in ammonium chloride solution in a glass jar. The porous pot holds powdered carbon mixed with manganese dioxide, and a carbon rod is dipped in this mixture. The zinc rod is the negative plate and the carbon rod is the positive plate. The e.m.f. is about 1.46 V.

Dry Cell

The dry cell is a modified Leclanché cell. A cylindrical zinc container holds a moist paste made of ammonium chloride, zinc chloride, manganese dioxide and powdered carbon. A carbon rod with a brass cap stands in the middle. The brass cap is the positive pole and the zinc container is the negative pole. Its e.m.f. is about 1.46 V.

Bichromate Cell

This cell is built in a flat-bottomed glass flask closed by a special stopper. The electrolyte is a solution of potassium dichromate in dilute sulphuric acid. Two carbon plates are the positive poles and one zinc plate is the negative pole. The zinc plate is fixed to the stopper through a screw arrangement so that it can be raised or lowered, and it is lifted out of the liquid when the cell is not being used. Potassium dichromate oxidises the hydrogen that forms. The e.m.f. of this cell is 2 V.

The eel fish of the Amazon region can produce electricity in its body, and a person who touches it gets an electric shock.

Other cells in use are the button cells, which run calculators and wrist watches, and the storage cells, which supply current to the headlights and engine of cars and motor cycles.

Electric Generator

A cell or small battery can run only small appliances. Homes and industries need far more electrical energy, and it is produced in power plants by an electric generator, which changes mechanical energy into electrical energy. A generator has two parts, a turbine and a dynamo. Flowing water or steam turns the turbine, which turns the dynamo connected to it, and the dynamo produces electricity. Cables then carry the electricity to cities and villages.

Series and Parallel Connections

The parts of a circuit can be joined in two ways: in series or in parallel.

Cells in Series

In a series connection, the positive terminal of one cell is joined to the negative terminal of the next. The free negative terminal of the first cell and the free positive terminal of the last cell are then joined to the bulb.

Fig. 6.13: cells joined in series (a) as pictured and (b) in a circuit diagram.

For cells in series, the total e.m.f. is the sum of the e.m.f. of the individual cells: \( E = E_1 + E_2 + E_3 + \dots \)

Example 1. Four cells of e.m.f. 1.5 V each are connected in series to form a battery. Find the e.m.f. of the battery.

Solution. \( E = E_1 + E_2 + E_3 + E_4 = 1.5 + 1.5 + 1.5 + 1.5 = 6 \) V.

Cells in Parallel

In a parallel connection all the positive terminals are joined together and all the negative terminals are joined together.

Fig. 6.14: cells joined in parallel, (A) as pictured and (B) in a circuit diagram.

When identical cells are joined in parallel, the total e.m.f. is the same as that of a single cell.

Bulbs in Series

Bulbs are in series when they are joined one after the other in a single path, with a battery and switch in the same loop.

Points to note about bulbs (or appliances) in series:

1. All the bulbs work together when the circuit is closed and all stop together when it is opened. If one bulb fuses, the rest also go off.
2. The bulbs do not glow brightly because the energy supplied by the battery is shared among them. Appliances in series therefore do not work at full capacity.

Bulbs in Parallel

Bulbs are in parallel when one end of every bulb is joined to one wire, the other end of every bulb is joined to a second wire, and these two wires lead to the battery.

Here every appliance is connected directly to the battery. Points to note:

1. Each bulb works independently of the others. If one bulb fuses, the remaining bulbs keep working normally.
2. Every bulb gets the full energy it needs, so all of them glow brightly.

Household Distribution of Electrical Energy

The electricity used in homes is produced in power plants and brought to us through cables. At a house, the cables from the transformer first reach the kilowatt hour meter (kW h meter), which records the electrical energy we use. Manufacturers state how much energy an appliance uses; the rate at which electrical energy is used is electric power, which is given in watt. Filament bulbs, for example, are rated 60 W, 100 W and so on.

The kW h meter measures the energy used in kilowatt hours, commonly called electrical units. An appliance of 1000 W power used for one hour consumes one kilowatt hour, or one unit.

Example 2. For how many hours can a 100 W bulb be used to consume one unit of electrical energy?

Solution. A 100 W bulb uses only \( \frac{1}{10} \) of the energy that a 1000 W appliance uses in the same time. So for the same energy it must be used 10 times as long. A 1000 W appliance uses one unit in one hour, so the 100 W bulb uses one unit in 10 hours.

Wiring of a House

From the supply company, two wires come through a transformer to the kW h meter. They are the live wire and the neutral wire. After the meter they pass through the main fuse and the main switch. From there they go either to appliances through switches or to the sockets in the rooms. There are two kinds of sockets, the two-pin socket and the three-pin socket. The three-pin socket has an extra pin for the earth connection. Appliances are connected to a socket through plugs and switches.

Electric Fuse

Every appliance is designed for a certain rated current, and a larger current can damage it. A fuse is a safety device that stops such an excess current from flowing through the appliance.

A wire heats up when current flows through it. Wires made from certain special materials melt quickly and break when the current is too large, and these are used as fuse wires. A fuse has a porcelain grip with two copper clamps, and the fuse wire is fixed between the clamps. Fuse wires differ in thickness, and a thicker wire can carry more current.

Every circuit or appliance has a maximum safe current. If the current goes beyond it, the excess current melts the fuse wire first. This breaks the circuit and saves the appliance. The main fuse beside the kW h meter also prevents unauthorised use of current.

Excess current is caused by short circuiting or by overloading. A short circuit happens when the insulation on wires wears off and the bare wires touch each other. Overloading happens when too many devices are plugged into one socket and together draw too much current.

Miniature Circuit Breakers (MCBs)

An MCB is a switch that turns itself off when the current in a circuit crosses the safe limit. It is joined in series with the live wire and is a good substitute for a fuse.

Precautions While Using Household Appliances

The human body conducts electricity, so we get a shock if current passes through us. We must make sure that no part of the body touches a live wire.

1. Use only appliances that carry the ISI mark.
2. For experiments in electricity use only cells, never the mains supply of the house.
3. Never put a finger or a metal wire into a socket.
4. Do not touch switches with wet hands.
5. Use a tester to check whether current is present.
6. Do not touch the metal parts of appliances or of an electric pole with bare hands.

Heating Effect of Electricity

A wire joined to a cell for a few minutes becomes warm. This is the heating effect of electric current: electrical energy is changed into heat energy. Electric irons, heaters, ovens and filament bulbs all work on this effect.

Such appliances contain a coil of wire called the element (heating element), which becomes red hot when the current flows. The heat produced in a wire depends on five things: its length, its thickness, the material it is made of, the amount of current and the time for which the current flows.

Different jobs need different wires. The filament of a bulb gets so hot that it glows, while ordinary connecting wires do not become hot in normal use. Filament bulbs also give out unwanted heat, which wastes electricity. Fluorescent tubes and compact fluorescent lamps (CFLs) give off much less heat, so using them for lighting saves electricity.

Electric Heater

The heating element of a heater is made of nichrome wire, usually coiled like a spring, and it is mounted on a frame of kaolin or asbestos. The frame is fixed in a metal or asbestos body, and a plug and wires carry the current to the nichrome. The wire resists the flow of current, so it becomes red hot and passes its heat to a vessel placed on the heater. A thick iron plate fixed over the element makes an electric hot plate, which is used in electric irons and in some kitchen appliances. The electric fuse also works on the heating effect of current.

Magnetic Effect of Electric Current

Hans Christian Oersted discovered that a wire carrying current behaves like a magnet. This is the magnetic effect of electric current. (Oersted made this discovery in 1820.)

To see it, wind insulated wire around a magnetic compass and join the free ends to a cell E through a switch S. Normally the compass needle points north-south. When the switch is closed, the needle turns away from its original position. When the switch is opened again, the needle comes back. The needle is a small magnet, so the turning shows that the wire carrying current acts as a magnet. This observation can be used to make an electromagnet.

Electromagnets

Wind insulated wire around an iron rod and join the ends to a cell E through a switch S. Keep a few iron nails below the rod. With the switch ON, the current makes the nails jump up and stick to the rod. With the switch OFF, they fall off.

The coil on the rod acts as a magnet only while a current flows through it, and this device is an electromagnet. Its strength can be increased by increasing the current in the coil.

Uses of electromagnets:

1. Electromagnetic cranes lift heavy loads.
2. Magnetic materials can be separated from scrap using an electromagnet.
3. An electromagnet is an important part of an electric bell.
4. Doctors use electromagnets to take out tiny magnetic particles that have fallen into the eye.

Electric Bell

The coil in the bell acts as an electromagnet. A soft iron strip with a hammer at one end is placed close to the coil. When current passes, the coil becomes a magnet and attracts the strip, so the hammer strikes the gong and a sound is produced. As the strip is pulled towards the coil it leaves the contact screw, so the circuit opens. With no current, the coil stops being a magnet and releases the strip, which springs back and touches the contact screw again. The circuit is now closed once more and the whole action repeats, so the bell rings continuously.

(Fig. 6.23: an electric bell.)

Chemical Effects of Electric Current

When current passes through certain liquids, such as a solution of copper sulphate, chemical changes take place in them. Such a liquid is called an electrolyte. The molecules of the electrolyte split into ions, and the ions travel towards the opposite electrodes. This process is electrolysis.

Electroplating

Electroplating means coating articles made of a cheap metal with a thin layer of gold or silver. Electrolysis is also used to purify metals: the impure metal is made the positive pole, pure metal collects on the negative pole, and a solution of a salt of the metal is the electrolyte. Electrotype setting in printing also works on the chemical effect of current.

Conservation of Electricity

Electricity from power plants is costly to produce, so it should be used only when it is really needed. We can save it as follows.

1. Switch off appliances when they are not in use.
2. Use good-quality wires and appliances so that less energy is wasted.
3. Remember the saying: energy saved is energy generated.

Points to Remember

1. There are two kinds of charge, positive and negative. Like charges repel and unlike charges attract.
2. A body can be charged by friction, by conduction and by induction. An electroscope detects a charge on a body.
3. Insulators (wood, plastic, air) do not let charge pass. Conductors (all metals) do.
4. Lightning is the discharge of the huge charge stored in clouds through air. A lightning conductor protects buildings from it.
5. A cell is a source of electric current and changes chemical energy into electrical energy. A combination of cells is a battery.
6. A closed circuit lets current flow; an open circuit does not.
7. In series, total e.m.f. = E₁ + E₂ + E₃ + … For identical cells in parallel, the total e.m.f. equals that of one cell.
8. Household appliances are connected in parallel.
9. A fuse is a safety device against excess current. An MCB is a switch that turns off automatically when the current is too large, and it can replace a fuse.
10. The incandescent bulb and the heater work on the heating effect of current. The electromagnet, the electric bell and the galvanoscope work on the magnetic effect of current.
11. An electric generator changes mechanical energy into electrical energy.
12. Electric power is the electrical energy used per second, measured in watt.
13. One unit, or one kilowatt hour, is the energy used by a 1000 W appliance in one hour: 1 unit = 1 kW h = 1000 × 60 × 60 J = 3.6 × 10⁶ J = 3.6 MJ.
14. Electrolysis is the splitting of an electrolyte into ions when current passes through it.
15. Electroplating deposits a thin layer of one metal on another using an electrolyte such as CuSO₄ or ZnSO₄. Electrotype setting works on the chemical effect of current.