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GRADE:-8 PHYSICS
CHAPTER- 8 MORE ABOUT ENERGY
Answer in short:
1) How is the potential energy different from kinetic energy?
Potential energy
Kinetic energy
a) It is defined as energy possessed by an object a) It is defined as energy possessed by an object
due to position or change in state.
by virtue of its motion.
b) P.E. = mgh
b) K.E. = 1 / 2 mv2
where
where
m = mass, v = velocity
m = mass, h = height,
g = gravitational acceleration
2) Define solar energy. Briefly explain how life on Earth depends on it.
a) Solar energy is the energy associated with the Sun.
b) (i) Brings rain: Heat energy of sun cause water to evaporate. (ii) Electricity: Flowing water has
kinetic energy due to solar energy. (iii) Wind energy: Solar energy warm up the air unevenly and
causes it to flow. (iv) Photosynthesis: Green plants absorb solar energy & convert it into the
chemical energy of food. This chemical energy of food is converted into heat & muscular energy in
our bodies when we eat food. (v) Chemical energy of fuels such as wood, coal & petroleum also
comes from solar energy.
3) What are non - renewable sources of energy? Give any 2 examples.
Non - renewable sources of energy are present in limited amount on Earth & cannot be replaced
once they get exhausted.
Ex: Coal, oil, natural gas.
4) What are renewable sources of energy? Give any 2 examples.
Renewable sources of energy are abundant in nature & are continually being replenished through
natural processes.
Ex: Solar energy, wind energy, bio - energy & geothermal energy.
5) What is photovoltaic cell? State its uses.
a) Photovoltaic cell is device made of thin layers of a substance such as silicon or gallium that
converts sunlight into electrical energy.
b) It is used to provide electrical energy to satellites & devices used in space, to run street lights, oil
rigs & lighthouses, electronic watches & calculators.
6) Why is the utilization of wind energy not always practical?
The utilization of wind energy is not always practical because winds are unpredictable.
7) What do you understand by the term nuclear fusion?
A nuclear reaction in which two or more atomic nuclei collide at a high speed & join to form a new
atomic nucleus is called nuclear fusion.
8) State 3 precautionary measures which must be taken while working at nuclear reactors.
a) Reactors should be constructed far from residential areas. b) Constructed properly so that there
is no leakage of nuclear radiation. C) Wear radiation – safe jackets.
9) What is geothermal energy? How can geothermal energy is used to generate electricity?
a) Heat energy obtained from hot rocks inside Earth is called geothermal energy.
b) The hot water from spring & the kinetic energy from steam is used to rotate a turbine & generate
electricity.
10) Why is it preferable to burn biogas rather than fossil fuels?
It is preferable to burn biogas rather than fossil fuels because it has higher energy efficiently than
fossil fuels.
Answer in detail:
1) What is biomass? How can we derive energy from it?
a) The animal & plant wastes derived from numerous sources, including the by – products from the
wood industry, agricultural crops, forests & household wastes are called biomass.
b) Biomass is mixed with water & poured into the slurry tank & taken to the digester tank that has no
air supply. It is allowed to decompose anaerobically. Biogas containing 50 – 70% methane is
produced.
2) What is nuclear fission? How can it be utilized to generate electrical energy?
a) Nuclear reaction in which the nucleus of an atom splits into smaller nuclei is called nuclear
fission.
b) A nucleus of uranium – 235 is bombarded with a neutron; hence barium, krypton & 3 free
neutrons are released.
Controlled nuclear fission reactions are carried out in a nuclear reactor using a nuclear fuel. The
heat so released is used to turn water into steam & generate electricity.
3) What is tidal energy? How it used to generate electricity?
a) The rising & falling tides in oceans possess energy called tidal energy.
b) At high tide, water can be trapped behind a tidal dam & then let out through a turbine to generate
electricity as the tide ebbs.
4) Write a note on hydroelectricity. State its 2 advantages & disadvantages.
a) The water stored in dams possesses potential energy, while falling it is converted into kinetic
energy. This water falls on large turbine which converts kinetic energy into mechanical energy.
Turbine is connected to a generator which converts mechanical energy into electricity.
Advantages: a) It do not cause air pollution. b) It is natural source and is free. c) Reliable than solar,
wind or tidal energy.
Disadvantages: a) Cost of construction is very high. b) Reservoir is needed to collect water. c)
Reservoirs interfere with flora, fauna and human inhabitation nearby.
5) Explain the construction of box – type solar cooker. What is the maximum temperature it
can attain? What are its advantages & disadvantages?
(Diagram from T.B)
a) It consists of a box made of an insulated material & is painted black inside for maximum
absorption of solar heat. A plane mirror is used to reflect sunlight into the cooker. The mirror can be
adjusted at a desired angle according to the position of the Sun. A glass lid is used to cover a box.
b) Maximum temperature it can attain is 100℃.
Advantages: a) Save fuel b) It does not cause pollution c) Environment friendly. d) Solar energy is
free of cost. E) Nutrients are preserved.
Disadvantages: a) Large surface area is required. b) Not effective at night or cloudy day. c)
Requires 3-4 hr to cook food.
Numerical:
1) Calculate the work done by a boy in lifting a 10 kg laptop from the ground & keeping it on
a shelf 1.5 m high. g = 10 m / s2.
P.E. = mgh = 10 * 10 * 1.5 = 150 J
2) A person does 5000 J of work in climbing a tree of height of 5 m. Calculate the mass of the
person.
P.E. = mgh
∴ 5000 = m * 10 * 5
∴m = 5000 / 50 = 100 kg.
3) A 900 kg compact car is moving at a certain speed. If its kinetic energy is 3,20,000 J, find
the velocity at which the car is travelling.
K.E. = 1 / 2 mv2
320000 = 1 / 2 * 900 * v 2
∴V 2 = 6400 / 9 = 80 / 3 = 26.66 m / s.
4) Two bodies of equal masses move with uniform velocities V and 3V, respectively. Find the
ratio of their kinetic energies.
K.E.1/ K.E. 2 = (1/2 m1v12) / (1/2 m2v22)
m1 = m2 = m
K.E.1/ K.E.2 = (1/2V12) / (1/2 V22)
= V2 / (3V)2
=1/9
CHAPTER – 9 STATIC ELECTRICITY
Answer in short:
1) How is a body charged by friction?
Whenever two objects are rubbed, free electrons get transferred from one object to the other. The
object that loses electrons gets positively charged while the one that gains electrons gets negatively
charged.
2) Define charging by conduction.
When an uncharged object is touched with another charged object, the uncharged object also gets
charged. The total charge on the charged object gets equally distributed between the two objects in
contact. The process is called charging by conduction.
3) Why does a glass rod get positively charged when rubbed with silk?
Because some free electrons flow from the glass rod to the silk, so a glass rod gets positively
charged & silk becomes negatively charged.
4) Explain why the leaves of a negatively charged electroscope collapse if a positively
charged glass rod is brought near the disc of electroscope.
Because the free electrons from the leaves move towards a positively charged glass rod. This
decrease in negative charge of the leaves decreases the divergence in the gold leaves.
Answer in detail:
1) Describe the structure of an atom with the help of diagram.
(Diagram from T.B)
a) Protons and neutrons are present in the core of the atom called the nucleus.
b) Electrons revolve around the nucleus in definite path called orbit. No. of orbits depends upon the
no. of electrons, an atom has.
c) The electrons in the outermost orbit are called valence electrons & can be easily knocked off to
other atoms. They can break losses & move freely inside a substance as free electrons.
d) If the atoms of an object gain electrons, the object becomes negatively charged & if atoms loses
an electron, it become positively charged.
e) The region around a charge within which it can exert an influence is called its electric field.
2) What is a conductor? Explain why a conductor will not produce static electricity.
a) Conductors are substances that allow electric charges to flow through them easily.
b) Electron transferred to an object at a given location quickly move across entire surface of the
object in conductors.
c) Electrons transferred to an insulator at a given location, the excess electrons will stay at that
initial location & do not move freely across the surface remaining static. This creates electrical
imbalance on the object.
3) State the basic property of electric charges.
a) Attractive property, b) Repulsive property
4) What is lighting? How is it caused?
a) Lighting is the high energy electric discharge & can cause great harm to human beings, animals,
trees & buildings.
b) When clouds move, the friction with surrounding air causes charges to develop on them. The
magnitude of charges increases as the clouds continue moving & hence lower part of a cloud
carries a negative charge while the upper part carries a positive charge.
c) When large clouds with negatively charged lower part passes over the electrically neutral ground,
a positive charge is induced on the ground. As air acts as an insulator, it does not allow these
charges to flow.
d) However, when the difference of charges becomes very large, the insulation of air breaks down
and electrons flow through air to neutralize this charge difference.
e) This generates a huge electric spark in the sky & is observed as lighting.
5) Explain why charging by induction transfers an opposite charge on to the conductor. How
is an electroscope charged by induction?
(Diagram from T.B)
a) Because when negatively charged object is brought near to the conductor, the electrons of the
conductor gets repelled by the electrons of the negatively charged object & gets collected at the
other end of the conductor, making that end negatively charged.
b) And protons get attracted by the negatively charged object & hence makes that end of the
conductor positively charged.
Charging by induction:
The charging object is brought close to, but not touched to the brass cap. This causes unlike
charges to accumulate on the brass cap, while like charges accumulate on the leaves. When the
brass cap is earthed, the like charges from the leaves flow into Earth. Now, if the earthing & the
charging object is removed, the unlike charges from the brass cap move to the leaves.
6) (a) Describe a gold- leaf electroscope with a diagram.
(Diagram from T.B)
i) It consists of a bell jar fitted on a wooden base.
ii) There is an insulator plug at the mouth of the bell jar, through which a brass rod passes. A seal
insulates the brass rod from the glass jar.
iii) A brass cap or disc is fixed on the upper end of the brass rod & the lower end has two thin,
flexible & sensitive gold leaves.
iv)There are 2 earthed metal foil stripes on either sides of the gold leaves. They increase the
sensitivity of the instrument by taking away any charge leakage from the glass jar. If an excessive
charge is applied to the gold leaves, they stretch out, touch the strips & get discharged.
v) The jar protects the leaves from currents.
(b) How can you use it to detect charge on an object?
(Diagram from T.B)
i) When a positively charged object is touched to the uncharged brass cap of the electroscope, free
electrons from the brass cap flow into the object. This causes the brass cap to acquire a positive
charge. As a result, the leaves also get positively charged, repel each other & hence diverge.
ii) When a negatively charged object is touched to the uncharged brass cap, free electrons flow from
the object to the brass cap. The brass cap & gold leaves then acquire a negative charge.
iii) When a charged object is brought in contract with the uncharged electroscope, the leaves
diverge in case of an uncharged object, there is no divergence.
(c) What is the purpose of the metal foil placed at the base of the electroscope?
To increase the sensitivity of the instrument by taking always any charge leakages from the glass
jar.
CHAPTER-10 CURRENT ELECTRICITY
Answer in short:
1) Define electric current.
Amount of charge flowing per unit time through a conductor is called electric current.
2) Express the mathematical relationship between charge flown, time taken & current.
I=Q/t
3) What type of connection is used to connect an ammeter in a circuit?
Series connection is used to connect an ammeter in a circuit.
4) Define electric potential.
Electric potential is defined as that property of a conductor which determines whether it will allow the
flow of electrons through it.
5) Which effect of current is used in an electric bulb to give out light?
Heating effect of current is used in an electric bulb to give out light.
Answer in short:
1) What do you understand by conventional electric current?
a) Conventional current flows from the positively charged conductor i.e. higher potential to the
negatively charged conductor i.e lower potential.
b) It flows in the direction opposite to the flow of electron current.
2) How does the electric potential govern the flow of charges in a circuit? What is the source
of potential difference in a simple electric circuit?
a) Flowing of charge depends upon the electric potential. When there is a electric potential
difference, charges flow & if there is no potential difference, flowing of charge is not possible.
Electric charge flows from lower potential to the higher potential i.e. from higher concentration to the
lower concentration of electrons & continue until both the potential have the same charge.
b) An electric cell or battery is the source of potential difference in a simple electric circuit.
3) Why is the glass structure of an electric bulb filled with an inert gas?
Because inert gas has low reactivity with other substances, hence it does not undergo chemical
reactions under a set of given conditions.
4) Why is tungsten found suitable to make the filament of a bulb?
Because tungsten has very high melting point (3422 ℃).
5) What is a fuse wire made up of?
A fuse wire is made up of an alloy of lead & tin because it has low melting point and high
resistance.
6) Name the material used to make the heating coils in electric heating device. Why is this
material found suitable for this use?
The material used to make the heating coils in electric heating device is nichrome because it has
high resistance.
Answer in detail:
1) What do you understand by heating effect of current? State the factors affecting the
amount of heat produced in a conductor, when electric current is allowed to flow through it.
a) When electric current is allowed to flow through a conductor, it gets heated.
b) Factors: i) Length of the conductor ii) Area of cross section iii) Material of the conductor iv)
Amount of electric current v) Time of current flow.
2) What is meant by electric potential difference? What is its SI unit? Name the instrument
used to measure it. In what manner should this device be connected in an electric circuit to
read its potential difference?
a) The difference between the electric potentials of two charged objects is called electric potential
difference.
b) SI unit – volt (V)
c) Instrument used- Voltmeter
d) Connected in parallel with the device whose potential difference is to be measured.
3) What is the difference between electron current & the conventional current? What is the
direction of the two currents in an electric circuit?
Electron current: Flows from lower potential to higher potential of electric potential.
Conventional current: Flows from higher potential to lower potential of electric potential.
a) The direction of both the currents is in the opposite direction to each other.
4) Describe the construction of an electric bulb. How does it function?
(Diagram from T.B)
Construction:
a) It consists of a hollow, sealed glass bulb filled with an inert gas.
b) It contains a thin, coiled tungsten filament fixed on nickel – iron wires attached to the neck of the
bulb.
c) Two ends of filament are connected to thick lead wires & drawn out to join the contact pins.
d) Oxygen is removed from bulb to prevent oxidation of tungsten oil at such high temperature.
Working:
a) Contact pins connect the wires through the bulb holder to electric mains & allow electric current to
flow through the filament. When current flows, temperature of the tungsten coil rises to nearly 3000
℃, it becomes red hot & starts glowing to give out heat & light.
5) Describe the construction of an electric fuse. How does it prevent the damage of electrical
systems?
Construction:
It consists of a small length of a wire made of lead - tin alloy.
Functions:
During an excessive flow of current the fuse wire gets heated & melts. This breaks the circuit
immediately & stops the flow of current to the appliance & prevents it from damage.
Numerical:
1) A charge of 200 C is allowed to pass through a conductor for 10 seconds. How much
electric current flows through the conductor?
Q= 200 C, t = 10 s.
I = Q / t = 200 / 10 = 20 A
2) 56 C of charge flows through a conductor for 8 seconds. How much electric current flows
through the conductor?
Q = 56 C, t = 8 s.
I = Q / t = 56 / 8 = 7 A
3) An electric heater consumes 5 A of electric current in 600 seconds. What is amount of
charge used by the heater?
I = 5 A, t = 600 s.
I=Q/t
∴Q = I * t = 5 * 600 = 3000 C
4) For how much time should a charge of 480 C flow through a point in a conductor to
constitute an electric current of 6 A?
I=Q/t
∴t = Q / I = 480 / 6 = 80 s
CHAPTER -11 MAGNETISM & ELECTRICITY
Answer in a word or two or in a sentence:
1) What do you understand by magnetic field?
The region in which the magnetic force of a magnet is felt by other magnets or magnetic materials is
called its magnetic field.
2) Name two appliances that use electromagnets.
Electric bell, electric motor.
3) What is an alternating current?
A current that changes direction at fixed intervals of time is called alternating current.
4) If a current is flowing downwards in a straight conductor, what is the direction of the
magnetic field?
The direction of the magnetic field is clockwise.
5) Why does a compass needle placed near a current - carrying conductor show deflection?
Because magnetic field is produced by the electric current.
Answer in short:
1) How are natural magnets different from artificial magnets?
Natural magnets
Artificial magnets
a) They are naturally occurring magnetic a) They are man- made.
materials.
b) Ex: lodestone
b) Ex: Iron, cobalt, nickel
c) They have weak magnetic property.
c) They have strong magnetic property.
2) What is a solenoid?
When a long insulated copper wire is wound in the form of a coil, & current passed through it, it
behaves as a magnet. It is then called a solenoid.
3) What do you understand by electromagnetic induction?
The phenomenon due to which a changing magnetic field within a closed conducting coil induces an
electric current in the coil is called electromagnetic induction.
4) Difference between an electric motor & a dynamo.
Electric motor
Dynamo
a) It converts electrical energy into a) It converts mechanical energy into electrical
mechanical energy.
energy.
b) It
works
on
the
principle
of b) It works on the principle of electromagnetic
electromagnetism.
induction.
c) It is used in fans, electric mixer grinders, c) It is used in generation of electricity.
and electric water pump.
Answer in detail:
1) Describe an electromagnet. How can the strength of an electromagnet be increased?
a) An electromagnet is a temporary magnet produced when an electric current is passed through a
wire wrapped around an iron core. It is a solenoid with a soft iron core. It works on the principle of
magnetic effect of electric current.
b) Strength can be increased by i) Increasing amount of current through the conductor, ii) Increasing
no of turns in the coil, iii) Increasing the length of the coil. Also, depends upon the nature of the
material of the core.
2) State the principle of an electric motor & describe its working with a diagram.
(Diagram from T.B)
Principle of electric motor is electromagnetism.
Working:
An armature with rectangular coil kept in the magnetic field of a U- shaped magnet, two split rings &
2 carbon brushes connected to a battery. When current flows through the rectangular coil, it
becomes an electromagnet. The magnetic fields of electromagnet & the U- shaped magnet interfere
with each other & result in making each side of the coil experience a force. These forces in turn
rotate the coil.
3) How is alternating current different from direct current?
Alternating current
Direct current
A current that changes direction at fixed intervals A current that always flows in same direction.
of time.
4) What is the function of a transformer? Differentiate between the two types of transformers.
Function: To increase or decrease the voltage of an alternating current.
Step - up transformer
a) Increase the magnitude of alternating voltage.
Step – down transformer
a) Decrease the magnitude of alternating
voltage.
b) No. of turns in primary coil is less than the no. b) No. of turns in primary coil is more than the
of turns in secondary coil.
no. of turns in secondary.
5) What is the principle of working of a dynamo? How does a dynamo work?
a) Principle of working of a dynamo is electromagnetic induction.
b) Working: As the coil ABCD is made to rotate, it cuts the magnetic lines of force of the horseshoe
magnet & an electric current is induced in the coil. The two slip rings also rotate with the coil,
thereby maintaining the contact during the rotations. The slip rings allow the direction of the induced
current to change after every half - rotation. Therefore, it produces an alternating current.
6) Explain the construction & working of an electric bell with a labelled diagram.
(Diagram from T.B)
Construction:
a) It consists of a U – shaped electromagnet, an armature, a contact spring, a contact adjustment
screw, a hammer & a gong.
b) One end of the wire is connected to the switch & other end to one end of the armature. The
armature is a soft iron rod & faces the poles of the electromagnet.
c) The contact adjustment screw consists of a brass screw with a tip made of silver cadmium alloy &
is attached to a contact spring.
d) A hammer made of steel to attached to the armature.
Working:
a) When switch is pressed, the iron strip comes in contact with the screw & current begins to flow
through the electromagnet.
b) The iron core gets magnetized & attracts the armature. The armature moves towards the poles of
electromagnet, guiding the hammer to move & strike the gong. This causes the ringing sound.
c) The movement of the armature towards the poles of the solenoid breaks the circuit & the current
stops flowing. The electromagnet loses its magnetism & the armature returns back to its original
position.
7) Why does a freely suspended magnet always rest in the north – south direction?
Because Earth itself behaves like a huge bar magnet. The magnetic north pole of Earth is close to
the geographical south pole, & the magnetic south pole is close to the geographical North Pole.
CHAPTER-7 PRESSURE & ARCHIMEDES PRINCIPLE
Answer in short:
1) Define pressure .Write the mathematical formula for it & state its SI unit.
Pressure: Force acting normally on a unit area of a surface.
Pressure (P) = Force (F) / Area (A)
SI unit: Pascal (Pa)
2) What do you understand by atmospheric pressure? How does it change with altitude?
Atmospheric Pressure: The force exerted by the atmosphere per unit area of the surface of Earth.
Atmospheric pressure decreases with an increase in the altitude. At higher altitudes, the pressure
inside our body is greater than the atmospheric pressure.
3) State the factors that affect the buoyant force exerted by a fluid on an object immersed in
it.
a) The density of the liquid.
b) The volume of the object or the volume of the liquid displaced by it.
4) Why can water not be used as a barometric liquid?
a) Its density is only 1 g/cm3. It will have to rise to a height of 10.5 m to balance atmospheric
pressure.
b) It is not easily available in its purest form & its transparent surface makes it difficult to read.
c) It wets & sticks to the walls of the glass tube & may therefore introduce errors in reading.
d) It vaporizes in vacuum & will never record the true atmospheric pressure.
5) List the factors on which liquid pressure depends.
a) Height of the liquid, b) Density of the liquid, c) Depth.
Answer in detail:
1) Describe why mercury is preferred as barometric liquid over water.
a) It has the highest density among liquids (13.6 g/cm3). It only has to rise up to 76 cm to balance
atmospheric pressure.
b) Obtained in a pure & dry state & opaque surface enables it to be read easily.
c) Does not wet or sticks to the walls of the glass tube, hence gives correct reading of atmospheric
pressure.
d) Does not vaporize in vacuum, thus giving correct atmospheric pressure.
2) State Archimedes principle. Describe a simple experiment to verify it.
Archimedes principle: If a body is partially or fully immersed in a fluid, it experiences an upthrust &
thus an apparent loss in weight, wherein the loss in weight is equal to the weight of the fluid
displaced by the submerged part of the body.
Experiment:
a) Attach the stone to the spring balance & note the reading in air (W 1). Also measure the weight of
the empty beaker.
b) Fill the can with water until it overflows. Place the empty beaker below the spout of the can.
c) Immerse the stone into the water & allow water to overflow from the spout. When the water stops
dripping, note the reading on the spring balance (W2). Weigh the water collected in the beaker (W 3).
Let the weight of the empty beaker by W 4 .
d) W 1 – W 2 is the loss of weight of the object.
e) W 3 – W 4 is the weight of water displaced.
f) Loss of weight of object = weight of liquid displaced.
Hence, verified.
3) Explain buoyancy & buoyant force. Also state the law of flotation.
Buoyancy: The property of a liquid to exert an upward force on an object immersed in it is called
buoyancy.
Buoyant force: The upward force exerted by a liquid when an object is fully or partially immersed in
it.
Law of flotation: Body floats in a liquid if the weight of the body is equal to the weight of the liquid
displaced by the immersed part of the body.
4) Describe the construction & working of a simple manometer.
(Diagram from T.B)
Construction:
a) U-shaped glass tube, open at both ends is fixed to wooden board with a scale on it.
b) Scale should be able to measure the readings marked on both sides of the glass tube.
c) A rubber pipe with a thistle funnel attached is connected to arm A. Mouth of the funnel is covered
with a stretched rubber membrane.
d) U - tube is half – filled with coloured water.
Working:
a) Initially, the level of water in both arms A & B is the same, but when some pressure is applied on
the stretched membrane of the funnel, the level falls in arm A. The difference in the levels of water
in both arms is a measure of the pressure exerted on the membrane.
5) What is a Torricellian barometer? How can it be constructed?
(Diagram from T.B)
The first barometer was invented by an Italian scientist Evangelisto Torricelli & is known as the
Torricelli barometer.
Construction:
a) It consists of a 100 - cm long tube, with a narrow uniform bore.
b) The tube is filled with pure & dry mercury. The open end is closed with a finger & the inverted
tube is carefully inserted into a trough containing mercury, immersing it completely.
c) The finger is removed, causing some mercury to flow out from the tube into the trough creating an
empty space in the tube called Torricellian vaccum.
6) Describe the function of a simple mercury barometer. State its disadvantages.
Function: It measures atmospheric pressure.
Disadvantages:
a) It is not compact & thus not portable.
b) The level of mercury in the open trough increases or decreases with the change in atmospheric
pressure. Hence, a permanent scale cannot be marked on the glass tube.
c) The trough is open & there are changes of impurities getting mixed with the mercury.
7) How are the disadvantages of a mercury barometer overcome in an aneroid barometer?
a) It is made of metal & therefore is light, compact & portable.
b) It consists of an airtight & partially evacuated metal chamber covered with a thin elastic metal
disc. Any change in the atmospheric pressure causes the disc to bulge inwards or outwards.
c) This displacement is amplified by a special arrangement of levers, which make a needle attached
to them to move over a graduated scale.
d) When pressure increases, needle rotates clockwise & when pressure decreases, needle rotates
anticlockwise.
8) Give reason of the following:
a) A dam is made broader at its base.
To withstand the greater lateral pressure of water at that depth.
b) Deep sea divers & astronauts have to wear special suits.
To protect themselves from being crushed by the high liquid pressure in the ocean depth, deep sea
diverse wear special suits. To counter the effects of the zero atmospheric pressure, astronauts have
to wear specially designed pressurized suits, otherwise their bodies may burst.
c) A rubber suction cap stays stuck to the wall.
Because when the cap is pressed against the wall, any air present inside the cap is pushed out
leaving behind a vaccum. The atmospheric pressure then acts on the vaccum, pushing the cap
firmly against the wall, thereby fixing it.
Numerical:
1) The volume of a 23.3 g piece of silver is 2.5 cm3. Find the density of silver in (a) CGS, (b)
MKS system.
Density = m/V
(a) CGS
D = 23.3 g/2.5 cm3 = 9.32 g/cm3.
(b) MKS:
1 g/cm3 = 1000 kg/m3
D = 9320 kg/m3
2) The density of a metal is 11.27 g/cm3. What is the volume of a 56.35 g piece of this metal?
D= 11.27 g/cm3, m = 56.35 g
D = m/V
∴ V = m/D = 56.35/11.27 = 5 cm3
3) The cuboidal block has volume of 10 cm3 & mass of 54.6 g. Calculate (a) density in CGS
system, (b) Density in MKS system, (c) Relative density.
(a) D = m/V = 54.6/10 = 5.46 g/cm3
(b) D = 5.46 g/cm3 = 5.46 * 1000 kg/m3 = 5460 kg/m3
(c) Relative density = Density of substance / Density of water at 4 ℃
= 5.46 g/cm3 ÷ 1 g/cm3
= 5.46