Electrostatics - Question Answers - Physics XII

Chapter – 12

Q.1: Repulsion is the sure test of electrification. Explain?
Ans: Electrostatic attraction is observed between oppositely charged bodies and also between a charged (+ve or -ve) and an uncharged body. But, however, only two charges of the same kind (both +ve or both -ve) can repel each other. Hence, repulsion is the sure test of electrification.

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Q.2: Will a solid metal sphere hold a large electric charge than a hollow sphere of the same diameter? Where the charge does resides in each case?
Ans: A solid metal sphere will hold the same amount of charge as is held by a hollow sphere of the same diameter. This is due to the fact that any excess electric charge resides only on the outer surface of a conductor.

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Q.3: Explain why it is so much easier to remove an electron from an atom of large atomic mass than it is to remove a proton?
Ans: In an atom of large atomic mass, the number of both protons and electrons is large. This big atom contains many orbits (or shells). So it is easier to remove an electron from its outermost orbit. The heavy positive nucleus exerts weaker coulomb attraction force on it as compared to an electron in the innermost shell (e.g., K shell). However, protons are in the nucleus which are held very strongly by strong nuclear forces.

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Q.4: Why is it not correct to say that potential difference is the work done in moving a unit positive charge between the points concerned?
Ans: The potential difference is the increase in electric potential energy (or work) per unit charge. So, if a small positive charge (q₀) is moved against the electric field between the two points, then the work divided by the amount of charge (w/q₀) gives the potential difference.

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Q.5: Why is it logical to say that the potential of an earth-connected object is zero? What can be said about the charge on the earth?
Ans: Practically, the earth is taken to be at zero potential. If a charged body is connected to the earth by a conductor, electron flow takes place such that the charge of the body is neutralized.

The earth is a reasonably good conductor. It is a huge neutral body. It is considered as an infinite sink to which electrons can easily migrate without changing its potential.

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Q.6: Can an electric potential exist at n point in a region where the field is zero? Can the potential be zero at a place where the electric field intensity is not zero? Give example to illustrate your reasoning.
Ans: Yes, electric potential can exist where the electric intensity is zero. The electric charge resides on the outer surface of a hollow sphere. At all points inside the sphere, the electric field intensity is zero. Otherwise, the field lines would link the charges of opposite sign in the sphere. Thus no work is done when a charge is moved between two points inside the sphere. Hence, potential is the same at all points throughout the sphere and equals that at the surface, i.e., potential is constant inside and on the surface.

• Electric potential can be zero at a point where electric intensity is not zero. For example, consider a point in the middle of two equal and opposite charges. There the electric potential is
  $V = K \, q/r + k \, -q/r = 0$

But the net electric intensity is toward the negative charge.

• Both the potential and intensity are zero for a point at infinity.

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Q.7: An air capacitor is charged to a certain potential difference. It is then immersed in oil. What happens to its (a) charge (b) potential and (c) capacitance?
Ans: The dielectric constant $\varepsilon_r$ of oil is greater than that of air. When an air capacitor is immersed in oil (after disconnecting the battery), then:

1. Its charge remains constant (since there is no path for charge transfer).
2. Potential difference between the plates decreases (and also the electric field is weakened) by a factor of $1/\varepsilon_r$.
3. The capacitance increases (since $C = q/V$) by a factor $\varepsilon_r$.

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Q.8: Two unlike capacitors of different potential and charges are joining in parallel. What happens to their potential difference? How are their charges distributed? Is the energy of the system affected?
Ans: When two unlike capacitors of different potentials and charges are joined in parallel, then:

1. The resultant potential difference will be less than the highest applied potential difference on one capacitor. This resultant potential difference will be the same for the two capacitors in parallel.
2. The charge is redistributed, and the capacitor of higher capacitance will have more charge (since $q = CV$).
3. The energy of the system will decrease. The missing energy is used in heating the wires.

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Q.9: Four similar capacitors are connected in series and joined to a 36 V battery. The mid-point of the group is earthed. What is the potential of the terminal of the group?
Ans: If two similar capacitors are connected in series, joined to a 36 V battery, and if the midpoint of the group is earthed, then there is no transfer of charge. This midpoint is between two oppositely charged plates (of $C_2$ and $C_3$). Hence the potential difference across the end of the group will remain the same (i.e., 36 V).

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Q.10: A point charge is placed at the center of a spherical Gaussian surface. Is the flux changed?

1. If the spherical Gaussian surface is replaced by a cube of the same volume.
2. If the sphere is replaced by a cube of 1/10 of this volume.
3. If the charge is moved from the center in the sphere.
4. If the charge is moved outside the sphere.

Ans:

1. No
2. No
3. No
4. Yes
5. Yes

Q.11: Four capacitors each 2μF connected in such a way that the total capacitance is also 2μF. Show what combination gives this value?
Ans: To get an equivalent capacitance of 2μF, the four capacitors, each of 2μF, can be combined in (i) two pairs of parallel combination or (ii) two pairs of series capacitors combined in parallel.

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Q.12: A capacitor is charged by a battery. The battery is disconnected and a slab of some dielectric is slipped between the plates. Describe what happens to the charge, potential difference, capacitance, and the stored energy?
Ans: When a capacitor is charged, the battery is disconnected, and a slab of some dielectric (of relative permittivity $\varepsilon_r$) is inserted, then:

1. The charge remains constant (since there is no path for transfer of the charge).
2. The potential difference decreases (and also the electric field is weakened) by a factor $\varepsilon_r$.
3. The capacitance increases (since $C = q/v$) by a factor $\varepsilon_r$.
4. The energy stored will decrease by a factor $1/\varepsilon_r$ (since, energy = $½ \, qV$), which is used in polarizing the dielectric.

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Q.13: Answer Question 12 if the battery is not disconnected?
Ans: When a capacitor is charged and the battery is not disconnected, and a slab of some dielectric (of relative permittivity $\varepsilon_r$) is increased, then:

1. The charge increases (additional charge is delivered by the battery).
2. The potential difference (and also the electric field) remains constant.
3. The capacitance increases (since $C = q/v$).
4. The energy stored will increase (since, energy $½ \, qV = ½ \, CV^2$).

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Q.14: A capacitor is connected across a battery. Why does each plate receive a charge of the same magnitude? Will it be true if the plates are of different sizes?
Ans: When a capacitor is connected to a battery, such that +ve, terminal ‘b’ is at a higher potential than the plate B, then electrons are drawn toward, ‘b’ from B. However, the -ve terminal ‘a’ is at potential than the plate A, so the electrons are drawn the plate A from ‘a’. Thus B is positively charged. The charging stops when $V_{AB} = V$. If the sizes of the plates are different, then the plate of larger area will receive more amount of charge.

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Q.15: Write electric field statements analogous to the following in gravitational field. 1. Water flows from a higher level to a lower level; 2. Water always maintains its level; 3. The total mass is conserved; 4. When a body falls through a height ‘h’ it loses potential energy and gains kinetic energy?
Ans: The analogous statements are:

1. Electric charge flows from a higher potential to a lower potential.
2. Charge always maintains its potential.
3. The total charge is conserved.
4. When a charge body falls through a potential difference, it loses its electrical potential energy and gains kinetic energy.

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Q.16: Is it true that “an alternating current can pass through a capacitor while a direct current”. Explain?
Ans: A capacitor is said to ‘block’ direct current or voltage. That is, there is no current through a capacitor by a steady direct voltage. However, when the capacitor is connected across an a.c supply, the capacitor (or plates) are continuously charged and discharged (during alternate quarter cycle), and charged the other way round by the alternating voltage. The current thus flows round the circuit. A capacitance that allows alternating current offers opposition (in ohms), called capacitive reactance (X-C).

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Q.17: The unit of permittivity $C^2 \, N^{-1} \, m^{-2}$ is the same as $F \, m^{-1}$ how?
Ans: $F = \frac{M}{L \, T^2} \times C^2 \times \frac{1}{m} = \frac{C^2}{J/C \times m} = \frac{C^2}{N.m} = \frac{C^2}{N.m^2}$.

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Q.18: What happens if a charge is moved in an electric field?
Ans: A charge is displaced in two ways:

1. Against the electric field (say from point A to B): then work is done on the charge. This increase electric potential energy thus i (P.E.) = qΔV.
2. In the direction of the electric field (from B to A): then p.c. decreases which appears as increase in K.E. thus (K.E.) = ½ mv^2.

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Q.19: What will be the flux through a closed surface which does not contain any charge?
Ans: As the surface encloses no charge, so the flux is zero. From Gauss's Law: $\phi = q/\varepsilon_0 = 0/\varepsilon_0 = 0$

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Q.20: What is the flux, electric field intensity and potential inside a charged sphere?
Ans: Both the flux and the intensity are zero inside a charged sphere. The potential inside a charged sphere is the same as at its surface.

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Q.21: An uncharged conducting spherical shell is placed in the field of a positive charge q. what will be the net flux through the shell? What is the unit of electric flux?
Ans: According to Gauss’ law, the net flux through the shell will be zero as then it contains no charge. The SI unit of electric flux is $\text{N m}^2 \text{ C}^{-1}$.

Q.22: Is electrical p.d. same as electrical p.e?
Ans: No, electrical p.e. is the total work done in moving a certain charge $q$ from one point to another against electric intensity.
Thus, $U_B - U_A = W_A \rightarrow B = \Delta W$

Electric p.d. is the work done in moving a unit positive charge from one point to another against electric intensity.
Since $V_B - V_A = \Delta W/q_0 = \Delta U/q_0$

Now, a relation between them is: $U = q_0 \, \Delta V$

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Q.23: Why is water not used as a dielectric?
Ans: The near impossibility of removing all impurities dissolved in water makes it unsuitable in practice as a dielectric.

HEAT

Questions Answer (Q/A)

Chapter - 11

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Q.1: How do you distinguish between temperature and heat? Give example?
Ans: Heat is the energy flowing between bodies and surroundings due to the difference of temperature. But temperature is a measure of the average translation kinetic energy of the molecules of a body. If we dip a red hot iron ball in the sea, heat will flow. The amount of internal energy possessed by the ball is very small as compared to the immense amount contained in the sea.

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Q.2: Why is the earth not in thermal equilibrium with the sun?
Ans: The earth is not in thermal equilibrium with the sun; because while the earth is being warmed by the absorbed radiant energy, it is also losing heat in various ways (e.g. re-radiation, conduction, convection and evaporation). Moreover, they are not in perfect thermal contact with each other. The average temperature of the earth is about 300 K.

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Q.3: Is temperature a macroscopic concept?
Ans: Yes, temperature is a macroscopic concept.

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Q.4: It is observed that when mercury in a glass thermometer is put in a flame, the column of mercury first descends and then rises. Explain.
Ans: When mercury in a glass thermometer is put in a flame, the glass bulb expands. So the column of mercury descends. But no sooner the heat reaches the mercury in the bulb, it expands and this expansion is greater than that of the glass bulb. So, now, the mercury rises in the column.

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Q.5: Is it correct that the unit for specific heat capacity (c) is m².s⁻².°C⁻¹?
Ans: Yes, it is correct. Because c = (ΔQ / mΔT)
The SI unit of c = Jkg⁻¹ °C⁻¹ = N.m.kg⁻¹ °C⁻¹ = kg m/sec² m.kg⁻¹ °C⁻¹ = m² s⁻² °C⁻¹

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Q.6: What is the standard temperature?
Ans: The standard temperature is the ice-point at S.T.P. i.e. 0°C or 273 K.

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Q.7: When a block with a hole in it is heated, why does not the material around the hole expand into the hole and make it small?
Ans: Thermal expansion of a homogeneous substance causes increase in all directions with the same linear thermal expansion coefficient. This increase in all directions causes an effective magnification of an object. So a hole in the block, on heating, expands outward (i.e. becomes big).

Q.8: A thermometer is placed in direct sunlight. Will it read the temperature of the air, or of the sun, or of something else?
Ans: This thermometer will record the temperature of the surroundings (thermometric substance).

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Q.9: Will one kilogram of hydrogen contain more than one kilogram of lead? Explain.
Ans: Yes, one kilogram of hydrogen will contain more atoms than one kilogram of lead because hydrogen atoms are much lighter than lead atoms (the atomic mass of hydrogen is 207 times less than the atomic mass of lead).

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Q.10: The pressure in a gas cylinder containing hydrogen will leak more quickly than if this containing oxygen. Why?
Ans: This is so because the hydrogen molecules are lighter than oxygen molecules (since the molecular mass of hydrogen is 16 times less than the molecular mass of oxygen). Molecular speed (and hence rate of diffusion) is inversely proportional to the molecular mass. Hence hydrogen will leak more quickly than oxygen.

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Q.11: What are some factors that affect the efficiency of an automobile engine?
Ans: The efficiency of an automobile engine depends upon:

• Temperature of hot reservoir
• Temperature of cold reservoir
• Friction and heat losses (dissipative effect)

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Q.12: What happens to the temperature of a room in which an air conditioner is left running on a table in the middle of the room?
Ans: When an air conditioner is left running on a table in the middle of a room, heat is removed from the room by the air conditioner. But, heat is radiated on the other side to the room by the coils (condenser) at the back of the refrigerator. The heat pumped out the back of the air conditioner and into the room is greater than the heat pulled into the front of the unit, as work done to remove the heat from cold hot puts into the room an additional amount of heat $Q_4 = Q_2 + W$. Consequently, the air conditioner warms the room.

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Q.13: When a sealed thermos bottle full of hot coffee is shaken? What are the changes, if any in?
Ans: The temperature of the coffee increase due to shaking.

• The internal energy of the coffee increases. Infect, the work done in shaking the coffee appears as increase in internal energy. Hence the temperature of the coffee increases (due to friction of walls of the flask).

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Q.14: When an object is heated, not all the energy it absorbs goes into increasing the velocity of the molecules? Explain where does the remaining energy go?
Ans: When an object is heated, not all the energy it absorbs goes into increasing the velocity of the molecules. Some goes into the rotational motion of the molecules and some into the internal vibration motion.

Q.15: If a pendulum clock has to keep correct time at different temperatures, will it be better to use aluminum or steel?
Ans: It is better to use steel in a pendulum clock that has to keep correct time at different temperatures because the coefficient of thermal expansion of steel is almost half of that of aluminum.

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Q.16: Why is the average velocity of the molecules of a gas zero but the average of the squares of the velocities is not zero?
Ans: Due to the random motion of the molecules in a gas, the number of molecules, on the average, moving with certain velocities along the positive x, y, and z axis is equal to the number of molecules moving along negative x, y, and z axis with the same velocities. Hence the average velocity of the molecules of a gas is zero. But however, the square of a negative component of velocity is also positive. Hence the square of the molecular velocities is not zero.

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Q.17: (a) What is kilo-mole of 72g of water? (b) What is the value of universal gas constant in J.k mole⁻¹ k⁻¹?
Ans:
a) Kilo-mole is the mass of a substance in kilogram and is numerically equal to the molecular mass of a substance in kilogram.

Number of kilo-mole, $n = \frac{\text{Mass of substance in kg}}{\text{Molecular mass in kg per mole}} = \frac{m}{M}$
For water, $n = \frac{72 \times 10^{-3} \, \text{kg}}{18 \, \text{kg/mole}} = 4 \times 10^{-3}$

b) $R = 8.314 \times 10^3 \, \text{J. k mol}^{-1} \text{k}^{-1}$

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Q.18: Why does the pressure of a gas in an automobile lyre increase if the automobile is driven for a while?
Ans: When an automobile is driven on road, it does work to overcome the friction between the tires and the road. So heat is produced. This, in turn, raises the temperature of the gas. Since pressure is directly proportional to temperature, hence pressure of the gas in the tire increases.

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Q.19: Under what condition can heat be added to a system without changing its temperature?
Ans: Heat can be added without changing the temperature of the system.

• For a gaseous system, it can be achieved in an isothermal process.
• For a liquid, it can be achieved at the boiling point of the liquid.
• For a solid, it can be achieved at melting point of the solid.

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Q.21: Is it possible to cool a room by keeping the refrigerator door open?
Ans: A room cannot be cooled by leaving the door of an electric refrigerator open. Whatever heat $Q_2$ is removed from the air directly in front of the open refrigerator is deposited directly back into the room at the rear of the unit. Also, work done to remove the heat.

Q.22: When does the entropy of a system decrease?
Ans: When it rejects to the surroundings.

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Q.23: Is it possible, according to the second law of thermodynamics, to construct an engine that is free from thermal pollution?
Ans: It is not possible to construct an engine free from thermal pollution since heat rejected to a sink is an essential requirement. This sink is the atmosphere to which the heat rejection results in thermal pollution. The small temperature changes have disruptive effects on the overall ecological balance. Thermal pollution is an invertible consequence of the second law of thermodynamics.

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Q.24: When two systems are in thermal equilibrium, do they have the same amount of kinetic energy?
Ans: Temperature is a measure of the average translational kinetic energy of the molecules of a system. However, the systems with the same average translational kinetic energy have the same temperature, even if one has greater internal energy (due to greater rotational and vibration energy).

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Q.25: What do you mean by “heat is energy in transit”?
Ans: Heat is not the energy that a body contains; it refers to the amount of energy transferred from a hot body to a cold body.

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Q.26: What is the nature of the graph between the length and temperature of a heated metal rod?
Ans: The graph between length and temperature is a straight line (linear relationship).

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Q.27: How much work should be done to produce 1 calorie of heat?
Ans: 4.2 J, since $W = JQ$. When work is in Joule and quantity of heat is measured in calories, then Joules constant $J = W/Q = 4.2 \, \text{J/cal}$.

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Q.28: Why are the values of molar heat capacities of substances (with a few exceptions) the same, i.e., almost mole⁻¹ k⁻¹?
Ans: In the case of molar heat capacity, we add heat energy to the same number of molecules, irrespective of the nature of the substance. Thus $C = cM = 25 \, \text{J mole}^{-1} \text{K}^{-1}$.

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Q.29: Why is the specific heat of polyatomic gases higher than that of monatomic gases? Calculate specific heat ratio ($\gamma$) of a monatomic gas.
Ans: In a monatomic gas, the whole of the supplied energy is used up in increasing the translational kinetic energy, i.e., its temperature. But in diatomic or polyatomic gases, the heat energy supplied is wasted in increasing the rotational kinetic energy and vibration kinetic energy. Thus to obtain the same range of temperature, more heat is required for polyatomic gases.

• Increase of KE of 1 molecule of monatomic gas = $3/2 \, k \Delta T$
• Increase of KE of 1 mole of monatomic gas = $N_A \, (3/2 \, k \Delta T)$
• Increase of KE of 1 mole of monatomic gas per Kelvin = $3/2 \, k \, N_A$
• Increase of K.E. of 1 mole of monatomic gas per Kelvin = $2/3 \, R$

$C_V = \frac{3}{2} R = 3/2 \times 8.3 = 12 \, \text{J mole}^{-1} \text{K}^{-1}$
$C_p = C_V + R = 3/2 \, R + R = 5/2 \, R = 20.75 \, \text{J mole}^{-1} \text{K}^{-1}$

$\gamma = \frac{C_p}{C_V} = \frac{20.75}{12} = 1.667$

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Q.30: Work can be converted completely into heat (W a Q), so can heat be converted completely into work?
Ans: A given amount of heat cannot be completely converted into work, as some of the heat energy is used up in increasing the internal energy of the system. If heat is converted completely to work, then efficiency of heat engine will be 100%. This violates the second law of thermodynamics.

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Q.31: Entropy has often been called as “time arrow”. Explain.
Ans: Entropy is called “time arrow,” because it tells us in which direction the time is going. The normal sequence of events is that in which disorder increase with time.

Physical Quantities and Measurement

 PHYSICS 9TH - Short / Detailed Question Answers

Q.1: What is Science?

Ans: Science:
The word science refers to the study of a fact by collecting information through observation, presenting it in a mathematical way, justifying the idea with experiment, and finally making a conclusion about the fact.

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Q.2: Define Physics.

Ans: Physics:
One of the most basic and ancient sciences is Physics. It can be defined as: Physics is the branch of science which observes the nature, represents it mathematically, and concludes with the experiment.

In other words, Physics is the branch of science that deals with studies of matter, its composition, properties, and interaction with energy.

It deals with the behavior and structure of matter and the energy that derives from matter. Physics is the branch of natural science that studies matter, its motion, its behavior through space and time, and the related entities of energy and force. Physics is one of the most fundamental scientific disciplines, and its main goal is to understand how the universe behaves.

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Q.3: Name and define the branches of Physics.

Ans:
Since the beginning of the universe, the structure of the universe is very straightforward. The classification of physics was not that easy, but as the physicists explained the universe, they classified Physics into many branches. These branches show the spectrum and scope of Physics around us and help scientists describe ideas in a well-organized way. The branches of Physics are classified on the basis of different areas of study with different approaches. The main branches of Physics are as follows.

Mechanics:
This branch of physics is mainly concerned with the laws of motion and gravitation.

Thermodynamics:
Thermodynamics deals with heat and temperature and their relation to energy and work.

Electricity:
Electricity is the study of the properties of charges in rest and motion.

Magnetism:
Magnetism is the study of the magnetic properties of materials.

Atomic Physics:
Atomic physics deals with the composition, structure, and properties of the atom.

Optics:
Optics studies physical aspects of light and its properties with the help of optical instruments.

Sound:
Sound is the study of production, properties, and application of sound waves.

Nuclear Physics:
Nuclear physics deals with the constituents, structure, behavior, and interaction of atomic nuclei.

Particle Physics:
Particle physics studies the elementary constituents of matter and radiation, and the interaction between them.

Astrophysics:
The study of celestial objects with the help of the laws of physics is known as Astrophysics.

Plasma Physics:
The study of the state of matter and its properties is known as Plasma Physics.

Geophysics:
The study of the internal structure of the Earth is known as Geophysics.

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Q.4: What is the importance of Physics?

Ans: Importance of Physics in Science, Technology, and Society:
Society's reliance on technology represents the importance of physics in daily life. Many aspects of modern society would not have been possible without the important scientific discoveries made in the past. These discoveries became the foundation on which current technologies were developed.

Discoveries such as magnetism, electricity, conductors, and others made modern conveniences, such as television, computers, smartphones, medical instruments, other business and home technologies possible. Moreover, modern means of transportation, such as aircraft and telecommunications, have drawn people across the world closer together, all relying on concepts of physics.

It is a matter of fact that Physics can be considered as the mother of all sciences. The beauty of physics lies in its laws that govern this whole universe, from an atom to large-scale galaxies, and in its experiments, from home to large-scale experiment labs.

Q.5: In how many categories physicists are categorized?

Ans:
Physicists are categorized into two categories: those who observe nature, solve its mysteries with available and missing information, and present their theories with a mathematical approach. They are known as theoretical physicists. Others who are more interested in testing those theories with experiments are known as experimental physicists.

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Q.6: What are physical quantities?

Ans:
Physics is much concerned with matter and energy and the interaction between them, which is explained with the help of describing the mathematical relations between various physical quantities. All physical quantities are important for describing the nature around us.

Definition:
Physics defines the mathematical relation between physical quantities. A physical quantity is a physical property of a phenomenon, body, or substance that can be quantified by measurement.

A physical quantity is a physical property of a phenomenon, body, or substance that can be quantified by measurement.

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Q.7: Define fundamental and derived physical quantities.

Ans:
Physical quantities are classified into two categories:

1. Fundamental quantities
2. Derived physical quantities

Fundamental Quantities:
Physical quantities that cannot be explained by other physical quantities are called fundamental physical quantities.

There are seven fundamental physical quantities listed in the following table along with their units:

Fundamental Quantities	S.I. Unit	Symbol of Unit
Length	meter	m
Mass	Kilogram	Kg
Time	Second	s
Electric current	Ampere	A
Temperature	Kelvin	K
Amount of substance	mole	mole
Luminous intensity	candela	cd

Derived Physical Quantities:
Physical quantities that are explained on the basis of fundamental physical quantities are called derived physical quantities.

Derived Quantities	S.I Unit	Symbol of Unit
Volume	Cubic Meter	m³
Velocity	meter per second	m/s
Force	Newton	N
Density	Kilogram per cubic meter	Kg/m³
Acceleration	meter per second square	m/s²

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Q.8: What do you know about scientific or physical instruments?

Ans:
All physical quantities are either calculated mathematically or measured through an instrument. Scientists, engineers, doctors, and others like blacksmiths, carpenters, and goldsmiths, even workers and ordinary human beings, measure those physical quantities with the help of instruments. For instance, our doctor uses a thermometer to tell our body temperature, a carpenter uses the inch-tape to measure the length of wood required for furniture.

A puncture mender uses air gauges to check the air pressure in the tire. Similarly, a chemical engineer uses a hydrometer for describing the density of a liquid.

Measuring physical quantities correctly with an instrument is not an easy task for scientists and engineers. Scientists are seriously concerned with the accuracy of the instrument and its synchronization. Moreover, the instruments they design, mostly for their own sake of research, often make their way into the commercial market. Many of the instruments we use today are inventions by pioneers of science. Usually, the basic physical quantities we use in our daily life are measured with basic and simple instruments.

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Q.9: Define least count.

Ans: Least Count:
The use of every instrument is restricted by the smallest measurement that it can perform, which is called least count.

Q.10: Define length and their units.

Ans:
If there is any measurement that has proven to be useful to humanity, it is length. For example, units of length include the inch, foot, yard, mile, meter, etc.

Definition of Length:
Length is defined as the minimum distance between two points lying on the same plane.

S.I. Unit of Length:
The meter (m) is the S.I. unit of length and is defined as: The length of the path traveled by light in a vacuum during the time interval of 1/299,792,458 of a second.

The basic measurement of length can be obtained with the help of a meter rod or an inch tape.

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Q.11: What is a meter rule? How can we use a meter rule to measure length?

Ans: Meter Rule:
A meter rule is a device used to measure the length of different objects. A meter rule of length 1 meter is equal to 100 centimeters (cm). On a meter rule, each cm is divided further into 10 divisions, which are called millimeters (mm). So, a meter rule can measure up to 1 mm as the smallest reading. It is made up of a long rigid piece of wood or steel.

The zero-end of the meter rule is first aligned with one end of the object, and the reading is taken where the other end of the object meets the meter rule.

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Q.12: What is a Vernier Caliper? How can we read it?

Ans: Vernier Caliper:
The Vernier Caliper is a precision instrument that can be used to measure internal and external distances extremely accurately. It has both an imperial and metric scale. A Vernier caliper has main jaws that are used for measuring external diameter, as well as smaller jaws that are used for measuring the internal diameter of objects. Some models also have a depth gauge. The main scale is fixed in place, while the Vernier scale is the name for the sliding scale that opens and closes the jaws.

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Q.13: What is a Micrometer Screw Gauge? How can we read it?

Ans: Micrometer Screw Gauge:
A screw gauge is extensively used in the engineering field for obtaining precision measurements. A micrometer screw gauge is used for measuring extremely small dimensions. A screw gauge can even measure dimensions smaller than those measured by a Vernier caliper.

Q.14: What do you understand by the term "the standard mass"?

Ans: The Standard of Mass:
The kilogram is the S.I. unit of mass and is equal to the mass of the international prototype of the kilogram, a platinum-iridium standard that is kept at the International Bureau of Weights and Measures.

A kilogram is a cylinder of special metal about 39 millimeters wide by 39 millimeters tall that serves as the world's mass standard. Each country that subscribed to the International Metric Convention was assigned one or more copies of the international standards; these are known as National Prototype Meter and Kilogram.

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Q.15: What is a physical balance? How does it work?

Ans: Physical Balance:
A physical balance is an instrument used for the measurement of mass. It is mostly used in a laboratory. It works on the principle of moments. It consists of a light and rigid beam of brass, a metallic pillar, a wooden base, two pans, a metallic pointer, and an ivory scale.

The plumb line indicates whether the balance is horizontal. In an ideal condition, the plumb line is aligned with the end of the knob fixed with the pillar. When the beam is horizontal, the pointer remains on the zero mark on the ivory scale. The whole box has leveling screws at the bottom to set it to horizontal. The device is enclosed in a glass box to avoid wind effects.

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Q.16: What is an electronic balance?

Ans: The Electronic Balance:
The digital mass meter is an electronic instrument configured with integrated circuits and works on the principle of balancing the forces.

The device is turned on and set to zero, then the object is placed on the plate. The reading on the screen gives the mass of the object. The electronic balance is available in different ranges of measurement, such as microgram, milligram, and kilogram, etc.

Q.17: Describe the standard of time and define the second.

Ans: The Standard of Time:
Before 1960, the standard of time was defined in terms of the mean solar day for the year 1900. The rotation of the Earth is now known to vary slightly with time, and this motion is not a good one to use for defining a time standard.

In 1960, the second was redefined to take advantage of the high precision attainable in a device known as an atomic clock, which uses the frequency of the caesium-133 atom as the "reference clock."

Second:
The second is now defined as 9,192,631,770 times the period of vibration of radiation from the caesium atom.

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Q.18: What is a stopwatch? What is meant by "human reaction time"?

Ans: Stop Watch:
A stopwatch is used to measure the time interval for two events. There are two types of stopwatches:

1. Mechanical stopwatch
2. Digital stopwatch

Mechanical / Analogue Stopwatch:
A mechanical stopwatch can measure a time interval of up to 0.1 second. It has a knob that is used to wind the spring that powers the watch. It can also be used as a start-stop and reset button. The watch starts when the knob is pressed once. When pressed the second time, the watch stops, while the third press brings the needle back to zero.

Digital Stopwatch:
A digital stopwatch can measure a time interval of up to 0.01 second. It starts to indicate the time elapsed as the start/stop button is pressed. As soon as the start/stop button is pressed again, it stops and indicates the time interval recorded by it between the start and stop of an event. A reset button restores its initial zero settings. Nowadays, almost all mobile phones have a stopwatch function.

Human Reaction Time:
As an analogue or digital watch is operated by human action, i.e., they have to be started or stopped by hand, this causes a random error in the measurement of time, called human reaction time. For most people, human reaction time is about 0.3 to 0.5 seconds. Therefore, for more accurate measurement of time intervals, light gates can be used.

Q.19: What instrument will you choose to measure the height of your friend?

Ans:
To measure the height of our friend, we can use a meter rule or inch tape. When our height is measured at the doctor's clinic, we usually stand next to a device called a stadiometer.

A stadiometer is a long ruler attached to the wall. It has a sliding horizontal headpiece that's adjusted to rest on top of our head. It's a quick way of accurately measuring our height.

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Q.20: Can you describe how many seconds there are in a year?

Ans:
One calendar common year has 365 days:

1 common year = 365 days (365 days) × (24 hours/day) × (3600 seconds/hour) = 31,536,000 seconds.

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Q.21: Which instrument will you choose to measure your mass?

Ans:
The scientific word for how much an object weighs on a scale is "mass." Here we can use the words "weight" and "mass" interchangeably because both are used in everyday language. For example, "I weigh 70 kg" or "the car’s mass is 1 tonne."

Bathroom scales are used to measure a person’s weight. They can be analogue or digital. Bathroom scales usually show units in kilograms and grams.

Q.22: Why prefixes are used for expressing units?

Ans: Prefixes:
The physical quantities are described by scientists in terms of magnitudes and units. Units play a vital role in expressing a quantity, either base or derived. Prefixes are useful for expressing units of physical quantities that are either very big or very small. A unit prefix is a specifier. It indicates multiples or fractions of the units.

Units of various sizes are commonly formed by such prefixes. The prefixes of the metric system, such as kilo and millimeters, represent multiplication by a power of ten. Historically, many prefixes have been used or proposed by various sources, but only a narrow set has been recognized by standards organizations.

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Table of Prefixes:

Prefix	Symbol	Meaning	Multiplier (Numerical)	Multiplier Exponential
tera	T	Trillion	1,000,000,000,000	10¹²
giga	G	Billion	1,000,000,000	10⁹
mega	M	Million	1,000,000	10⁶
kilo	K	Thousand	1,000	10³
hecto	H	Hundred	100	10²
deka	da	Ten	10	10¹
Unit	1		1
*deci	d	Tenth	0.1	10⁻¹
*centi	c	Hundredth	0.01	10⁻²
*milli	m	Thousandth	0.001	10⁻³
*micro	μ	Millionth	0.000001	10⁻⁶
*nano	n	Billionth	0.000000001	10⁻⁹
pico	p	Trillionth	0.000000000001	10⁻¹²
femto	f	Quadrillionth	0.000000000000001	10⁻¹⁵
atto	a	Quintillionth	0.000000000000000001	10⁻¹⁸

Q.23: Can you tell if the size of a nucleus is up to 1W1m, what prefix shall we use to describe its size?

Ans:
Femtometer (fm)
$10^{-15}$ meters

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Q.24: Describe scientific notation.

Ans: Scientific Notation:
Scientific notation or the standard form is a simple method of writing very large numbers or very small numbers. In this method, numbers are written as powers of ten. Thus, the calculation of very large or very small numbers becomes easy.

Numbers in Scientific Notation are made up of three parts:

1. The Coefficient: The coefficient must be equal to or (not zero) greater than one.
2. The Base: The base must be 10.
3. The Exponent: The exponent can be negative or positive.

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Q.25: Why are physicists concerned about the property of matter, which may help to define the nature of the matter in terms of its mass and space?

Ans:
The three common phases or states of matter are solid, liquid, and gas. A solid maintains a fixed shape and a fixed size, even if the same force is applied; it does not readily change its volume. A liquid does not maintain a fixed shape; it takes on the shape of its container. But, like a solid, it is not readily compressible, and its volume can be changed significantly only by a large force. However, gas has neither a fixed shape nor a fixed volume—it will expand to fill its container.

Often we find large-weight woods floating on the surface of water. However, an iron needle sinks into the water. We say iron is "heavier" than wood. This cannot really be true; rather, we should say iron is "denser" than wood. Physicists are concerned with a physical quantity, a property of matter that may help to define the nature of the matter in terms of its mass and space.

Q.26: How can we measure the volume of solids, liquids, and gases?

Ans: Measuring the Volume:
For density to be measured or calculated, we first need to find the volume of substances. Most solid shapes have formulas for their volume, which are obtained through different parameters such as radius, height, depth, width, base, and length. For irregular objects, liquids, and gases, this approach is unusual. The volume of liquids can be measured with the help of cylinders and beakers.

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Q.27: How can we use a measuring cylinder to measure the volume of liquids?

Ans: Measuring Cylinder:
A measuring cylinder is a glass or plastic cylinder with a scale graduated in cubic centimeters or milliliters (ml). It is used to find the volume of liquids. When a liquid is poured, it rises to a certain height in the cylinder. The level of liquid in the cylinder is noted, and the volume of the liquid is obtained. To read the volume correctly, we should keep the eye level with the bottom of the meniscus of the liquid surface.

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The Volume of Liquid:
A volume of about a liter or so can be measured using a measuring cylinder. When the liquid is poured into the cylinder, the level on the scale gives the volume. Most measuring cylinders have scales marked in milliliters (ml) or cubic centimeters (cm³). It should be noted that while recording the value from the cylinder, the eyes should maintain the level with the value. Angular observation may result in a false reading of the volume.

Q.28: How can we measure the volume of a regular and irregular solid?

Ans:

Regular Solid:
If an object has a regular shape, its volume can be calculated. For instance:

• Volume of a rectangular block = length × width × height
• Volume of a cylinder = π × radius² × height

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Irregular Solid:
For an irregular solid, its volume is calculated by lowering the object in a partially filled measuring cylinder. The rise in the level on the volume scale gives the volume of that object. Thus, the volume of an irregular solid is calculated by subtracting the original volume of liquid from the raised volume of liquid.

The total volume is found. The volume of the solid is measured in a separate experiment and then subtracted from the total volume.

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Quick Lab:

• Take a measuring cylinder of 1-liter capacity at the full place and set it in a beaker.
• Fill the cylinder full with water.
• Pour a stone of irregular shape into it gradually.
• As you pour the stone into the cylinder, the water from the cylinder drops into the beaker.
• Drop the stone in the cylinder completely.
• Calculate the volume of water ejected out of the cylinder.
• The volume of water ejected is the volume of the stone.

Q.29: Define density. How can we measure density?

Ans: Density:
The term density of a substance is defined as the mass of a substance (m) per unit volume (V). It is denoted by the Greek letter ρ (rho).

$$ρ = \frac{m}{V}$$

Density is a characteristic property of any pure substance. Objects made of a particular pure substance such as pure gold can have any size or mass, but its density will be the same for each. Following the above equation, the mass of a substance can be expressed as:

$$m = ρV$$

S.I. Unit of Density:
The S.I. unit for density is kg/m³ (kgm⁻³). Sometimes, the density of substances is given in g/cm³. The density of aluminum is 2.70 g/cm³, which is equal to 2700 kg/m³.

Measuring the Density:
It is to be noted that there are two ways of finding the density of a substance, either mathematically or experimentally by taking the density of water at 4°C as a reference, which is sometimes known as relative density or specific gravity. It has no unit. It is a number whose value is the same as that of the density in g/cm³.

$$\text{Relative density} = \frac{\text{density of substance}}{\text{density of water}}$$

More Information:
In Jordan, there is a sea known as the 'Dead Sea.' The human in that sea, while swimming, does not sink! This is because the water of the sea is much more salty than normal, which raises the density of the water.

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Q.30: Can you tell how a hot air balloon works?

Ans:
As air is heated, it becomes less dense than the surrounding cooler air. The less-dense hot air has enough lifting power to cause the balloon to float and rise into the air.

Q.31: Define and describe significant figures.

Ans: Significant Figures:
Engineers and scientists around the world work with numbers representing a large or small magnitude of a physical quantity. Engineers are, however, more interested in the accuracy of a value as they mostly work on estimation, but scientists, especially physicists, are more concerned with the accuracy of these numbers. For instance, an engineer records the speed of the wind and explains it as an average. On the other hand, for the physicist, the speed of the Earth on its course, the speed of light in a vacuum, the mass or charge on an electron is not just a matter of numbers but accurate numbers.

Definition:
The numbers of reliably known digits in a value are known as significant figures.

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Rules for determining significant figures:

Rule	Example
1. All non-zeros are significant.	2.25 (3 significant figures)
2. Leading zeros are not significant.	0.0000034 (2 significant figures)
3. Trailing zeros are significant ONLY if an explicit decimal point is present.	200 (1 significant figure), 2000 (3 significant figures), 2.00 (3 significant figures)
4. Trapped zeros are significant.	0.00509 (3 significant figures), 2045 (4 significant figures)

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Q.32: A pendulum swings as shown in the given figure from X to Y and back to X again.

(i) What would be the most accurate way of measuring time for one oscillation with the help of a stopwatch?
(a) Record time for 10 oscillations and multiply by 10
(b) Record time for 10 oscillations and divide by 10
(c) Record time for one oscillation
(d) Record time for X to Y and double it

(ii) Suggest an instrument for measuring time period more accurately.

Ans:
(i) A simple way to measure the pendulum’s period fairly precisely is to start the pendulum swinging and measure the time required for a large number of full swings — 40, 50, or so. Choose the number of swings so that the total time for the measurement is 40 seconds or more.

Dividing the total time by the number of full swings will give the period.
(a) to (d) For these questions, we need readings, so you have to perform practical work to answer them.

(ii) Today, the usual measuring instruments for time are clocks and watches. For a highly accurate measurement of time, an atomic clock is used. For more accurate measurement of time intervals, light gates can be used.

DIRECT AND INDIRECT SPEECH: COMMANDS AND REQUESTS

ENGLISH 10TH - Language Practice

UNIT # 8.3

Q.
A relative pronoun is used to connect a clause or phrase to a noun or pronoun. The clause modifies or describes the noun. The most common relative pronouns are who, whom, whose, which, and that. Sometimes, ‘when’ and ‘where’ can be used as relative pronouns as well.

SAFETY MEASURES THAT CAN SAVE YOUR LIFE

ENGLISH 10TH - Short Question Answers

UNIT # 8.1

Q.1: What are the main causes of road accidents?
Ans. One of the key reasons for road accidents is over-speeding; the other is the disregard for traffic rules.

Q.2: What two important safety measures can save people’s lives?
Ans. Two important safety measures that can save people’s lives and protect them from disability are wearing helmets and fastening seat belts.