std 8 Electric current

11 ELECTRIC CUrrent

points to remember :

1. Electric current : A flow of electric charges is called an electric

current.

2. Conversion of electrical energy into light : When the positive

and negative ends (or poles) of a dry cell (an electrochemical

cell) are connected to an electric bulb with copper wires, the cell

moves the negative stationary charges in the wires by applying

a force on them. The resulting electric current passes through

the filament of the bulb. The resulting rise in the temperature of

the filament makes it glow. Thus, electrical energy is converted

into light.

3. Electromotive force : The force applied by an electrochemical

cell on electric charges is called the electromotive force. It sets

electrons in motion.

4. Types of electrochemical cell : The simple cell, Leclanche cell,

dry cell, nickel-cadmium cell, button cell (lithium cell) are

various types of electrochemical cell.

5. Magnetic field of an electric current : A current carrying

conductor behaves like a magnet. A magnetic field is produced

around it by the electric current.

6. Electromagnetic induction : If there is a change in the

magnetic field associated with a coil of wire, an electric current

is produced in the coil. It is called an induced current and the

corresponding phenomenon is called electromagnetic induction.

In electromagnetic induction (1) an electric current flows

through the coil as long as the magnet is moving (or as long as

the magnetic field is changing) (2) the direction of the current

depends upon the direction of motion of the magnet (or on the

way the magnetic field changes).

The phenomenon of electromagnetic induction was

discovered by Michael Faraday.

7. Electric motor : The production of magnetic field by an electric

current is made use of in an electric motor.

137

8. Electromagnet : The production of magnetic field by an electric

current is made use of in an electromagnet. Electromagnets are

used in (1) big cranes for moving heavy loads from one place to

another (2) toys.

9. Electrical conductors and insulators : A substance through

which electricity can flow is called an electrical conductor.

Examples : Iron, copper.

A substance through which electricity cannot flow is called

an electrical insulator. Examples : Rubber, wood.

Questions and Answers

Q. 1 Answer the following questions :

(1) When silk is rubbed on glass, what type of charge is

developed on glass and silk ? Why are they called static electric

charges?

Ans. When silk is rubbed on glass, positive electric charge is

developed on glass, while negative electric charge is developed on

silk. As these charges remain still on the bodies, they are called

static electric charges.

(2) What is electric current?

Ans. When a force is applied on electric charges, they start

moving. A flow of electric charges is called an electric current.

(3) When does electric bulb give light ?

Wire

+

–

Electric

bulb

Electric

cell

Fig. 11.1

Ans. When an electric current is passed

through the filament of an electric bulb, the

temperature of the filament rises. As the

filament becomes hot, it glows and the bulb

gives light.

[Note : In this case, chemical energy in the cell is

converted into electrical energy which is then converted

into heat. A part of heat is converted into light. ]

 (4) What is electromotive force ?

Ans. The force applied by an electrochemical cell on electric

charges, setting them in motion, is called the electromotive force.

(5) With a neat labelled diagram, explain the working of

the simple electric cell.

Zn Cu

Dilute sulphuric acid

Bulb

Glass

trough

Key

Fig. 11.2 : Simple electric cell

Ans. In a simple electric

cell, dilute sulphuric acid is

taken in a glass trough and one

plate of zinc (Zn) and another

plate of copper (Cu) are dipped

in it as shown in the figure. The

plates are separated by some

distance. The Cu plate acts as

the positive pole while the

Zn plate acts as the negative

pole.

Using connecting wires of copper, a circuit consisting of

the cell, an electric bulb and a key is formed. When the key is closed,

the circuit is completed and an electric current flows through it

from the positive pole to the negative pole through the connecting

wires. The current is produced by the chemical reactions in the cell.

The bulb glows due to the current.

This cell is called Volta’s cell.

Answers to the Questions given on page 105 of the textbook

* (1) When an electric current flows, what is the

direction of motion of electrons ?

Ans. When an electric current flows, electrons move from

the negative pole to the positive pole through the conducting wire.

[Note : Electrons move in the direction opposite to that of electric current.]

* (2) What is the advantage of having a key in the

circuit ?

Ans. The advantage of having a key in the circuit is that

a current can be started by closing the key and stopped

by opening the key.

[Note : If a circuit is connected without a key, there will be a continuous flow

of current and the cell may be discharged soon.]

 (6) Describe the construction of the Leclanche cell.

Ans. In the Leclanche cell, a mixture of powdered manganese

dioxide and carbon is packed in a cylindrical porous container of

porcelain. The container acts as the positive pole. A solution of

ammonium chloride is taken in a large glass container and the

porous pot and the zinc rod are placed in it. Electric current is

produced due to the chemical reaction between manganese dioxide,

zinc and ammonium chloride.

(7) With a neat labelled diagram, describe the

construction of a dry cell.

+

Metal cap

Carbon rod

Paste of zinc chloride

and ammonium chloride

Manganese dioxide

and graphite

_

Fig. 11.3 : Dry cell

Ans. A dry cell consists

of a cylindrical zinc

container acting as the

negative pole. At the

centre, there is a carbon

rod which acts as the

positive pole. Around the

rod, there is a mixture of

finely powdered manganese

dioxide (MnO

_

) and graphite (C) in a bag of thin material. The

space between the bag and the zinc container is filled with a moist

paste of zinc chloride (ZnCl

_

) and ammonium chloride (NH

_

Cl). The

carbon rod is fitted with a metal cap. The cell is sealed.

[ Note : In this cell, the chemical reaction giving rise to electric current is very

slow. Hence, it has a longer life than other cells.]

Answers to the Questions given on page 107 of the textbook

* (1) Why was the ‘Dry cell’ given this name?

Ans. The chemicals used in the dry cell are dry powders

and thick moist paste. As the dry cell is sealed, the moist paste

in the cell does not come out of it. Hence, the name dry cell.

* (2) What is the direction of the electron current in a

dry cell ?

Ans. In a dry cell, electrons flow from the zinc electrode

towards the carbon rod.

 (8) State the positive and negative poles in the nickelcadmium

cell. Name the alkali used in this cell.

Ans. In the nickel-cadmium cell, nickel (Ni) forms the positive

pole while cadmium (Cd) forms the negative pole. Potassium

hydroxide is used as alkali in this cell.

[Note : This cell is made air-tight.]

(9) What is the main advantage of the nickel-cadmium

cell ?

Ans. The main advantage of the nickel-cadmium cell is that it

can be recharged again and again.

(10) Where is the nickel-cadmium cell used ?

Ans. The nickel-cadmium cells are used in some portable

machines that run on electricity like a drilling machine or a

gardening tool.

(11) What are the positive and negative poles of a button

cell ? Which salt is used in the space between the poles ?

Ans. In a button cell, carbon is the positive pole while lithium

iron phosphate is the negative pole. Lithium salt is filled in the

space between the poles.

[Note : It is called a lithium cell. It cannot be recharged.]

(12) Where is the button cell (lithium cell) used ?

Ans. A button cell (lithium cell) is used in watches, toys,

cameras, calculators, etc.

[Note : It is small in size and easy to handle. As it does not leak, there is no

possibility of damage to the delicate parts of a device in which this cell is used.]

(13) With a neat labelled diagram, describe a simple

experiment to show that an electric current produces a

magnetic field.

– +

Matchbox

tray

Magnetic needle

Key

Electric cell

Turns of copper wire

Fig. 11.4 : Magnetic effect of

electric current

Ans. (1) Place a freely turning

magnetic needle in an empty matchbox

tray and wind ten to twelve turns of a

copper wire around the tray. Connect

the ends of the wire to a dry cell through

a key as shown in the figure. Let the key

be open in the beginning. (2) The needle

is initially at rest in the north-south

direction. Bring a bar magnet near it.

You will find that the needle is

deflected. Take the magnet away. The needle will return to its

initial position. (3) Close the key and observe the magnetic needle.

You will find the needle gets deflected (from its initial position of

rest in the north-south direction) as the current passes in the

circuit. (4) Open the key. You will find that the needle comes to rest

in the north-south direction when there is no current in the circuit.

This shows that when an electric current flows through a

conductor, it acts as a magnet. A magnetic field is produced around

the conductor due to which the needle is deflected.

(14) What is induced current? What is electromagnetic

induction?

Ans. If there is a change in the magnetic field associated with a

coil of wire, an electric current is produced in the coil. It is called an

induced current. This phenomenon of production of induced

current due to a change in magnetic field is called electromagnetic

induction.

[Note : The phenomenon of electromagnetic induction was discovered by

Michael Faraday.]

(15) With a neat labelled diagram, describe the basic

experiment related to electromagnetic induction.

G

N S

Coil of insulated Bar magnet

copper wire

Galvanometer

Fig. 11.5 : Electromagnetic induction

Ans. (1) Take a coil of

insulated copper wire such that

the diameter of the coil is 1 cm

and the number of turns is very

large. Connect the ends of the

wire to a galvanometer (G)

(Fig. 11.5). The deflection of the

needle (or pointer) in the galvanometer gives the direction and the

strength of the current through the galvanometer. (2) Hold a bar

magnet stationary near the coil. You will find that the needle (or

pointer) remains steady. This shows that there is no current in the

circuit. Take the magnet away. (3) Move the north pole of the

magnet swiftly towards the coil. You will find that the needle is

deflected as long as the magnet is in motion. The direction of the

current developed in the circuit can be found from the direction of

deflection of the needle. The strength of the current can be found

from the extent of the deflection. (4) Now, move the north pole of

142 NAVNEET GENERAL SCIENCE DIGEST : STANDARD VIII

the magnet swiftly away from the coil. You will find that the needle

is deflected as long as the magnet is in motion but the deflection is

in the opposite direction compared to that in (3). This shows that

now the direction of the current is opposite to that in (3).

Inference : (1) An electric current flows through the coil as

long as the magnet is in motion. (2) The direction of the current

depends upon the direction of motion of the magnet. When the

magnet is moved towards the coil, the current flows in one direction

and when the magnet is moved away from the coil, the current flows

in the opposite direction.

The current developed is called the induced current and the

phenomenon is called the electromagnetic induction. This

experiment was done by Michael Faraday.

Answers to the Questions given on page 109 of the textbook

* (1) What difference will you see in the electric

current if the magnet approaches the coil with greater

speed?

Ans. If the magnet approaches the cell with greater speed,

there will be an increase in the strength of the electric current.

* (2) Will there be an induced electric current if the

magnet remains at rest ?

Ans. There will be no induced electric current if the

magnet remains at rest.

(16) With reference to the figure given below answer the

following questions :

Electric cells

Horseshoe

magnet

Coil of insulated

copper wire

S N

–+ –+

Fig. 11.6

 (i) What will you observe if the ends of the wire of the coil

are connected in the circuit and disconnected immediately?

Ans. It will be observed that the coil is deflected.

(ii) How do you explain this observation ?

Ans. When an electric current flows through the coil, it acts as

a magnet. One face of the coil behaves as a south pole and the other

as a north pole. When the north pole of the coil faces the north pole

of the magnet, there is a mutual repulsion and hence the deflection

(rotation) of the coil.

(iii) How is this phenomenon [see (2) and (3)] used in the

working of an electric motor?

Ans. In an electric motor, the direction of the current through

the armature (coil) is changed after every half rotation (using a

commutator). Therefore, the armature continues to rotate as there

is always repulsion between the like poles of the armature and the

magnets as they face each other. Otherwise, there will be attraction

between the unlike poles of the armature and the magnet as they

face each other and the armature will stop rotating.

* (17) Why is the direction of the current in an electric

motor reversed after every half round?

Ans. For reference, see the answer to Q. 16 (3).

* (18) What is the principle of the electric motor?

Ans. By sending an electric current through the armature coil

of the electric motor and using a commutator and a permanent

magnet, the armature can be set in rotational motion continuously.

This is the principle of the electric motor.

(19) What is an electromagnet?

Ans. When a current is passed through a conductor, it acts as a

magnet. It is called an electromagnet.

(20) With a neat labelled diagram, explain how a simple

electromagnet can be made.

Ans. (1) Take an insulated and fine copper wire about 50 cm

long and coil it around an iron nail. Connect the ends of the wire to

a dry cell through a key. Let the key be open in the beginning

+

_

Nail

Key

Turns of

the wire

Electric

cell

(Dry cell)

Fig. 11.7 : Simple electromagnet

(Fig. 11.7). (2) Spread some

iron pins near the nail. Close

the key. There will be a current

in the circuit. Hence, pins will

be attracted by the nail. Some

of these will stick to the nail.

(3) Now, open the key. When

there is no current in the

circuit, the nail will not act as a

magnet. Hence, the pins will

not be attracted by it and those sticking to the nail earlier will get

separated.

This shows that the magnetism developed in the nail is only

temporary (as long as there is a current through the wire wound

around the nail.)

Answers to the Questions given on page 112 of the textbook

* (1) What do you think will happen if the number of

turns around the coil is increased ?

Ans. If the number of turns around the nail is increased

(keeping the strength of the electric current the same), there

will be an increase in the strength of the electromagnet.

* (2) What will be the effect of using a thicker wire ?

Ans. If a thicker wire (of the same length) is used and the

same dry cell is connected in the circuit, there will be an

increase in the current through the wire. Hence, there will be

an increase in the strength of the electromagnet.

* (21) How can the strength of an electromagnet be

increased?

Ans. The strength of an electromagnet can be increased by

increasing the current producing the magnetic field.

(22) State uses of an electromagnet.

Ans. Electromagnets are used in (1) big cranes for moving

heavy loads from one place to another (2) toys that run on electric

cells (3) electric doorbell, etc.

 (23) With a neat labelled diagram, describe the

construction and working of an electric doorbell.

Striker

Contact

screw

Horseshoe

magnet

Soft iron strip

+ –

Gong

Switch

Electric cell

Ans. (1) The construction of an

electric doorbell is shown in the figure.

The circuit consists of an electromagnet,

(with horseshoe shaped iron

core) electric cell, a switch, a flexible

strip of soft iron with a striker and a

contact screw. The figure also shows the

metal striker and the metal gong.

(2) When the switch is closed, a current

flows in the circuit. Hence, the iron

strip is attracted by the electromagnet,

the metal striker strikes the metal gong

and sound is produced. Now, a gap is

produced between the contact screw and the iron strip. Hence, there

is no current in the circuit and the electromagnet no more attracts

the iron strip. The strip being flexible returns to its normal position

making contact with the screw. Therefore, the circuit is completed

and sound is produced as explained earlier. The making and

breaking of the circuit takes place a large number of times every

second and the bell continues to ring as long as the switch is closed.

Answers to the Questions given on page 113 of the textbook

* (1) What is the use of the flexible strip ?

Ans. Due to the flexible strip, the making and breaking of

the circuit occur alternately and the bell continues to ring as

long as the switch is closed.

* (2) What will happen if the strip loses its flexibility ?

Ans. If the strip loses its flexibility, when the circuit

breaks and the electromagnet loses its magnetism, the strip

will not return to its normal position of contact with the screw.

Hence, the circuit will not be completed and the bell will not

ring.

* (24) Draw a circuit diagram using the symbols for an

electric cell, key, electric bulb and conducting wires.

146 NAVNEET GENERAL SCIENCE DIGEST : STANDARD VIII

Ans.

_ +

Electric cell

Key

Electric bulb

Conducting wire

Fig. 11.9 : Circuit diagram

(25) Name the components necessary in a circuit.

Ans. The components necessary in a circuit are electric cell,

electric bulb/gadget, conducting wires, a key to open and close the

circuit.

* (26) Why is there a key in an electrical circuit ?

Ans. Depending on the requirement, an electrical circuit can

be completed (closed) by closing the key and broken by opening the key.

* (27) Is the mechanism of a shirt button like that of a key in

an electric circuit ?

Ans. In a way, yes. When one feels very hot, one can open the

shirt button to allow the body heat to escape. When one feels very

cold, one can close the shirt button to maintain the body temperature.

Answers to the Questions given on pages 113 & 115

of the textbook

* (1) What is the function of the switches in our homes?

Ans. A switch in our home is like a key in an electrical

circuit. We can switch on or switch off a fan, tubelight, etc.,

using the switch.

* (2) What will happen if there is no key in a circuit ?

Ans. If there is no key in a circuit, it will be difficult to

make or break the circuit at will; everytime, we will have to

connect or disconnect the conducting wires in the circuit.

* (3) Name the components of the electrical circuit in

the diagram below.

_ +

Ans. Electric cell

( + _ ) electric bulb

coil of wire

(for use as a magnet),

key ( ( and

Fig. 11.10 connecting wire (—)

 (28) What is an electrical conductor? Give two

examples.

Ans. A substance through which electricity can flow is

called an electrical conductor.

Examples : Iron, copper.

(29) What is an electrical insulator ? Give two examples.

Ans. A substance through which electricity cannot flow is

called an electrical insulator.

Examples : Rubber, wood.

Answer to the Question given on page 115 of the textbook

* What is an insulating tape made from?

Ans. An insulating tape is made from an insulating

material such as cotton, plastic or rubber.

Q. 2 Give scientific reasons :

* (1) The copper wire used in an electromagnet is insulated.

Ans. (1) To make an electromagnet, insulated copper wire is

wound around a rod of soft iron. The ends of the wire are connected

to an electric cell through a key. As the wire is insulated, there is no

electrical contact between the wire and the rod. (2) If the wire is

bare i.e., not insulated, there will be electrical contact between the

wire and the rod and consequently between the adjacent turns of

the wire. This will reduce the electrical resistance in the circuit

significantly. If the key is closed under such condition, the wire

will carry large current and become very hot. In a short time,

practically all the electrical energy in the cell will be converted into

heat. To avoid this the copper wire used in an electromagnet is

insulated.

* (2) In an electric bell, the electric current stops again and

again.

Ans. (1) When the flexible iron strip in an electric bell is in

contact with the contact screw, the circuit is completed. Due to the

current, the electromagnet starts functioning and attracts the iron

strip. Therefore, the metal striker strikes the metal gong and sound

is produced. (2) As the strip bends, it loses contact with the screw,

thereby breaking the circuit. Therefore, the electromagnet loses its

magnetism. The iron strip is no more attracted by it and being

flexible, returns to its normal position of contact with the screw.

As the making and breaking of the circuit occurs a large number of

times every second, the electric current stops again and again.

* (3) Electrical switches are made of plastic.

Ans. Plastic is an electrical insulator. Our body and the

ground can conduct electricity. To prevent electrical contact and

subsequent shock while using (operating) an electrical switch, the

switch is made of plastic.

* Q. 3 Answer the following questions in one sentence

each :

(1) What do you call the force exerted by an electric cell on

electric charges?

Ans. The force exerted by an electric cell on electric charges is

called electromotive force.

(2) Give two examples of electrical cell.

Ans. Examples of electrical cell : Volta’s cell, Leclanche’s cell.

(3) Name a device/appliance in which an electromagnet

is used.

Ans. An electromagnet is used in an electric doorbell.

(4) Give one example each of an electrical conductor and

an electrical insulator.

Ans. Copper is an electrical conductor while wood is an

electrical insulator.

Q. 4 Find the odd man out and give the reason :

(1) Rubber, Wood, Plastic, Iron.

(2) Plastic, Copper, Iron, Aluminium.

Ans. (1) Iron. Iron is an electrical conductor while rubber,

wood and plastic are electrical insulators. (2) Plastic. Plastic is an

electrical insulator while copper, iron and aluminium are electrical

conductors.

Q. 5 State whether the following statements are True or

False. If a statement is false, correct it and rewrite :

(1) In Volta’s cell, an electric current is produced due to a

chemical reaction.

(2) In the simple electric cell, the copper plate acts as the

negative pole.

 (3) In the Leclanche electric cell, the zinc rod acts as the

negative pole.

(4) The outer covering of a dry cell is made of zinc.

(5) The nickel-cadmium cell cannot be recharged.

(6) The lithium cell cannot be recharged.

(7) The phenomenon of electromagnetic induction was

discovered by Michael Faraday.

Ans. (1) True. (2) False. In the simple electric cell, the copper

plate acts as the positive pole. (3) True. (4) True. (5) False. The

nickel-cadmium cell can be recharged. (6) True. (7) True.

Q. 6 Fill in the blanks :

(1) The force applied by an electric cell on electric charges is

called ……… force.

(2) In the simple electric cell, ……… acts as the negative pole.

(3) In the Leclanche cell, a solution of ……… is taken in the

glass trough.

(4) In the dry cell, ……… acts as the positive pole.

(5) The phenomenon of electromagnetic induction was

discovered by ……… .

Ans. (1) electromotive (2) zinc plate (3) ammonium chloride

(4) carbon rod (5) Michael Faraday.

* Q. 7 Match the following :

‘A’ ‘B’

(1) Carbon, zinc

(2) Copper, zinc

(3) Electric motor

(4) Protection from electric current

(a) Armature

(b) Insulator

(c) Dry cell

(d) Simple cell

Ans. (1) Carbon, zinc – Dry cell (2) Copper, zinc – Simple cell

(3) Electric motor– Armature (4) Protection from electric

current – Insulator.