Electricity

Class 7 | Science — Physics | Unit 2

Class 7 Unit 2 Science

Unit 2 - ELECTRICITY

Learning Objectives

• Understanding the flow of electric current and learning to draw the circuit diagram.
• Understanding the difference between conventional current and electron flow.
• Understanding the different types of circuit based on flow of electricity and the connection of bulbs in a circuit.
• Distinguishing a cell and a battery
• Understanding the effects of electric current and factors affecting the effect of electric current.
• Applying their knowledge in identifying the components of electrical circuits.
• Understanding the discrimination between different type of circuits.
• Doing numerical problems and drawing the circuit diagram of their own.

Introduction

In 1882, when it was sun set in the west that miracle happened in New York city. When Thomas Alva Edison gently pushed the switch on 14,000 bulbs in 9,000 houses suddenly got lighted up. It was the greatest invention to mankind. From then the world was under the light even in the night. Many countries began using electricity for domestic purposes. Seventeen years after the New York, in 1899 electricity first came to India. The Calcutta Electric Supply Corporation Limited commissioned the first thermal power plant in India on 17 April 1899. Around 1900s, a thermal power station was set up at Basin Bridge in Madras city and power was distributed to the government press, general hospital, electric tramways and certain residential areas in Madras.

Today electricity is a common household commodity. In your class 6, we learned about electricity and their sources. From operating factories, running medical equipments like ventilator, communications like mobile, radio and TV, drawing water to the agricultural field and light up homes electricity is important.

What is electricity? We can see that it is a form of energy, like heat and magnetism. We have learnt that all materials are made up of small particles called atoms. The centre of the atom is called the nucleus. The nucleus consists of protons and neutrons. Protons are positively charged. Neutrons have no charge. Negatively charged electrons revolve around the nucleus in circular orbits. Electricity is a form of energy that is associated with electric charges that exists inside the atom.

Activity 1

Comb your dry hair. Immediately after combing the dry hair, bring the comb closer to the bits of paper. what will you observe?

When you are getting up from the plastic chair, the nylon shirt seems to be stuck to the chair and make crackling sound. What is the reason for the creation of the sound?

A balloon sticks to wall without any adhesive after rubbing on your hand. Do you know the reason for all?

In all the above activities, when a body is rubbed against some other body become charged.

Electric charge is measured in a unit called coulomb. One unit of coulomb is charge of approximately 6.242 × 10¹⁸ protons or electrons. Electrical charges are generally denoted by the letter 'q'.

2.1. Electric Current

The flow of electric charges constitute an electric current. For an electrical appliance to work, electric current must flow through it. An electric current is measured by the amount of electric charge moving per unit time at any point in the circuit. The conventional symbol for current is 'I'.

Unit of Electric Current

The SI unit for measuring an electric current is the ampere, which is the flow of electric charge across a surface at the rate of one coulomb per second.

I = q/t

Where I represents current (in Ampere A), q represents charge (in coulomb C), t represents time taken (in seconds s).

Worked example 2.1

If 30 coulomb of electric charge flows through a wire in two minutes, calculate the current in the wire?

Solution:

Given Charge (q) = 30 coulomb

Time (t) = 2 minutes × 60 seconds = 120 seconds

Current I = q/t = 30 C/120 seconds = 0.25 A.

2.1.1. Conventional Current and Electron Flow

Before the discovery of electrons, scientists believed that an electric current consisted of moving positive charges. This movement of positive charges is called conventional current. After the electrons were discovered, it was known that electron flow actually takes place from the negative terminal to the positive terminal of the battery. This movement is known as electron flow.

Important Note

Conventional current is in the direction opposite to electron flow.

2.1.2. Measurement of electric current

Electric current is measured using a device called ammeter. The terminals of an ammeter are marked with plus and minus sign. An ammeter must be connected in series in a circuit. Instruments used to measure smaller currents, in the milliampere or microampere range, are designated as milliammeters or microammeters.

Unit Conversions

1 milliampere (mA) = 10⁻³ ampere = 1/1000 ampere

1 microampere (μA) = 10⁻⁶ ampere = 1/1000000 ampere

Worked Examples 2.2

If 0.002A current flows through a circuit, then convert the current in terms of microampere?

Solution:

Given that the current flows through the circuit is 0.002A

We know that 1 A = 10⁶ μA

0.002A = 0.002 × 10⁶ μA = 2 × 10⁻³ × 10⁶ μA = 2 × 10³ μA

0.002A = 2000 μA.

2.2. Potential difference (V)

Electrical charges need energy to push them along a circuit. Water always flows from higher to lower ground. Similarly, an electric charge always flows from a point at higher potential to a point at lower potential. An electric current can flow only when there is a potential difference (V) or P.D.

Definition

The potential difference between any two points in the circuit is the amount of energy needed to move one unit of electric charge from one point to the other.

2.2.1. Unit of potential difference

Did you ever notice the precautionary board with symbol of downward bent arrow mark, while crossing the railway track and the electrical transformer? What does the word high voltage denotes? The term mentioned in the board volt is the measurement for the electric potential difference. The SI unit of potential difference is volt (V). potential difference between two points is measured by using a device called voltmeter.

Analogy

The electric current flow from the higher potential level to the lower potential level is just like the water flow.

2.2.2. Electrical conductivity and Resistivity

Resistance (R)

An electrical component resists or hinders the flow of electric charges, when it is connected in a circuit. In a circuit component, the resistance to the flow of charge is similar to how a narrow channel resists the flow of water. The higher the resistance in a component, the higher the potential difference needed to move electric charge through the component.

We can express resistance as a ratio. Resistance of a component is the ratio of the potential difference across it to the current flowing through it.

R = V/I

The SI unit of resistance is ohm. Greater the ratio of V to I, the greater is the resistance.

Electrical conductivity (σ)

Electrical conductivity or specific conductance is the measure of a material's ability to conduct an electric current. It is commonly represented by the Greek letter σ (sigma). The SI Unit of electrical conductivity is Siemens per meter (S/m).

Electrical resistivity (ρ)

Electrical resistivity (also known as specific electrical resistance, or volume resistivity) is a fundamental property of a material that quantifies how strongly that material opposes the flow of electric current. The SI unit of electrical resistivity is the ohm metre (Ω m).

Material	Resistivity (ρ) (Ω m) at 20°C	Conductivity (σ) (S/m) at 20°C
Silver	1.59 × 10⁻⁸	6.30 × 10⁷
Copper	1.68 × 10⁻⁸	5.98 × 10⁷
Annealed Copper	1.72 × 10⁻⁸	5.80 × 10⁷
Aluminium	2.82 × 10⁻⁸	3.5 × 10⁷

2.2.3. Analogy of Electric Current with Water Flow

An electric current is a flow of electrons through a conductor (like a copper wire). We can't see electrons, however, we can imagine the flow of electric current in a wire like the flow of water in a pipe. Let us see the analogy of flow of electric current with the water flow.

Water flowing through pipes is pretty good mechanical system that is a lot like an electrical circuit. This mechanical system consists of a pump pushing water through a closed pipe. Imagine that the electrical current is similar to the water flowing through the pipe. The following parts of the two systems are related:

• The pipe is like the wire in the electric circuit and the pump is like the battery.
• The pressure generated by the pump drives water through the pipe.
• The pressure is like the voltage generated by the battery which drives electrons through the electric circuit.
• Suppose, there are some dust and rust that plug up the pipe and slow the flow of water, creating a pressure difference from one end to the other end of the pipe. In similar way, the resistance in the electric circuit resists the flow of electrons and creates a voltage drop from one end to the other. Energy loss is shown in the form of heat across the resistor.

2.3. Sources of Electric current - Electrochemical cells or electric cells

Definition

An electric cell is something that provides electricity to different devices that are not fed directly or easily by the supply of electricity.

Activity 2 - Shall we produce electricity at our home?

Materials required: Zinc and copper electrodes, a light bulb, connecting wires, and fruits such as lemons, orange, apples, grapes, and bananas.

Procedure:

1. Set up a circuit as shown in figure 2. Note the brightness of the bulb when the circuit is connected to a lemon.
2. Repeat the experiment using the other fruits listed above.

Do you notice the differences in the brightness of the bulb when it is connected to different fruits?

Which fruit gives the greatest brightness? Why?

Inference: In the above activity what makes enabled the bulb to glow. Why there is a difference in the brightness of the bulb? The reason is that the fruits which you have connected to the bulb produces the electric energy at different levels. The sources which produce the small amount of electricity for shorter periods of time is called as electric cell or electrochemical cells. Electric cell converts chemical energy into electrical energy.

In addition to electrochemical, we use electrothermal source for generating electricity for large scale use. It has two terminals. When electric cells are used, a chemical reaction takes place inside the cells which produces charge in the cell.

2.3.1. Types of cell - primary cell and secondary cell

In our daily life we are using cells and batteries for the functioning of a remote, toys cars, clock, cell phone etc. Even though all the devices produce electrical energy, some of the cells are reusable and some of them are of single use. Do you know the reason why? Based on their type they are classified into two types namely primary cell and secondary cell.

Primary cell

The dry cell commonly used in torches is an example of a primary cell. It cannot be recharged after use.

Secondary cells

Secondary cells are used in automobiles and generators. The chemical reaction in them can be reversed, hence they can be recharged. Lithium cylindrical cells, button cells and alkaline cells are the other types that are in use.

Activity 3

2.3.2. Difference between primary cell and secondary cell

PRIMARY CELL	SECONDARY CELL
1. In primary cell the chemical reaction inside the primary cell is irreversible	In secondary cell the chemical reaction inside the secondary cell is reversible
2. primary cell cannot be recharged.	Secondary cell can be recharged
3. Primary cell is used to operate small devices such as, tape recorder, cycles, toys, hand torch light	Secondary cell is used to operate devices such as mobile phones, cameras, computers, and emergency lights.
4. Examples for primary cell are simple voltaic cell, Daniel cell, and Leclanché cell and dry cell	Examples of secondary cells are lead accumulator, Edison accumulator and Nickel Iron accumulator.

2.3.3. Primary cell simply Dry cell

A dry cell is a type of chemical cell commonly used in the common form batteries for many electrical appliances. It is a convenient source of electricity available in portable and compact form. It was developed in 1887 by Yei Sakizo of Japan. Dry cells are normally used in small devices such as remote control for Television, torch, camera and toys. A dry cell is a portable form of a leclanché cell. It consists of zinc vessel which acts as a negative electrode or anode. The vessel contains a moist paste of saw dust saturated with a solution of ammonium chloride and zinc chloride. The ammonium chloride acts as an electrolyte.

Definition

Electrolytes are substances that become ions in solution and acquire the capacity to conduct electricity.

The purpose of zinc chloride is to maintain the moistness of the paste being highly hygroscopic. The carbon rod covered with a brass cap is placed in the middle of the vessel. It acts as positive electrode or cathode. It is surrounded by a closely packed mixture of charcoal and manganese dioxide (MnO₂) in a muslin bag. Here, MnO₂ acts as depolarizer. The zinc vessel is sealed at the top with pitch or shellac. A small hole is provided in it to allow the gases formed by the chemical action to escape. The chemical action inside the cell is the same as in Leclanché cell.

2.3.4. Batteries

Batteries are a collection of one or more cells whose chemical reactions create a flow of electrons in a circuit. All batteries are made up of three basic components: an anode (the 'plus' side), a cathode (the 'minus' side), and some kind of electrolyte. Electrolyte is a substance that chemically reacts with the anode and cathode.

2.3.5. Invention of the Battery

One fateful day in 1780, Italian physicist, physician, biologist, and philosopher, Luigi Galvani, was dissecting a frog attached to a brass hook. As he touched the frog's leg with an iron scalpel, the leg twitched. Galvani theorized that the energy came from the leg itself, but his fellow scientist, Alessandro Volta, believed otherwise. Volta hypothesized that the frog's leg impulses were actually caused by different metals soaked in a liquid. He repeated the experiment using cloth soaked in brine instead of a frog corpse, which resulted in a similar voltage. Volta published his findings in 1791 and later created the first battery, the voltaic pile, in 1800. The invention of the modern battery is often attributed to Alessandro Volta. It actually started with a surprising accident involving the dissection of a frog.

2.4. ELECTRIC SWITCH

Our country faces a shortage of electricity. So wastage of electricity means you are depriving someone else of electricity. Your electricity bill goes up. So, we must use electricity very carefully and only when it is needed. We must use the electricity as long as we need it in our household activities. Can you remember what you did last year to turn the current on or off? This time, we shall use a switch to turn the current on or off. You may have used different kinds of switches to turn your household electric appliances on or off. Switches help us to start or stop the appliances safely and easily.

Activity 4 - Make your own switch

Let us make a switch of our circuits. Take 10 cm long iron strip. Bend it twice as shown in figure. Now drive a nail into the bend of the wooden block. Nail one end of the strip to the other end of the wooden block so that its free end rests just above the first nail without touching it. Your switch is ready. Would you like to test your switch? To do so, first set up the circuit as shown in the figure. How would you use the switch to open or close the circuit.

If the bulb in your circuit glows when the metal strip of your switch is pressed on the nail and turns off when it is not, then your switch is working. The switch you made is a simple one. You may have seen many different types of switches on switchboards and appliances at your home and school. The switches are designed according to their usage, convenience and safety. But all of them work on the same principle. Switch is a mechanical component that consists of two or more terminals that are internally connected to a metal strip.

2.5. Electric circuit

It is difficult to draw a realistic diagram of this circuit. The electrical appliances you use at home have even more difficult circuits. Can you draw realistic diagrams of such circuits which contain many bulbs, cells, switches and other components? Do you think it is easy? It is not easy. Scientists have tried to make the job easier. They have adopted simple symbols for different components in a circuit. We can draw circuit diagrams using these symbols. Symbols for bulbs, cells and switches are shown in figure. In a cell, the longer line denotes the positive (+) terminal and the short line denotes the negative (-) terminal. We shall use these symbols to show components in the circuits we draw. Such diagrams are called circuit diagrams.

2.5.1. Types of electrical circuits

In the above experiment, we make a circuit with a bulb and a cell. We make only one kind of the circuit with a cell and a bulb. But we can make many types of circuits if we have more than one bulb or cells by connecting these components in different ways.

2.5.2. Series circuit

Two kinds of circuits can be made with two bulbs and a cell. In this experiment we shall make one of them and study it. Look at the circuit with two bulbs, and a cell and a switch given here (Figure) It is clear from the circuit diagram, that the two bulbs are connected one after the other. The circuit diagram shows the sequence of the bulbs and cell, not their real position. The way in which the bulbs have been connected in this circuit is called series connection. Now make the circuit by joining the two bulbs and cell. Do both the bulbs light up? Do both glow equally bright? If one glows less bright, will it shine more brightly if we change its place in the sequence? Change the sequence of bulbs and notice. Sometimes bulbs appear to be similar can differ from each other. So, similar looking bulb do not always glow equally bright when connected in series. The circuit can be broken at several places. For example, between the cell and the bulb, between the two bulbs etc.

2.5.3 Parallel Circuit

Figure shows a circuit in which two bulbs are connected in different places. This is a second type of circuit. Two bulbs in this circuit are said to be connected in parallel and such circuits are called parallel circuits.

2.5.4. Similarity and Difference between Series and Parallel Circuit

Features of Series Circuit

• Single loop connection
• Bulbs dimmer
• Bulbs share power
• All bulbs go out if one goes out

Features of Parallel Circuit

• Connected by branches
• Bulbs brighter
• Each bulb fully powered
• All bulbs stay lit if one goes out

Common features

The common feature for both circuit is energy source and wires.

Science to mind pricking

If an electrician attending an electrical fault at your home gets current shock, will you touch him in order to get rid off him from current risk? Will you use the wet stick to beat him to avoid further effects of electric shock? Why do the electric line man are wear rubber gloves in their hands while doing electrical works on a electrical pole? We know that all materials are made up of the basic building block, the 'atom'. An atom, in turn, contains electrically charged particles. Many of these particles are fixed to the atoms but in conductors (such as all metals) there are lots of particles that are not held to any particular atom but are free to wander around randomly in the metal. These are called 'free charge'.

Do You Know?

Short circuit - You might have observed the spark in the electric pole located nearby your house. Do you know the cause of this electric spark? This is due to the short circuiting of electricity along its path. A short circuit is simply a low resistance connection between the two conductors supplying electrical power to any circuit. Arc welding is a common example of the practical application of the heating due to a short circuit.

2.6. Conductors And Insulators

Based on the property of conductance of electricity, substances are classified into two types, namely, Conductors and Insulators (or) bad conductors of electricity. The electrons of different types of atoms have different degrees of freedom to move around. With some types of materials, such as metals, the outermost electrons in the atoms are loosely bound and they chaotically move in the space between the atoms of that material. Because these virtually unbound electrons are free to leave their respective atoms and float around in the space between adjacent atoms, they are often called as free electrons.

Let's imagine that we have a metal in the form of a wire. When a voltage is connected across the ends of the metal wire, the free electrons drift in one direction. So, a really good conductor is one that has lots of free charges while those who don't have enough 'free charges' would not be good at conducting electricity or we can say that they would be 'poor conductors' of electricity.

2.6.1. Conductors

Conductors are the materials whose atoms have electrons that are loosely bound and are free to move through the material. A material that is a good conductor gives very little resistance to the flow of charge (electron) on the application of external voltage. This flow of charge (electron) is what constitutes an electric current. A good conductor has high electrical conductivity in the above activity. In general, more the free electrons, the better the material will conduct (for a certain applied voltage).

2.6.2. Insulators

Those materials which don't have enough 'free electrons' are not good at conducting electricity or we can say that they would be 'poor conductors' of electricity and they are called insulators.

Do You Know?

This is the material used in SIM Cards, Computers, and ATM cards. Do you know by which material I am made up off? The chip which are used in SIM Cards, Computers, and ATM cards are made up of semiconductors namely, silicon and germanium because of their electrical conductivity lies between a conductor and an insulator.

An insulator gives a lot of resistance to the flow of charge (electron). During the drift of the electrons in an object when an external voltage is applied, collisions occur between the free electrons and the atoms of the material also affect the movement of charges. These collisions mean that they get scattered. It is a combination of the number of free electrons and how much they are scattered that affects how well the metal conducts electricity. The rubber eraser does not allow electric current to pass through it. So rubber is a non-conductor of electricity. Rubber is an insulator. Most of the metals are good conductors of electricity while most of the non-metals are poor conductors of electricity.

Do You Know?

Wires made of copper, an electrical conductor, have very low resistance. Copper wires are used to carry current in households. These wires are in turn enclosed in electrical insulators, or materials of high electrical resistance. These materials are usually made of flexible plastic.

2.7. Effects of Electric Current

You performed many experiments with electricity in Class 6 and learned quite a few interesting facts. For example, you saw that a bulb can be made to light up by making electricity flow through it. The light of the bulb is thus one of the effects of electricity. There are several other important effects of electricity. We shall study some of these effects in this chapter. There are 3 main effects of electricity as:

• Heating effect
• Magnetic effect (Magnetism)
• Chemical effect

2.7.1. Heating effect

When an electric current passes through a wire, the electrical energy is converted to heat. In heating appliances, the heating element is made up of materials with high melting point. An example of such a material is nichrome (an alloy of nickel, iron and chromium). The heating effect of electric current has many practical applications. The electric bulb, geyser, iron box, immersible water heater are based on this effect. These appliances have heating coils of high resistance.

Definition

Generation of heat due to electric current is known as the heating effect of electricity.

Factors affecting Heating Effect of current

1. Electric Current
2. Resistance
3. Time for which current flows

Electric Fuse

Electric fuse is a safety device which is used in household wiring and in many appliances. Electric fuse has a body made of ceramic and two points for connecting the fuse wire. The fuse wire melts whenever there is overload of the current in the wire. This breaks the circuit and helps in preventing damage to costly appliances and to the wiring. In electrical devices, a glass fuse is often used. This is a small glass tube, in which lies the fuse wire.

MCBs (Miniature Circuit Breaker)

MCBs have been replacing electric fuse from wirings at most of the places. The electric fuse has a big practical problem. Whenever the wire fuses, one needs to replace the wire to resume electric supply. More often than not, this proves to be a cumbersome task. Miniature circuit breakers break the circuit automatically. One just needs to switch it on to resume the electric supply. Many models of MCBs have a built in mechanism by which the electric supply is automatically resumed.

2.8. Magnetic Effect of electricity

The next effect of electric current is Magnetism. In 1819, Hans Christian Oersted discovered the electricity that has a magnetic effect. The experiment in activity-5 will help you understand the magnetic effect of electric current.

2.8.1. Application of magnetic effect of electric current

Electromagnet

Magnetic effect of electric current has been used in making powerful electromagnets. Electromagnets are also used to remove splinters of steel or iron in hospitals dealing with eye injuries. Electromagnets are used in many appliances that we use in our day to day life, namely, electric bell, cranes and telephone. Let us know how the magnetic effect of electric current is applied in telephones.

Activity 5

Materials required:

• Iron nail
• Battery & Switch
• Wire

Take around 75 cm long piece of insulated flexible wire and an iron nail say about 8 to 10 cm long. Wind the wire tightly around the nail in the form of a coil. Connect the free ends of the wire to the terminals of a cell as shown. Place some pins on or near the end of the nail. Now switch on and switch off the current, What happens?

When the switch is kept in on position the pins starts to cling to the end of the nail. When the electric current is switched off the coil generally loses its magnetism. Such coils are called as electromagnets. The polarities of both ends of the coil changes according to the direction of electric current passes.

2.8.2. Telephone

In telephones, a changing magnetic effect causes a thin sheet of metal (diaphragm) to vibrate. The diaphragm is made up a metal that can be attracted to magnets.

1. The diaphragm is attached to spring that is fixed to the earpiece.
2. When a current flows through the wires, the soft iron bar becomes an electromagnet.
3. The diaphragm becomes attracted to the electromagnet.
4. As the person on the other end of the line speaks, his voice cause the current in the circuit to change. This causes the diaphragm in the earpiece to vibrate, producing sound.

Thomas Alva Edison (1847 - 1931)

Thomas Alva Edison was affected by scarlet fever and hence he joined the school at Fort Huron in America only at the age of eight. When he was a child his hearing capacity was reduced. One day his teacher scolded him vehemently, On that day, he dropped out of the school. After leaving the school, his mother who was a teacher taught lessons at home for three years. Since the age of seven, Edison was interested towards domestic electrical devices. At the age of 9, he read the book, "Natural and Experimental Philosophy" written by Richard Parker. At the age of 21, he read deeply Michal Faraday's "Experimental Researches in Electricity". Edison worked as a telegraph operator in a railway station. He was the dynamic telegraph operator. His first invention was electrical telegraph and its related instruments.

Edison invented an advanced instrument Gramophone in 1877. Edison used a platinum wire coil in a vacuum glass and discovered the first electric bulb in 1879. Thomas Alva Edison invented a commercially viable electric bulb. This was exhibited in 1897. By using mechanical power in a battery, electric power was generated by providing the voltage. Edison proved that voltage is given in the ends of battery. The same was transferred into an electric motor which provided mechanical energy. By extending Kinetoscope into 50 feet film strip, he made first talkie film by using electric motor and magnifying glass in 1891. As a mark of respect to Edison on his death, the light of "Statue of Liberty" in New York was turned off. Except the road lights of Chicago and Broadway, all the lights in the city were turned off.

Edison was an American Scientist and Industrialist. He invented many instruments like Electric bulb, electric motor, gramophone and kinetoscope. He was known as for taking the world of darkness to brightness crossing all the obstacles in life.

2.9. Chemical Effects of Electricity

Chemical reactions happens, when electricity passes through various conducting liquids. This is known as chemical effects of electricity. You will learn chemical effect of electricity in your higher classes.

POINTS TO REMEMBER

• An electric current is a flow of electric charge or the amount of charge flowing through a given cross section of a material in unit time.
• Conventional current is in the direction opposite to electron flow.
• One ampere is defined as the flow of electric charge across a surface at the rate of one coulomb per second.
• An electric cell is something that provides electricity to different devices that are not fed directly or easily by the supply of electricity.
• A dry cell is a portable form of a Leclanché cell.
• Batteries are a collection of one or more cells whose chemical reactions create a flow of electrons in a circuit.
• The cell is the basic single electrochemical unit which converts chemical energy to electrical energy.
• Ammeter, An instrument for measuring the flow of electrical current in amperes. Ammeters are always connected in series with the circuit to be tested.
• Ampere (A), A unit of measure for the intensity of an electric current flowing in a circuit. One ampere is equal to a current flow of one coulomb per second.
• Circuit, A closed path in which electrons from a voltage or current source flow. Circuits can be in series, parallel, or in any combination of the two.
• Current (I), The flow of an electric charge through a conductor. An electric current can be compared to the flow of water in a pipe. Measured in ampere.
• Fuse, A circuit interrupting device consisting of a strip of wire that melts and breaks an electric circuit if the current exceeds a safe level.
• Conductor, Any material where electric current can flow freely. Conductive materials, such as metals, have a relatively low resistance. Copper and aluminium wire are the most common conductors.
• Insulator, Any material where electric current does not flow freely. Insulation materials, such as glass, rubber, air, and many plastics have a relatively high resistance. Insulators protect equipment and life from electric shock.
• Parallel Circuit, A circuit in which there are multiple paths for electricity to flow. Each load connected in a separate path receives the full circuit voltage, and the total circuit current is equal to the sum of the individual branch currents.
• Series Circuit, A circuit in which there is only one path for electricity to flow. All of the current in the circuit must flow through all of the loads.
• Short Circuit, When one part of an electric circuit comes in contact with another part of the same circuit, diverting the flow of current from its desired path.
• One unit of coulomb is charge of approximately 6.242 × 10¹⁸ protons or electrons.
• The potential difference between any two points is the amount of energy needed to move one unit of electric charge from one point to the other.
• Electrical conductivity or specific conductance is the measures a material's ability to conduct an electric current.
• Electrical resistivity is the property of a material that quantifies how strongly that material opposes the flow of electric current.
• The sources which produce the small amount of electricity for shorter periods of time is called as electric cell or electrochemical cells.
• Electrolytes, A substance that dissociates into ions in solution and acquires the capacity to conduct electricity. Sodium, potassium, chloride, calcium, and phosphate are examples of electrolytes.

EVALUATION

Exercise I. Choose the correct answers

1. In the circuit diagram below, 10 units of electric charge move past point x every second. What is the current in the circuit?

a) 10 A b) 1 A c) 10 V d) 1 V

2. In the circuit shown, which switches (L, M or N) must be closed to light up the bulb?

a) switch L only b) switch M only c) Switch M and N only d) either switch L or switches M and N

3. Small amounts of electrical current are measured in milliampere (mA). How many milliampere are there in 0.25 A?

a) 2.5 mA b) 25 mA c) 250 mA d) 2500 mA

4. In which of the following circuits are the bulb connected in series?

a) Diagram A b) Diagram B c) Diagram C d) Diagram D Exercise II. Fill in the blanks

1. The direction of conventional current is to electron flow.

2. One unit of coulomb is charge of approximately protons or electrons.

3. is used to measure the electric current.

4. In conducting materials electrons are bounded with atoms.

5. SI unit of Electrical conductivity of a conductor is .

Exercise III. True or False, If False give the correct answer 1. Electron flow is in the same direction to conventional current flow. 2. The fuse wire does not melt whenever there is overload in the wiring. 3. In a parallel circuit, the electric components are divided into branches. 4. The representation of the electric current is A. 5. The electrical conductivity of the semiconductor is in between a conductor and an insulator. Exercise IV. Match the following

1. Cell
2. Switch
3. Circuit
4. Miniature circuit
5. Fuse

Options: Used to open or close a circuit, Safety device used in a electric circuit, A complete path for the flow of an electric current, Reset by hand, circuit becomes complete once again, A device which converts chemical energy into electrical energy

Exercise V. Analogy

1. Water in pipe : Electric current is .

2. Copper is conductor : Wood is .

3. Length is metre scale : Current is .

4. milliampere is microampere : 10⁻³ A is .

Exercise VI. Assertion and Reason

1. Assertion (A) Copper is used to make electric wires.
Reason (R) Copper has very low electrical resistance.

A. Both Assertion and Reason are true and Reason is the correct explanation of Assertion. B. Both Assertion and Reason are true but Reason is NOT the correct explanation of Assertion. C. Assertion is true but Reason is false. D. Assertion is false but Reason is true. E. Both Assertion and Reason are false.

2. Assertion (A): Insulators do not allow the flow of current through themselves.
Reason (R) They have no free charge carriers.

A. both Assertion and Reason are true and the Reason is correct explanation of Assertion. B. both Assertion and Reason are true but Reason is not a correct explanation of Assertion. C. Assertion is true and Reason is false. D. both Assertion and Reason are false. Exercise VII. Very short answer

1. What is the speed of electric current?

2. What is the SI unit of electrical conductivity?

3. Name the device used to generate electricity.

4. Define fuse.

5. Name some devices that run using heat effect of electric current

6. Name few insulators.

7. What is a battery?

Exercise VIII. Short Answer

1. Define an electric current.

2. Differentiate parallel and serial circuits.

3. Define electrical conductivity.

Exercise IX. Long Answer

1. Explain the construction and working of a Telephone.

2. Explain the heating effect of electric current.

3. Explain the construction and working of a dry cell.

Exercise X. Higher Order Question

A student made a circuit by using an electric cell, a switch, a torch bulb (fitted in the bulb holder) and copper connecting wires. When he turned on the switch, the torch bulb did not glow at all. The student checked the circuit and found that all the wire connections were tight. What could be the possible reason for the torch bulb not glowing even when the circuit appears to be complete?

Exercise XI. Picture based Questions

1. Three conductors are joined as shown in the diagram. The current in conductor RS is 10 A. The current in conductor QR is 6 A. What will be the current in conductor PR?

a) 4 A b) 6 A c) 10 A d) 16 A

2. Draw the circuit diagram for the following series connection

3. Study the electric circuit below. Which of the following switches should be closed so that only two bulbs will light up

a) S1, S2 and S4 only b) S1, S3 and S5 only c) S2, S3 and S4 only d) S2, S3 and S5 only

4. Study the three electric circuits below. Each of them has a glass rod (G), a steel rod (S), and a wooden rod (W). In which of the electric circuits would the bulb not light up?

a) A only b) C only c) A and B only d) A, B and C

ICT Corner

Electricity

This activity helps the students to understand about the Parallel and series circuit.

PROCEDURE

Step 1: Type the URL link given below in the browser or scan the QR code. A page opens with a battery, some cables, two sets for circuit and two bulbs.

Step 2: Ask the students to fix the wires to the battery and the circuit.

Step 3: Let the students do it and understand the concept with different combinations.

Electricity URL: http://www.physics-chemistry-interactive-flash-animation.com/electricity_electromagnetism_interactive/components_circuits_association-series_parallel.htm

Pictures are indicative only
*If browser requires, allow Flash Player or Java Script to load the page.

📝 Summary

1. Electricity is a form of energy associated with electric charges that exist inside atoms, which consist of a nucleus containing protons (positively charged) and neutrons (neutral), with negatively charged electrons revolving around the nucleus.
2. Electric current is the flow of electric charges, measured in amperes (A), and is calculated using the formula I = q/t, where I is current, q is charge in coulombs, and t is time in seconds.
3. The history of electricity began with Thomas Edison's breakthrough in 1882 when 14,000 bulbs were lit in New York City, and electricity came to India in 1899 with the first thermal power plant in Calcutta.
4. Electric charge is measured in coulombs, where one coulomb equals the charge of approximately 6.242 × 10^18 protons or electrons, and charges are denoted by the letter 'q'.
5. Electricity can be created through rubbing different materials together, as demonstrated by combing dry hair and attracting paper bits, or rubbing a balloon against your hand to make it stick to walls.