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CBSE Questions for Class 8 Physics Some Natural Phenomena Quiz 7 - MCQExams.com
CBSE
Class 8 Physics
Some Natural Phenomena
Quiz 7
The amplitude of vibrations measured on the Richter's scale increase by steps of about
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10
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20
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30
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40
Explanation
Answer is A.
The amplitude of vibrations measured on the Richter's scale increase by steps of about 10.
The Richter magnitude scale was developed in 1935 by Charles F. Richter of the California Institute of Technology as a mathematical device to compare the size of earthquakes.
The magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded by seismographs
. Adjustments are included in the magnitude formula to compensate for the variation in the distance between the various seismographs and the epicenter of the earthquakes. On the Richter Scale, magnitude is expressed in whole numbers and decimal fractions. For example, a magnitude of 5.3 might be computed for a moderate earthquake, and a strong earthquake might be rated as magnitude 6.3. Because of the logarithmic basis of the scale,
each whole-number increase in magnitude represents a tenfold increase in measured amplitude;
as an estimate of energy, each whole number step in the magnitude scale corresponds to the release of about 31 times more energy than the amount associated with the preceding whole number value.
Earthquakes are measured on
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Ruler
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Barometer
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Richter scale
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All of these
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None of these
Explanation
The magnitude of intensity of earthquakes are measured on Richter scale.
State whether the given statement is True or False :
Depending on which material it is rubbed with, objects can acquire either positive or negative charge.
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True
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False
Explanation
A body can be charged by rubbing it with another body. The type of charge the body will acquire depends on the materials with which they are rubbed.
As an example: When a glass rod is rubbed with a silk cloth, the glass rod acquires positive charges and the silk cloth acquires negative charges.
The air in the path of lightning goes up to a temperature of about
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$$300^oC$$
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$$3000^oC$$
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$$300000^oC$$
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$$30000^oC$$
Explanation
Answer is D.
The air in the path of lightning goes up to a temperature of about 30,000 degrees C.
Thunder is caused by lightning. The bright light of the lightning flash caused by the return stroke represents a great deal of energy. This energy heats the air in the channel to above 30,000 degrees C in only a few millionths of a second. The air that is now heated to such a high temperature had no time to expand, so it is now at very high pressure. The high-pressure air then expands outward into the surrounding air compressing it and causing a disturbance that propagates in all directions away from the stroke. The disturbance is a shock wave for the first 10 yards, after which it becomes an ordinary sound wave or thunder. Thunder can seem like it goes on and on because each point along the channel produces a shock wave and sound wave.
Earthquakes can occur because of ............ .
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Movement of tectonic plates
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Volcanic eruption
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Mine blasts
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All of these
Waves produced due to the earthquake are known as
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seismic waves
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shock waves
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infrasonic waves
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Tectonic waves
Explanation
Events like, tectonic plate movement, underground explosion or volcanic explosion can produce tremors on the surface of earth. The tremors produce waves on the surface of the earth which are capable of covering large distances. These are called seismic waves.
The lightning rod should be taller than the building
with a pointed top end.
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True
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False
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Ambigious
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Data insufficient
The seismic zones are those areas which are
less prone to earthquakes.
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True
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False
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Ambigious
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Data insufficient
Explanation
Earthquakes are caused by the movement of plates, the boundaries of the plates are the weak zones where earthquakes are more likely to occur. The weak zones are also known as seismic or fault zones.
Which of the following four persons will stand the
minimum risk of getting struck by lightning?
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A person holding umbrella.
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A person squatting low on the ground.
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A person riding a bicycle.
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A person standing in an open field.
The first seismograph was invented in
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Germany by Albert Einstein.
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China by Zhang Heng.
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Germany by Emmy Noether.
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None of the above.
Explanation
A Chinese mathematician, named Zhang Heng, invented and constructed the first seismograph in A.D. 132.
In 1752, scientist Benjamin Franklin showed that lightning and sparks from our clothes are ______ phenomena.
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Same
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Different
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Related
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None of these
Explanation
In 1752, Benjamin Franklin discovered and showed that sparks produced from our clothes and lighting are essentially the same phenomena. Both lightning and sparks are caused by the accumulation of charges in the clouds and clothes respectively.
Fill in the blank:
Air is a/an ________ conductor of electricity.
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Good
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Poor
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Extremely good
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Neutral
Explanation
Normally air is a poor conductor of electricity as molecules are very loosely bound to each other and there is ample space between molecules.
Opening up of strips of an electroscope is an indication of:
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Proximity of a conductor.
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Proximity of a charged body.
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Proximity of an uncharged body.
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Earth connection.
Explanation
When a charged body comes close to an electroscope, charges are induced to the electroscope. These charges flow down to the metal strips which open up due to repulsion for having similar charges.
If lightning strikes you, why would you feel electric shock?
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Because charges are in motion
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Because of heating effect of current
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Because of magnetic effect
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Both a & b
Explanation
Electric shock occurs when a person comes in direct contact with any source of electric current. As lightning is basically charges in motion, the moving charges in the lightning pass through our body acting as a conductor, consequently giving us an electric shock.
When, on large scale, negative and positive charges meet in clouds, we experience:
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streak of bright light and sound
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thunderstorm
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lighting
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all of the above
Explanation
Positive and negative charges accumulate on the surfaces of the clouds.
There is accumulation of positive charges near the ground also. When the magnitude of the accumulated charges becomes very large, the air which is normally a poor conductor of electricity, is no longer able to resist their flow. Negative and positive charges meet, producing streaks of bright light and explosive sound. We see that streaks as lightning and hear the sound of thunderstorms.
The objects which acquires small electric charges, when rubbed with each other, are called:
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electric objects
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charged objects
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neutral objects
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none of the above
Explanation
In the process of rubbing objects with one another, both acquire small electric charges. These objects are called charged objects.
Why is a car or a bus safe place during lightening?
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Because it does not rain inside the bus.
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Because lightening cannot travel through the glass.
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Because a car or a bus is insulated from the Earth.
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None of the above.
Explanation
A car or a bus is safe during lightning because the lightning will travel around the surface of the vehicle and then go to ground.
The metallic frame of the car, being a good conductor, would direct the current around the objects and discharge it safely to the ground while shielding the objects inside the car.
What happens when you touch the water pipeline in the building during a thunderstorm?
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Your body will conduct electricity
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Charges will pass through your body
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Your body provides earthing
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All of the above
Explanation
When we touch the water pipeline in the building during a thunderstorm, if a lightning strikes the building there may be a chance of getting electric shocks because water and our body, both are
good conductors of electricity. As a result, charges pass through our body to the earth and get discharged over there. In other words, our body acts as an earthing to the building.
Other than lightning conductor, what are the objects which indirectly protect building from lightning / thunderstorm?
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Metal column used during construction.
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Electrical wires in the building.
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Water pipe in the building.
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All of the above.
Explanation
The metal columns used during construction, electrical wires and water pipes, all are good conductor of electricity and well earthed (connected to the ground) hence they provide earthing to the building by aiding in carrying the electric discharge to ground.
Why are lightning conductors pointed in shape?
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To reduce mass.
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It attracts more electric charge due to very small radius and hence small surface area.
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It allows more current.
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It repels the incident lightning discharge.
Explanation
Pointed lightning conductors attract more charges because of the small cross-sectional area. When a charged cloud passes over the lightning rod, an opposite charge gets induced on it. As the surface area of the rod is very small, the surface density of charge is hence increased considerably. Consequently, the rod attracts the lightning without letting it fall on any other part of the building.
Lightning strikes building when induced charge on top building is too high. This statement is:
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True
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False
Explanation
Lightning is nothing but electric current through air from clouds to earth.Lightning takes shortest path between clouds to earth.And there is less gap between top of tall buildings and clouds so lightnings prefers to strikes buildings (due to short path between cloud and top of building) and flow to ground.
The lightning rods present at the top of the high buildings are very good conductors and conduct the lightning through heavy wiring to the ground without damaging the building.
A lightning conductor consists of
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several sharp metal spiked connected to a thick copper strip.
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a metal plate buried deep inside the ground connected to the ground end of copper strip.
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a conductor fixed on top of the building.
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all of the above
Explanation
A lightning conductor consists of a conductor fixed on top of the building. The conductor consists of several sharp metal spikes connected to a thick copper strip. The other end of the copper strip is fixed to a metal plate buried deep inside the ground.
This provides a low resistance, conducting path for the electric discharge from the lightning to the earth.
The spark seen inside a switch is:
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electrical discharge
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due to heating effect of current
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an indicator glowing
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a sudden fire
Explanation
When a switch is put off, between two conducting plates inside, an electrical discharge occurs. This phenomenon is similar to lightning between the clouds.
It's not uncommon to see a small spark coming from inside a switch when you turn it off. This is normal in most cases, and it doesn't indicate a problem. It's caused by a load arc that occurs when the electricity jumps between the contacts as they pull away from each other.
The sparks caused due to friction between two machine parts are:
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similar to lighting
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not similar to lightning
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lightning on a very small scale
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none of the above
Explanation
Lightning is an electrical discharge between clouds or cloud and ground due to accumulation of heavy amount of electrostatic charge. The sparks caused in machines due to friction are out of local heating and not connected to electrical discharge.
Tremors on the earth can also be caused when?
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Valcano erupts
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Metero hits the earth
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Underground nuclear explosion
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All of the above
Explanation
Tremors on the earth are majorly caused by earthquakes. Tremors on the earth can also be caused when a volcano erupts, or a meteor hits the earth or an underground nuclear explosion.
Clouds get electrically charged when :
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The UV rays of sun falls on them
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Air molecules rub against water molecules
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Cold currents of air rub against hot currents of air
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Both B and C
Explanation
The heat from the sun and warm air cause water from water bodies to vaporize. These vapour accumulate in the sky to form clouds. When these vapour molecules rub against the air molecules or other vapour molecules, they acquire charge due to friction.
Lightning occurs when an electrical charge is built up within a cloud, due to static electricity generated by supercooled water droplets colliding with ice crystals near the freezing level. When a large enough charge is built up, a large discharge will occur and can be seen as lightning. The temperature of a lightning bolt can be five times hotter than the surface of the sun.
Both B and C are correct.
Option D
Lightning is a more common phenomenon during rainy season because of:
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high atmospheric density
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high humidity
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moderate temperature
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solar electrostatic disturbances
Explanation
During lightning, electrical discharge is between clouds and more clouds are formed in rainy season due to more humidity.
Why shouldn't one stand under a tree or a tall building during a thunderstorm?
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Lightning can strike trees and buildings,
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Trees and buildings may fall causing damage,
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Trees and buildings do not offer enough escape ground,
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None of the above,
Explanation
Sometimes electric discharge can take place between the charged cloud and Earth. While passing over a tree or a tall building an opposite charge may be induced causing a lightning to strike the tree or a building and if the charge is large it may cause severe damage and may prove to be fatal. Thus, it is advised to stay away from tall buildings or tree during thunderstorms.
The lightning conductor used in building, protects the building by:
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Dissipating the electric charge away from the building
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Conducting the lightning safely to the ground
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Absorbing the electric charge
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None of these
Explanation
Lightning Conductor is a device used to protect buildings from the effect of lightning. A metallic rod, taller than the building, is installed in the walls of the building during its construction. One end of the rod is kept out in the air and the other is buried deep in the ground. The rod provides an easy route for the transfer of electric charge to the ground.
The lightning conductor protects the building by conducting the lightning safely to the ground.
When a glass rod is rubbed with a piece of silk cloth the rod:
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and the cloth both acquire positive charge.
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becomes positively charged while the cloth has a negative charge.
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and the cloth both acquire negative charge.
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becomes negatively charged while the cloth has a positive charge.
Explanation
The two objects charged by friction gets oppositely charged.
Charge acquired by the glass rod is positive and charge acquired by the cloth is negative.
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