CBSE Class X

Question 1:

Name two energy sources that you would consider to be renewable. Give reasons

for your choices.

Answer:

Two renewable sources of energy are as follows:

(a) Sun: The energy derived from the Sun is known as solar energy. Solar

energy is produced by the fusion of hydrogen into helium, fusion of helium

into other heavy elements, and so on. A large amount of hydrogen and

helium is present in the Sun.

Therefore, solar energy can replenish on its own. The Sun has 5 billion years

more to burn. Hence, solar energy is a renewable source of energy.

(b) Wind: Wind energy is derived from air blowing with high speed. Wind

energy is harnessed by windmills in order to generate electricity. Air blows

because of uneven heating of the Earth. Since the heating of the Earth will

continue forever, wind energy will also be available forever.

Question 2:

Hydrogen has been used as a rocket fuel. Would you consider it a cleaner fuel

than CNG?

Why or why not?

Answer:

Hydrogengas is cleaner than CNG. CNG contains hydrocarbons. Therefore, it

has carbon contents. Carbon is a form of pollutant present in CNG. On the

other hand, hydrogen is waste-free. The fusion of hydrogen does not produce

any waste. Hence, hydrogen is cleaner than CNG.

Question 1:

Can any source of energy be pollution-free? Why or why not?

Answer:

No source of energy can be pollution-free. It is considered that solar cells are

pollution- free. However, even their making causes environmental damage

indirectly.

Also, in the case of nuclear energy, there is no waste produced after the fusion

reactions. However, it is not totally pollution-free. To start the fusion reactions,

approximately 10⁷ K temperature is required, which is provided by fission reactions.

The wastes released from fission reactions are very hazardous. Hence, no source

of energy is pollution-free.

Question 4:

What are the advantages of nuclear energy?

Answer:

The advantages of nuclear energy are as follows:

(a) Large amount of energy is produced per unit mass.

(b) It does not produce smoke. It is a clean energy.

(c) Fission of one atom of uranium produces 10 million times the energy

released by burning of one atom of carbon.

(d) Fusion of four hydrogen atoms produces huge amount of energy approximately

equal to 27 MeV.

Question 3:

What is geothermal energy?

Answer:

Geothermal power plants use heat of the Earth to generate electricity. This

heat energy of the Earth is known as geothermal energy.

When there are geological changes, the molten rocks present in the core of

the earth are pushed to the earth’s crust. This forms regions of hot spot.

Steam is generated when the underground water comes in contact with

these hot spots forming hot springs. This trapped steam is used to

generate electricity in the geothermal power plants.

Question 2:

What are the limitations of the energy that can be obtained from the oceans?

Answer:

The forms of energy that can be obtained from the ocean are tidal energy,

wave energy,and ocean thermal energy. There are several limitations in

order to harness these energies.

(i) Tidal energy depends on the relative positioning of the Earth, moon, and

the Sun.

(ii) High dams are required to be built to convert tidal energy into electricity.

(iii) Very strong waves are required to obtain electricity from wave energy.

(iv) To harness ocean thermal energy efficiently, the difference in the

temperature of surface water (hot) and the water at depth (cold) must

be 20℃ or more.

Question 1:

What kind of mirror − concave, convex or plain − would be best suited for use

in a solar cooker? Why?

Answer:

A solar cooker uses heat of the sunlight to cook and heat food. A mirror is

used in order to reflect and focus sunlight at a point. A concave mirror is

used in a solar cooker for this purpose. The mirror focuses all the incident

sunlight at a point. The temperature at that point increases, there by

cooking and heating the food placed at that point.

Question 3:

How has the traditional use of wind and water energy been modified for our

convenience?

Answer:

Traditionally, waterfalls were used as a source of potential energy which was

converted to electricity with the help of turbines. Since waterfalls are few in

number, water dams have been constructed in large numbers. Nowadays,

hydro-dams are used in order to harness potential energy of stored water.

In water dams, water falls from a height on the turbine, which produces

electricity.

Earlier, the windmills were used to harness wind energy to do mechanical

work such as lifting/drawing water from a well. Today, windmills are used

to generate electricity. In windmills, the kinetic energy of wind is harnessed

and converted into electricity. The rotatory motion of the blades turns the

turbine of the electric generator to generate electricity.

Question 2:

Why are we looking at alternate sources of energy?

Answer:

Fossil fuels, which have been traditionally used by human beings as an energy

sources,are non-renewable sources of energy. These sources of energy are

limited and cannot replenish on their own. They are being consumed at a large

rate. If this rate of consumption continues, then the fossil fuels would be

exhausted from the Earth.

Therefore, we have to conserve the energy sources. Hence, we should look

for alternate sources of energy.

Question 1:

What are the disadvantages of fossil fuels?

Answer:

The disadvantages of fossil fuels are as follows:

(a) Burning of coal and petroleum produces a lot of pollutants causing air

pollution.

(b) Fossil fuels release oxides of carbon, nitrogen, sulphur, etc. that cause

acid rain, which affects the soil fertility and potable water.

(c) Burning of fossil fuels produce gases such as carbon dioxide that causes

global warming.

Question 3:

If you could use any source of energy for heating your food, which one would

you use and why?

Answer:

Natural gas can be used for heating and cooking food because it is a clean

source of energy. It does not produce huge amount of smoke on burning.

Although it is highly inflammable, it is easy to use, transport, and it

produces a huge amount of heat on burning.

Question 2:

What is a good fuel?

Answer:

A good fuel produces a huge amount of heat on burning, does not produce a lot

of smoke, and is easily available.

Question 1:

What is a good source of energy?

Answer:

A good source of energy fulfils the following criteria:

(I)It produces a lot of heat per unit mass.

(II) It does a huge amount of work per unit mass.

(III) It is easily accessible.

(IV) It is easy to store and transport.

(V)It is economical.

(VI)It produces less amount of smoke.

Question 18:

What is the function of an earth wire? Why is it necessary to earth metallic

appliances?

Answer:

The metallic body of electric appliances is connected to the earth by means of

earth wire so that any leakage of electric current is transferred to the ground.

This prevents any electric shock to the user. That is why earthing of the

electrical appliances is necessary.

Question 17:

When does an electric short circuit occur?

Answer:

If the resistance of an electric circuit becomes very low, then the current flowing

through the circuit becomes very high. This is caused by connecting too many

appliances to a single socket or connecting high power rating appliances to the

light circuits. This results in a short circuit.

When the insulation of live and neutral wires undergoes wear and tear and then

touches each other, the current flowing in the circuit increases abruptly. Hence,

a short circuit occurs.

Question 16:

Explain the underlying principle and working of an electric generator by drawing

a labelled diagram. What is the function of brushes?

Answer:

An electric generator converts mechanical energy into electrical energy.

The principle of working of an electric generator is that when a loop is moved in a

magnetic field, an electric current is induced in the coil. It generates electricity by

rotating a coil in a magnetic field. The following figure shows a simple AC generator.

 

MNST → Rectangular coil

A and B → Brushes

C and D → Two slip rings

X → Axle, G → Galvanometer

If axle Xis rotated clockwise, then the length MN moves upwards while length ST

moves downwards. Since the lengths MN and ST are moving in a magnetic field, a

current will be induced in both of them due to electromagnetic induction. Length

MN is moving upwards and the magnetic field acts from left to right. Hence,

according to Fleming’s right hand rule, the direction of induced current will be

from M to N. Similarly, the direction of induced current in the length ST will be

from S to T.

The direction of current in the coil is MNST. Hence, the galvanometer shows a

deflection in a particular direction. After half a rotation, length MN starts moving

down whereas

length ST starts moving upward. The direction of the induced current in the coil

gets reversed as TSNM. As the direction of current gets reversed after each half

rotation, the produced current is called an alternating current (AC).

To get a unidirectional current, instead of two slip rings, two split rings are used,

as shown in the following figure.

 

In this arrangement, brush A always remains in contact with the length of the coil

that is moving up whereas brush B always remains in contact with the length that

is moving down. The split rings C and D act as a commutator.

The direction of current induced in the coil will be MNST for the first rotation and

TSNM in the second half of the rotation. Hence, a unidirectional current is produced

from the generator called DC generator. The current is called AC current.

Question 15:

State the rule to determine the direction of a (i) magnetic field produced around

a straight conductor-carrying current, (ii) force experienced by a current-carrying

straight conductor placed in a magnetic field which is perpendicular to it, and (iii)

current induced in a coil due to its rotation in a magnetic field.

Answer:

(i) Maxwell’s right hand thumb rule

(ii) Fleming’s left hand rule

(iii) Fleming’s right hand rule

Question 14:

Two circular coils A and B are placed closed to each other. If the current in the

coil A is changed, will some current be induced in the coil B? Give reason.

Answer:

Two circular coils A and B are placed close to each other. When the current in

coil A is changed, the magnetic field associated with it also changes. As a result,

the magnetic field around coil B also changes. This change in magnetic field lines

around coil B induces an electric current in it. This is called electromagnetic

induction.

Question 13:

A coil of insulated copper wire is connected to a galvanometer. What will happen

if a bar magnet is (i) pushed into the coil, (ii) withdrawn from inside the coil, (iii)

held stationary inside the coil?

Answer:

A current induces in a solenoid if a bar magnet is moved relative to it. This is the

principle of electromagnetic induction.

(i) When a bar magnet is pushed into a coil of insulated copper wire, a current is

induced momentarily in the coil. As a result, the needle of the galvanometer

deflects momentarily in a particular direction.

(ii) When the bar magnet is withdrawn from inside the coil of the insulated copper

wire,a current is again induced momentarily in the coil in the opposite direction. As

a result, the needle of the galvanometer deflects momentarily in the opposite

direction.

(iii) When a bar magnet is held stationary inside the coil, no current will be induced

in the coil. Hence, galvanometer will show no deflection.

Question 12:

Name some devices in which electric motors are used?

Answer:

Some devices in which electric motors are used are as follows:

(a) Water pumps

(b) Electric fans

(c) Electric mixers

(d) Washing machines