Nuclear Radiations - Question Answers - Physics XII

 Q.1: Explain how you would test whether the radiation from a radioactive source is α, β or Gamma radiation?

Ans: When radiations are allowed to pass through a magnetic field, the α and β particles are deflected while γ-rays pass through undeflected. This technique helps to identify the radiation.

Q.2: A particle which produces more ionization is less penetrating. Why?
Ans: When a particle ionizes an atom, it loses a part of its energy. Since the greater the ionizing power, the greater is the loss of energy; and hence, the smaller is its penetrating power.

THE ATOMIC NUCLEUS

Chapter – 19

Q.1: How many neutrons and protons do the following nuclei contain?

Nuclide	Protons	Neutrons
$^{27}_{13}Al$	13	$27 - 13 = 14$
$^{40}_{18}Ar$	18	$40 - 18 = 22$
$^{138}_{56}Ba$	56	$138 - 56 = 82$
$^{207}_{82}Pb$	82	$207 - 82 = 125$
$^{28}_{14}Si$	14	$28 - 14 = 14$
$^{238}_{92}U$	92	$238 - 92 = 146$

Q.2: Do $\alpha$, $\beta$, and gamma rays come from the same element? Why do we find all three in many radioactive samples?
Ans: A radioactive element either emits $\alpha$-particles or $\beta$-particles, but never both. Gamma radiations generally accompany $\beta$-emission and, in some cases, with $\alpha$-emission.

THE ATOMIC SPECTRA

Q.1: The Bohr’s theory of hydrogen atom is based upon several assumptions. Do any of these assumptions contradict classical physics?
Ans: The assumption in Bohr’s theory that an electron moving around the nucleus in a certain orbit does not radiate energy is contrary to classical electrodynamics.

Q.2: Why does the hydrogen gas produced in the laboratory not glow and emit radiations?
Ans: A spectrum is given by the light emitted from an incandescent gas or vapour e.g., electric discharge through a gas or hydrogen-filled discharge tube.

Advent of Modern Physics - Question Answers - Physics XII

  ADVENT OF MODERN PHYSICS

Chapter - 17

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Q.1: What do you understand by a frame of reference? What is the difference between inertial frame and non-inertial frame?
Ans: The position and motion of a body can be located with reference to some coordinate system, called the frame of reference.

The frame of reference that is either at rest or moves with uniform velocity is called an inertial frame. It has zero linear or rotational acceleration. Newton’s laws hold well in such a frame. All inertial frames of references are equivalent.

Electromagnetic Waves and Electronics - Question Answers - Physics XII

Chapter - 16

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Q.1: Under what circumstances does a charge radiate electromagnetic waves?
Ans: A charge radiates electromagnetic (e.m.) waves when it is accelerated.

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Q.2: In an electromagnetic wave, what is the relationship, if any, between the variation in the magnetic and electric fields?
Ans: In an electromagnetic wave, the transverse sinusoidal oscillating electric field and magnetic field are propagated at right angles to each other and to the direction of motion.

ELECTRICAL MEASURING INSTRUMENTS

Chapter - 15

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Q.1: What is the function of the concave pole pieces and the coaxial soft iron cylinder in the moving coil galvanometer?
Ans: The concave magnetic poles and the cylindrical core make the magnetic field radial and stronger (so the current becomes directly proportional to the deflection).

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Q.2: Why is it necessary to have some form of controlling couple in the moving coil galvanometer?
Ans: Controlling couple is necessary to control the motion of the coil, it is proportional to the current to be measured. It is produced by using a spring control method, which consists of two hair springs attached to a spindle wound in the opposite directions. As the coil rotates the spring winds up and produces a counter torque. The coil comes to rest (the final deflection of the pointer is given) when the deflecting torque (or magnetic torque) is counterbalanced by the controlling torque (or restoring torque).

MAGNETISM & ELECTROMAGNETISM

Chapter – 14

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Q.1: What is flux density and how is it related to the number of lines of induction expressed in Webers?
Ans: Magnetic flux density $B$ is the magnetic flux per unit area $(B = \Phi / A)$. The unit of flux density is Weber per $m^2$ (or tesla, $T$). Magnetic flux is the total number of magnetic lines of induction passing perpendicularly through an area $(\Phi = B \times A)$. Its SI unit is ‘Weber’ $(1 Wb = 1 T \cdot m^2)$. Hence magnetic flux density refers to the number of lines of induction (in Webers) per square meter.

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Q.2: Charged particles fired in a vacuum tube hit a fluorescent screen. Will it be possible to know whether they are positive or negative?
Ans: Yes, the charge on particles in motion can be found by applying a magnetic field perpendicular to the motion of the charges and by observing the deflection. A positive charge in an inward perpendicular magnetic field is deflected upward. In an electric field, a positive charge will be deflected towards the negative side (plate).

CURRENT ELECTRICITY

Chapter – 13

Q.1: Electrons leave a dry cell and flow through a lamp back to the cell. Which terminal the positive or negative is the one from which electrons leave the cell? In which direction is the conventional current?
Ans: Electrons leave the negative terminal of the cell and move towards the positive terminal. However, as a convention, the conventional current is assumed to be the consisted of positive electric charges moving from a positive terminal to the negative terminal change flowing through the area per unit time (I = q/t).

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Q.2: Both p.d. and e.m.f. are measured in volts. What is the difference between these concepts?
Ans: P.d. is the work done per unit charge across resistor in a closed circuit. But e.m.f. the total p.d. across the external and internal resistance, it refers to a source of current and is greater than the potential drop in an external circuit. (e.m.f. p.d. + internal resistance drop)

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.

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.