Class -7, Science, Chapter:- 14 Electric Current & it's Effects Notes, NCERT Solutions & Frequently Asked Questions

Science

Class:-7, Chapter:- 14

 Electric Current & it's Effects 

Notes, NCERT Solutions & Frequently Asked Questions

-- Notes --

⭐  Introduction to charge:-

• Electric charge is described as the basic physical properties of matter, which allows it to experience a special kind of force when kept under the influence of electromagnetic field.

• There are two types of charges → positive (+ve) and negative (-ve).

• Like charges repel each other and unlike charges attract each other

• The SI unit for the electric charge is Coulomb.

                { 1 Coulomb = 6.242 × 10¹⁸ e }

• The Value of charge of an electron is equal to  −1.602 × 10-¹⁹ C.

⭐ Conductors:- Substances or materials that offer comparatively less opposition to electric current through them are called as conductors. 

• Eg.- copper, iron, water, etc.

⭐ Insulators:- Substances, which offer larger opposition to electric current through them are called as insulators. 

• Eg.- rubber, wood, sand, etc.

⭐ Electric Circuits:- 

• A continuous conducting path (consisting of wires, bulb, switch, etc.) between the two terminals of a cell or battery along with an electric current flows, is known as an electric circuit.

⭐ Circuit Diagram:- A circuit diagram tells us how the various components in an electric circuit have been connected by using the electrical symbols of the components.

• When the bulb glows in an electric circuit, the switch is closed or the switch is said to be in the ON position.

• When the bulb does not glows in an electric circuit, the switch is open or the switch is said to be in the OFF position.

⭐ Symbols of electric components:- Some of the commonly used symbols for electric components are shown here:- 

⭐ Electric Cells:- The common source of electricity to run the number of devices (torches, radio, electric clocks and watches, toys, etc.) is an electric cell. 

⭐ Electric Battery / Combination of electric Cells:- Higher voltage can be obtained by combining a number of cells in series. 

• So, when the positive terminal of one cell is joined with the negative terminal of the other cell, then the cells are said to be joined in series. 

• So, a battery can be defined as the group of cells joined together in the series. 

⭐ Connection of Cells/Battery:- ‘+’ and ‘-‘ symbols are printed in the battery in order to have an exact placement of the cells in their respective battery compartment.

• The switch or key can be placed anywhere in the circuit.

⭐ Heating effects of electric current:- 

• A current carrying wire gets hot whenever electric passes through it. This is the heating effect of electric current.

• This effect was discovered by James Prescott Joule, therefore it is also known as Joule's Law of Heating Effect.

• The amount of heat produced in the wire depends on the material, thickness and length of the conductor.

• If a large amount of electric current passes through the wire, the wire may become so hot that it may melt and break.

• Heating effects of electricity are applied for many household appliances such as electric heaters, toasters, water heaters.

• They are also used in bulbs and used as safety devices(fuses).

⭐ Factors on Which the Heating Effect of Current Depends:- There are three factors on which the heating effect of current depends:

i) Resistance of wire:- Greater the resistance of a wire, greater will be the heat produced in it by a given current. 

-- Note --

• Resistance of nichrome wire is more than that of the other wires so nichrome wire is used to make heating elements of electric heating appliances such as electric room heater, electric iron, etc. 

• Resistance of a wire depends on the material of the wire, length of wire and thickness of the wire.

ii) Magnitude of current passed through a given wire:- If the magnitude of current passed through a given wire is greater, then the heat produced in it will also be greater. 

iii) Time for which the current flows through the wire:- If the current passes for a long time through a given wire, then the heat produced in it will also be greater.

iv) Cross-sectional area:- Thinner the wire, more is the heating effect.

⭐ Applications of the Heating Effect of Current:- Some of the important applications of heating effect of electric current are-

• For the production of light, the heating effect of electric current is utilised in the electric bulbs.

• For the working of electrical heating appliances such as water heater, electric room heater, electric iron, etc., the heating effect of electric current is utilised.

• The heating effect of electric current is utilised in a safety device called ‘electric fuse’.

⭐ Electric bulb:- 

• Coils of wire that utilise the heating effects of electric current are called as elements. These coils can be found in an electric room heater or electric iron box or any other electric appliances.

• Two important appliances based on the heating effects of electric current, which are widely used are the electric bulb and electric fuse.

• The filament of an electric bulb becomes so hot when an electric current passes through, that it starts glowing.

⭐ Compact Fluorescent Lamps (CFLs):- An electric bulb is basically used for producing light but it also releases the heat which is not desirable because a major part of the electricity consumed by the filament of a bulb is converted into heat and results in the wastage of electricity. So, this wastage can be decreased by using fluorescent tube light or CFLs in place of the bulbs.

• However, before purchasing bulbs, tubes or CFL’s we should look for the ISI mark (ISI- Bureau of Indian standard). It is because the ISI mark ensures that the appliance is safe and wastage of energy is minimum.

⭐ Electric Fuse:-  When wires are made of a material that melts on the passage of electric current are called as a fuse. 

• Electrical fuses are used as a safety device that breaks a circuit in times of overloading. They prevent the cause of fire and damage to electrical circuits.

-- Note --

We should not use a thick wire as a fuse wire because it will have low resistance and thus it will not get heated to its melting point ‘ when a large current passes through it.

⭐ Cause of Large Current Flow in Household Electric Wiring:- An extremely large current can flow in the household electric wiring circuits under two circumstances overloading and short circuit. 

a) Overloading:- It is a situation when too many electrical appliances are connected to a single socket, they draw an extremely large amount of current from the household circuit. 

• The flow of large current due to overloading may heat the copper wires of household wiring to a very high temperature and fire may be started.

b) Short Circuit:- Electric current is supplied by household through two insulated wires which run together and reach each and every electrical appliances. 

• One insulated wire is called live wire and the other insulated wire is called neutral wire and both these wires are necessary for the working of an electrical appliance.

• So, if the plastic insulation of the live wire and the neutral wire gets worn, then the two naked wires touch each other & this direct touching of wires is known as a short circuit. 

• Due to which a large current flows through the household wiring and this large’ current may heat the wires to a dangerously high temperature and a fire may be started.

-- Note -- 

Always use proper fuses which have been specified for particular applications carrying ISI mark Never use just any wire or strip of metal in place of a fuse.

⭐ Magnetic effects of electric current:- 

• When electric current flows through a current carrying conductor, it produces a magnetic field around it. 

• This can be seen with the help of a magnetic needle, which shows deflection. The more the current, the higher the deflection.

• If the direction of current in the circuit is reversed, the direction of deflection of the magnetic needle is also reversed.

• The magnetic needle experiences deflection only if there is an electric current flowing through the wire.

• This effect was discovered by Hans Christian Oersted.

⭐ Lightning and magnetism:- 

• During thunderstorms, air currents move upwards and water droplets move downwards. This causes separation of charges between clouds and between clouds and earth.

• When the magnitude of charges increases, air (normally a bad conductor) starts conducting and allows the flow of electricity. This is called as lightning, as this flow of charge is accompanied by bright streaks of light and sound.

• Lightning also makes naturally occurring magnets called as lodestones.

⭐ Electromagnets:- An electromagnet is an artificial magnet, which produces a magnetic field around a conductor due to the passage of electric current through it.

• This magnetic field disappears when there is no current flowing through the conductor.

• There are two factors through which an electromagnet can be made stronger-

           -- by increasing the amount of current                  used in the coil.

           -- by increasing the number of turns                      forming the coil.

⭐ Uses of Electromagnets:- 

• These magnets are used in electrical appliances such as an electric bell, electric fan, electric motor.

• These magnets have their utilisation in electric generators where the very strong magnetic field is required.

• For deflecting electron beam of the picture tube of TV electromagnets are used.

• For the magnetic separation of iron ores from the earthly substances, electromagnets are used.

• For preparing strong permanent magnets, electromagnets are used.

⭐ Difference between electromagnets and permanent magnets:- 

• Permanent magnets maintain their magnetism for long periods of time

• Electromagnets have their own magnetic field as long as current is flowing through them. Once the circuit is broken, they lose their magnetism.

⭐ Advantages of Electromagnets over Permanent Magnets:- An electromagnet is a temporary form of the magnet because its magnetism is only for the duration of current flowing in its coil. There are some of the advantages of the electromagnets over the permanent magnets-

• The magnetism of an electromagnet can be switched ON or switched OFF as desired. While it is not possible with a permanent magnet.

• By increasing the number of turns in the coil and by increasing the current passing through the coil an electromagnet can be made very strong. On the other hand, a permanent magnet cannot be made so strong.

⭐ Miniature Circuit Breakers (MCBs):-  It is increasingly being used these days in place of fuses. MCB does not work on the heating effect of current as it works on the magnetic effect of current. These switches automatically turn OFF when the current in a circuit exceeds the safe limit. We turn them ON and the circuit is once again completed.

⭐ Hazards of electricity:- 

• Electricity can be hazardous as it can cause a fire due to heating or loss of life.

• Electric circuits must be properly insulated. If the insulation breaks and wires come in contact with each other, they can cause a short circuit, which may give electric shocks when touched.

⭐ Electric bell:- An electric bell works on the magnetic effect of current. It has an electromagnet in it. 

• Construction of Electric Bell:- 

-- The electric bell has a U-shaped electromagnet. 

-- There is a small iron bar called armature which is held in front of the poles of the electromagnet. 

-- The lower end of the iron bar is attached to a flat spring and the flat spring is itself fixed to a metal bracket. 

-- The upper end of the iron bar has a clapper attach to it. 

-- A metal gong is fixed near the clapper.

• Working of Electric Bell:- 

-- In order to ring the bell, first of all, we press the push button switch in order to ring the bell. 

-- When we press the switch, then the electric circuit of the bell is completed and a current passes through the coil of the electromagnet and it gets magnetised. 

-- The electromagnet attracts the iron armature towards itself. So the armature moves towards the poles of the electromagnet, the clapper attached to it strikes the gong and produces a ringing sound. It implies that the bell rings.

-- When the armature moves towards the magnet, its contact with the contact screw is broken. 

-- Due to this, the electric circuit breaks and no current flows in the electromagnet coil. The electromagnet loses its magnetism for a moment and the armature is no longer attracted by it. 

-- The flat spring brings back the iron armature to its original position and the clapper also moves away from the gong.

-- As soon as the armature comes back and touches the contact screw the circuit is completed and current starts flowing in the electromagnet coil again. 

-- The electromagnet attracts the iron armature once again and the clapper strikes the gong again producing a ringing sound.

-- So, this process of ‘make and break’ of the electric circuit continues as long as we are pressing the switch. 

-- Due to this, the armature vibrates forwards and backwards rapidly each time making the clapper strike the gong.

-- Thus, the clapper strikes the gong rapidly producing an almost continuous sound.

-- NCERT Solutions --

Exercise Questions

Question 1.- Draw in your notebook the symbols to represent the following components of electrical circuits: Connecting wires, switch in the ‘OFF’ position, bulb, cell, switch in the ‘ON’ position, and battery.

Solution:-

Question 2.- Draw the circuit diagram to represent the circuit shown in Fig.14.21.

Solution:-

Question 3.- Fig.14.22 shows four cells fixed on a board. Draw lines to indicate how you will connect their terminals with wires to make a battery of four cells.

Solution:-

Question 4.- The bulb in the circuit shown in Fig.14.23 does not glow. Can you identify the problem? Make necessary changes in the circuit to make the bulb glow.

Solution:- In the circuit above, the bulb is connected on either side.

Question 5.- Name any two effects of electric current.

Solution:-

i) Heating effect of electric current

ii) Magnetic effect of electric current

Question 6.- When the current is switched on through a wire, a compass needle kept nearby gets deflected from its north-south position. Explain.

Solution:- When the current is switched on through a wire, the magnetic field is created around it hence we see deflection in the compass needle kept nearby.

Question 7.- Will the compass needle show deflection when the switch in the circuit shown by Fig.14.24 is closed?

Solution:- No, the compass needle does not show deflection when the circuit is closed, and the magnetic field is not created until the current is flowing through the circuit.

Question 8.- Fill in the blanks:

a) Longer line in the symbol for a cell represents its ________ terminal.

b) The combination of two or more cells is called a _________.

c) When the current is switched ‘on’ in a room heater, it _______.

d) The safety device based on the heating effect of electric current is called a ______.

Solution:-

a) positive 

b) battery

c) produces heat

d) fuse

Question 9.- Mark ‘T’ if the statement is true and ‘F’ if it is false:

a) To make a battery of two cells, the negative terminal of one cell is connected to the negative terminal of the other cell. 

b) When the electric current through the fuse exceeds a certain limit, the fuse wire melts and breaks. 

c) An electromagnet does not attract a piece of iron. 

d) An electric bell has an electromagnet. 

Solution:-

a) False

b) True

c) False

d) True

Question 10.- Do you think an electromagnet can be used for separating plastic bags from a garbage heap? Explain.

Solution:- No, because plastic does not have the magnetic property to get attracted to a magnet; hence magnet cannot be used to separate plastic bags.

Question 11.- An electrician is carrying out some repairs in your house. He wants to replace a fuse with a piece of wire. Would you agree? Give reasons for your response.

Solution:- It is not a wise idea to replace the fuse with a piece of wire, as it has a very low melting point. In the case of the metal piece, the melting point will be high, and the circuit will be intact in case there is overload or overheating.

Question 12.- Zubeda made an electric circuit using a cell holder shown in Fig. 14.4, a switch and a bulb. When she put the switch in the ‘ON’ position, the bulb did not glow. Help Zubeda to identify the possible defects in the circuit.

Solution:- The reasons may be as follows:

i) The connecting wire may be loose

ii) The electric cell may be used up

iii) Switch may not be functioning well

iv) Cell power has been exhausted

Question 13.- In the circuit shown in Fig. 14.25.

i) Would any of the bulb glow when the switch is in the ‘OFF’ position?

ii) What will be the order in which the bulbs A, B and C will glow when the switch is moved to the ‘ON’ position?

Solution:-

i) No, the bulb will not glow as the circuit is not complete when the switch is off.

ii) If the switch is ‘ON’, all the bulbs glow simultaneously.

-- Frequently Asked sked Questions -- 

Question 1.- What is an ‘Electric current’?

Answer:- An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space.

Question 2.- What is an ‘Electromagnet’?

Answer:- An electromagnet is a type of magnet in which the magnetic field is produced by an electric current.

Question 3.- What are the benefits of Electricity?

Answer:- 

1.- Lighting rooms with lamps 

2.- Electronic gadgets 

3.- Healthcare industry/medical devices 

4.- Engineering industry 

-- By Durgesh Pandey

( Eklavya Coaching Institute)