2 Marks Questions

1.When a planet X is at a distance of 824.7 million kilometers from earth its angular diameter is measured to be 35.72¹¹ of arc. Calculate the diameter of ‘X’.

Ans:

Diameter

2.A radar signal is beamed towards a planet from the earth and its echo is received seven minutes later. Calculate the velocity of the signal, if the distance between the planet and the earth is 6.3x10¹⁰m?

Ans:

3.Find the dimensions of latent heat and specific heat?

Ans:(1) Latent Heat  

Latent Heat =

(2) Specific heat =

4.in Vander Waal’s equation

Ans:

5.E, m, l and G denote energy, mass, angular momentum and gravitational constant respectively. Determine the dimensions of 

Ans:

Thus, it is dimension less.

6.Calculate the time taken by the light to pass through a nucleus of diameter 1.56 10^-16 m. (speed of light is 310⁸ m/s)

Ans:

7.Two resistances R₁ = 100 and R₂ = 200 are connected in series. Then what is the equivalent resistance?

Ans:

8.If velocity, time and force were chosen the basic quantities, find the dimensions of mass?  

Ans:

9.A calorie is a unit of heat or energy and it equals about 4.2 J where . Suppose we employ a system of units in which the unit of mass equals α kg, the unit of length equals  m, the unit of time is y s. Show that a calorie has a magnitude in terms of the new units.

Ans. Given that,

1 calorie = 4.2 (1 kg)

New unit of mass = α kg

Hence, in terms of the new unit, 1 kg =

In terms of the new unit of length,

And, in terms of the new unit of time,

10.Explain this statement clearly:

“To call a dimensional quantity "large" or "small" is meaningless without specifying a standard for comparison”. In view of this, reframe the following statements wherever necessary:

(a) atoms are very small objects

(b) a jet plane moves with great speed

(c) the mass of Jupiter is very large

(d) the air inside this room contains a large number of molecules

(e) a proton is much more massive than an electron

(f) the speed of sound is much smaller than the speed of light.

Ans. The given statement is true because a dimensionless quantity may be large or small in comparision to some standard reference. For example, the coefficient of friction is dimensionless. The coefficient of sliding friction is greater than the coefficient of rolling friction, but less than static friction.

(a) An atom is a very small object in comparison to a soccer ball.

(b) A jet plane moves with a speed greater than that of a bicycle.

(c) Mass of Jupiter is very large as compared to the mass of a cricket ball.

(d)The air inside this room contains a large number of molecules as compared to that present in a geometry box.

(e) A proton is more massive than an electron.

(f) Speed of sound is less than the speed of light.

11.Which of the following is the most precise device for measuring length:

(a) a vernier callipers with 20 divisions on the sliding scale

(b) a screw gauge of pitch 1 mm and 100 divisions on the circular scale

(c) an optical instrument that can measure length to within a wavelength of light ?

Ans. A device with minimum count is the most suitable to measure length.

(a) Least count of vernier callipers

= 1 standard division (SD)  1 vernier division (VD)

(b) Least count of screw gauge =

(c) Least count of an optical device = Wavelength of light  cm

= 0.00001 cm

Hence, it can be inferred that an optical instrument is the most suitable device to measure length.

12. Answer the following:

(a) You are given a thread and a metre scale. How will you estimate the diameter of the thread?

(b) A screw gauge has a pitch of 1.0 mm and 200 divisions on the circular scale. Do you think it is possible to increase the accuracy of the screw gauge arbitrarily by increasing the number of divisions on the circular scale?

(c) The mean diameter of a thin brass rod is to be measured by vernier callipers. Why is a set of 100 measurements of the diameter expected to yield a more reliable estimate than a set of 5 measurements only?

Ans. (a) Wrap the thread on a uniform smooth rod in such a way that the coils thus formed are very close to each other. Measure the length of the thread using a metre scale. The diameter of the thread is given by the relation,

Diameter=

(b) It is not possible to increase the accuracy of a screw gauge by increasing the number of divisions of the circular scale. Increasing the number divisions of the circular scale will increase its accuracy to a certain extent only.

(c) A set of 100 measurements is more reliable than a set of 5 measurements because random errors involved in the former are very less as compared to the latter.

13. The mass of a box measured by a grocer"s balance is 2.300 kg. Two gold pieces of masses 20.15 g and 20.17 g are added to the box. What is (a) the total mass of the box, (b) the difference in the masses of the pieces to correct significant figures?

Ans. Mass of grocer"s box = 2.300 kg

Mass of gold piece I = 20.15g = 0.02015 kg

Mass of gold piece II = 20.17 g = 0.02017 kg

(a) Total mass of the box = 2.3 + 0.02015 + 0.02017 = 2.34032 kg

In addition, the final result should retain as many decimal places as there are in the number with the least decimal places. Hence, the total mass of the box is 2.3 kg.

(b) Difference in masses = 20.17 - 20.15 = 0.02 g

In subtraction, the final result should retain as many decimal places as there are in the number with the least decimal places.

14. A physical quantity P is related to four observables a, b, c and d as follows:

The percentage errors of measurement in a, b, c and d are 1%, 3%, 4% and 2%, respectively. What is the percentage error in the quantity P? If the value of P calculated using the above relation turns out to be 3.763, to what value should you round off the result?

Ans.

Percentage error in P = 13 %

Value of P is given as 3.763.

By rounding off the given value to the first decimal place, we get P = 3.8.

15. The unit of length convenient on the atomic scale is known as an angstrom and is denoted by. The size of a hydrogen atom is aboutwhat is the total atomic volume in of a mole of hydrogen atoms?

Ans.  Radius of hydrogen atom, r = 0.5 =

Volume of hydrogen atom =

1 mole of hydrogen contains hydrogen atoms.

∴ Volume of 1 mole of hydrogen atoms =

=

16. Explain this common observation clearly: If you look out of the window of a fast moving train, the nearby trees, houses etc. seem to move rapidly in a direction opposite to the train"s motion, but the distant objects (hill tops, the Moon, the stars etc.) seem to be stationary. (In fact, since you are aware that you are moving, these distant objects seem to move with you).

Ans. Line of sight is defined as an imaginary line joining an object and an observer"s eye. When we observe nearby stationary objects such as trees, houses, etc. while sitting in a moving train, they appear to move rapidly in the opposite direction because the line of sight changes very rapidly.

On the other hand, distant objects such as trees, stars, etc. appear stationary because of the large distance. As a result, the line of sight does not change its direction rapidly.

17. Precise measurements of physical quantities are a need of science. For example, to ascertain the speed of an aircraft, one must have an accurate method to find its positions at closely separated instants of time. This was the actual motivation behind the discovery of radar in World War II. Think of different examples in modern science where precise measurements of length, time, mass etc. are needed. Also, wherever you can, give a quantitative idea of the precision needed.

Ans. It is indeed very true that precise measurements of physical quantities are essential for the development of science. For example, ultra-shot laser pulses (time interval  s) are used to measure time intervals in several physical and chemical processes.

X-ray spectroscopy is used to determine the inter-atomic separation or inter-planer spacing.

The development of mass spectrometer makes it possible to measure the mass of atoms precisely.

18. When the planet Jupiter is at a distance of 824.7 million kilometers from the Earth, its angular diameter is measured to be of arc. Calculate the diameter of Jupiter.

Ans. Distance of Jupiter from the Earth, D =
Angular diameter =
Diameter of Jupiter = d
Using the relation,

19. A LASER is a source of very intense, monochromatic, and unidirectional beam of light. These properties of a laser light can be exploited to measure long distances. The distance of the Moon from the Earth has been already determined very precisely using a laser as a source of light. A laser light beamed at the Moon takes 2.56 s to return after reflection at the Moon"s surface. How much is the radius of the lunar orbit around the Earth?
Ans. Time taken by the laser beam to return to Earth after reflection from the Moon = 2.56 s
Speed of light =  m/s
Time taken by the laser beam to reach Moon =
Radius of the lunar orbit = Distance between the Earth and the Moon =  =
m = 3.84  km

20. A SONAR (sound navigation and ranging) uses ultrasonic waves to detect and locate objects under water. In a submarine equipped with a SONAR the time delay between generation of a probe wave and the reception of its echo after reflection from an enemy submarine is found to be 77.0 s. What is the distance of the enemy submarine? (Speed of sound in water = 1450).
Ans. Let the distance between the ship and the enemy submarine be.
Speed of sound in water = 1450 m/s
Time lag between transmission and reception of Sonar waves = 77 s
In this time lag, sound waves travel a distance which is twice the distance between the ship
and the submarine (2S).
Time taken for the sound to reach the submarine
∴ Distance between the ship and the submarine (S) = 1450 × 38.5 = 55825 m = 55.8 km

3 Marks Questions

1.  Just as precise measurements are necessary in science, it is equally important to be able to make rough estimates of quantities using rudimentary ideas and common observations. Think of ways by which you can estimate the following (where an estimate is difficult to obtain, try to get an upper bound on the quantity):

(a) the total mass of rain-bearing clouds over India during the Monsoon

(b) the mass of an elephant

(c) the wind speed during a storm

(d) the number of strands of hair on your head

(e) the number of air molecules in your classroom.

Ans. (a) During monsoons, a metrologist records about 215 cm of rainfall in India i.e., the height of water column, h = 215 cm = 2.15 m

Area of country, A =

Hence, volume of rain water, V = A × h =

Density of water, p =

Hence, mass of rain water = p× V =  kg

Hence, the total mass of rain-bearing clouds over India is approximately  kg.

(b) Consider a ship of known base area floating in the sea. Measure its depth in sea (say).

Volume of water displaced by the ship,

Now, move an elephant on the ship and measure the depth of the ship () in this case.

Volume of water displaced by the ship with the elephant on board,

Volume of water displaced by the elephant =

Density of water = D

Mass of elephant = AD

(c) Wind speed during a storm can be measured by an anemometer. As wind blows, it rotates. The rotation made by the anemometer in one second gives the value of wind speed.

(d) Area of the head surface carrying hair = A

With the help of a screw gauge, the diameter and hence, the radius of a hair can be determined. Let it be r.

∴Area of one hair =

Number of strands of hair

(e) Let the volume of the room be V.

One mole of air at NTP occupies 22.4 l i.e., volume.

Number of molecules in one mole =

∴Number of molecules in room of volume V

==

=

2.  The unit of length convenient on the nuclear scale is a fermi: 1 f = m. Nuclear sizes obey roughly the following empirical relation:

where r is the radius of the nucleus, A its mass number, and is a constant equal to about, 1.2 f. Show that the rule implies that nuclear mass density is nearly constant for different nuclei. Estimate the mass density of sodium nucleus. Compare it with the average mass density of a sodium atom obtained in Exercise. 2.27.

Ans. Radius of nucleus r is given by the relation,

… (i)

= 1.2 f =  m

Volume of nucleus, V=

Now, the mass of a nuclei M is equal to its mass number i.e.,

M = A amu = kg

Density of nucleus,

p =

This relation shows that nuclear mass depends only on constant. Hence, the nuclear mass densities of all nuclei are nearly the same.

Density of sodium nucleus is given by,

3.  A great physicist of this century (P.A.M. Dirac) loved playing with numerical values of Fundamental constants of nature. This led him to an interesting observation. Dirac found that from the basic constants of atomic physics (c, e, mass of electron, mass of proton) and the gravitational constant G, he could arrive at a number with the dimension of time. Further, it was a very large number, its magnitude being close to the present estimate on the age of the universe (~15 billion years). From the table of fundamental constants in this book, try to see if you too can construct this number (or any other interesting number you can think of). If its coincidence with the age of the universe were significant, what would this imply for the constancy of fundamental constants?

Ans. One relation consists of some fundamental constants that give the age of the Universe by:

Where,

t = Age of Universe

e = Charge of electrons =

= Absolute permittivity

= Mass of protons = kg

= Mass of electrons = kg

c = Speed of light =  m/s

G = Universal gravitational constant =

Also,

 Substituting these values in the equation, we get

4 Marks Questions

1.A book with many printing errors contains four different formulas for the displacement y of a particle undergoing a certain periodic motion:

(a)

(b) y = a sin vt

(c)

(d)

(a = maximum displacement of the particle, v = speed of the particle. T = time-period of motion). Rule out the wrong formulas on dimensional grounds.

Ans. (a) Ans.: Correct

Dimension of y =

Dimension of a =

Dimension of =

Dimension of L.H.S = Dimension of R.H.S

Hence, the given formula is dimensionally correct.

(b) Ans.: Incorrect

y = a sin vt

Dimension of y =

Dimension of a =

Dimension of vt =

But the argument of the trigonometric function must be dimensionless, which is not so in the given case. Hence, the given formula is dimensionally incorrect.

(c) Ans.: Incorrect

Dimension of y =

Dimension of =

Dimension of=

But the argument of the trigonometric function must be dimensionless, which is not so in the given case. Hence, the formula is dimensionally incorrect.

(d) Ans.: Correct

Dimension of y =

Dimension of a =

Dimension of =

Since the argument of the trigonometric function must be dimensionless (which is true in the given case), the dimensions of y and a are the same. Hence, the given formula is dimensionally correct.

2.One mole of an ideal gas at standard temperature and pressure occupies 22.4 L (molar volume). What is the ratio of molar volume to the atomic volume of a mole of hydrogen? (Take the size of hydrogen molecule to be about 1). Why is this ratio so large?

Ans. Radius of hydrogen atom, r = 0.5 = m

Volume of hydrogen atom =

Now, 1 mole of hydrogen contains  hydrogen atoms.

∴ Volume of 1 mole of hydrogen atoms,

Molar volume of 1 mole of hydrogen atoms at STP,

Hence, the molar volume is  times higher than the atomic volume. For this reason, the inter-atomic separation in hydrogen gas is much larger than the size of a hydrogen atom.

3.The nearest star to our solar system is 4.29 light years away. How much is this distance in terms of parsecs? How much parallax would this star (named Alpha Centauri) show when viewed from two locations of the Earth six months apart in its orbit around the Sun?

Ans. Distance of the star from the solar system = 4.29 ly

1 light year is the distance travelled by light in one year.

1 light year = Speed of light × 1 year

=  m

∴4.29 ly =  m

1 parsec =  m

∴4.29 ly = = 1.32 parsec

Using the relation,

Where,

Diameter of Earth’s orbit, d=

Distance of the star from the Earth,D=

But, 1 sec = rad

∴

4.Estimate the average mass density of a sodium atom assuming its size to be about 2.5. (Use the known values of Avogadro"s number and the atomic mass of sodium). Compare it with the density of sodium in its crystalline phase: 970 kg. Are the two densities of the same order of magnitude? If so, why?

Ans.  Diameter of sodium atom = Size of sodium atom = 2.5

Radius of sodium atom, r =

=  m

Volume of sodium atom, V =

According to the Avogadro hypothesis, one mole of sodium contains  atoms and has a mass of 23 g or  kg.

∴ Mass of one atom =

Density of sodium atom, p =

It is given that the density of sodium in crystalline phase is 970 kg.

Hence, the density of sodium atom and the density of sodium in its crystalline phase are not in the same order. This is because in solid phase, atoms are closely packed. Thus, the inter-atomic separation is very small in the crystalline phase.

5. It is a well known fact that during a total solar eclipse the disk of the moon almost completely covers the disk of the Sun. From this fact and from the information you can gather from examples 2.3 and 2.4, determine the approximate diameter of the moon.

Ans.

The position of the Sun, Moon, and Earth during a lunar eclipse is shown in the given figure.

Distance of the Moon from the Earth =  m

Distance of the Sun from the Earth = m

Diameter of the Sun =  m

It can be observed that ΔTRS and ΔTPQ are similar. Hence, it can be written as:

Hence, the diameter of the Moon is  m.

5 Marks Questions

1. Just as precise measurements are necessary in science, it is equally important to be able to make rough estimates of quantities using rudimentary ideas and common observations. Think of ways by which you can estimate the following (where an estimate is difficult to obtain, try to get an upper bound on the quantity):

(a) the total mass of rain-bearing clouds over India during the Monsoon

(b) the mass of an elephant

(c) the wind speed during a storm

(d) the number of strands of hair on your head

(e) the number of air molecules in your classroom.

Ans. (a) During monsoons, a metrologist records about 215 cm of rainfall in India i.e., the height of water column, h = 215 cm = 2.15 m

Area of country, A =

Hence, volume of rain water, V = A × h =

Density of water, p =

Hence, mass of rain water = p× V =  kg

Hence, the total mass of rain-bearing clouds over India is approximately  kg.

(b) Consider a ship of known base area floating in the sea. Measure its depth in sea (say).

Volume of water displaced by the ship,

Now, move an elephant on the ship and measure the depth of the ship () in this case.

Volume of water displaced by the ship with the elephant on board,

Volume of water displaced by the elephant =

Density of water = D

Mass of elephant = AD

(c) Wind speed during a storm can be measured by an anemometer. As wind blows, it rotates. The rotation made by the anemometer in one second gives the value of wind speed.

(d) Area of the head surface carrying hair = A

With the help of a screw gauge, the diameter and hence, the radius of a hair can be determined. Let it be r.

∴Area of one hair =

Number of strands of hair

(e) Let the volume of the room be V.

One mole of air at NTP occupies 22.4 l i.e., volume.

Number of molecules in one mole =

∴Number of molecules in room of volume V

==

=

2.  The unit of length convenient on the nuclear scale is a fermi: 1 f = m. Nuclear sizes obey roughly the following empirical relation:

where r is the radius of the nucleus, A its mass number, and is a constant equal to about, 1.2 f. Show that the rule implies that nuclear mass density is nearly constant for different nuclei. Estimate the mass density of sodium nucleus. Compare it with the average mass density of a sodium atom obtained in Exercise. 2.27.

Ans. Radius of nucleus r is given by the relation,

… (i)

= 1.2 f =  m

Volume of nucleus, V=

Now, the mass of a nuclei M is equal to its mass number i.e.,

M = A amu = kg

Density of nucleus,

p =

This relation shows that nuclear mass depends only on constant. Hence, the nuclear mass densities of all nuclei are nearly the same.

Density of sodium nucleus is given by,

3.  A great physicist of this century (P.A.M. Dirac) loved playing with numerical values of Fundamental constants of nature. This led him to an interesting observation. Dirac found that from the basic constants of atomic physics (c, e, mass of electron, mass of proton) and the gravitational constant G, he could arrive at a number with the dimension of time. Further, it was a very large number, its magnitude being close to the present estimate on the age of the universe (~15 billion years). From the table of fundamental constants in this book, try to see if you too can construct this number (or any other interesting number you can think of). If its coincidence with the age of the universe were significant, what would this imply for the constancy of fundamental constants?

Ans. One relation consists of some fundamental constants that give the age of the Universe by:

Where,

t = Age of Universe

e = Charge of electrons =

= Absolute permittivity

= Mass of protons = kg

= Mass of electrons = kg

c = Speed of light =  m/s

G = Universal gravitational constant =

Also,

 Substituting these values in the equation, we get