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Advent of Modern Physics - Question Answers - Physics XII

ADVENT OF MODERN PHYSICS

Chapter - 17

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.

The frame of reference which possesses acceleration is known as a non-inertial frame. Laws of motion do not remain valid in such a frame.

Q.2: Explain why the Compton Effect is not observable with visible light?
Ans: In Compton’s experiment of x-rays of wavelength - 1 Å, equivalent to energy - 140 eV, were directed on a graphite block, where binding energies of bounded electrons were 102 eV. If visible light is used, it possesses low frequency, and these photons have energies - 0.1 eV. This energy is too small to be given to loosely bound electrons to get them scattered.

Q.3: What phenomena require wave description of light? What phenomena required particle picture of light? How are the two aspects related putatively?
Ans: The convincing evidences that light are a wave phenomena are:

Interference of light
Diffraction of light
Polarization of light
Production of electromagnetic waves
The optical Doppler Effect.

The idea of quantum nature of light, i.e., photon (which has a particle nature), was introduced due to the following evidences:

Black body radiation
The photoelectric emission
Compton scattering
X-ray production

The wave and particle aspects are related in de Broglie equation as: $λ = \frac{h}{m v}$ . Thus a particle of non-zero rest mass moves as if it were guided by an associated matter wave. Nevertheless, the ‘particle waves’ are waves of probability. Confirmation of de Broglie wavelength came in 1927 by C.J. Division and L.H. German and, independently, G.P. Thomson. It is astounding to note that Thomson (J.J.), the father, was awarded the Nobel prize in 1906 for having shown that the electron is a particle; and 31 years later Thomson (J.P.), the son, for having shown that the electron is a wave.

Q.4: In what way do the particles of light (photons) differ from the particles of matter, such as electrons and protons?
Ans:
Particles of matter (e.g., electron, proton, etc.) possess certain characteristics:

Non-zero rest mass.
They possess inertia and contain no energy 'packets.'
Their speed is always less than $c$ (speed of light).
Their energy is proportional to the square of the speed ( $E = \frac{1}{2} m v^{2}$ ).
They may be charged or uncharged.
When in motion, they are guided by matter waves.

Particles of light (photons) possess the following distinct characteristics:

Zero rest mass.
They consist of waves in packets of discrete amounts (called 'energy packets' or quanta).
They travel with speed equal to that of light.
Their energy is proportional to frequency ( $E = h ν$ ).
They are always electrically neutral.
They are not guided by matter waves.

Q.5: In the photoelectric effect, the energy of a photoelectron is less than that of an incident photon. Explain?
Ans: When radiation (a photon) strikes a metal surface, it deposits its entire energy on some electron in the absorbing surface. If the energy of the photon (by $h ν$ ) exceeds the energy required by the electron in work against the force binding it to the surface ( $ϕ_{0}$ ), it will be emitted with some energy. As $K . E . = h ν - ϕ_{0}$ , hence $k . e . < h ν$ .

Q.6: How did de Broglie hypothesis help to explain the stability of the atom?
Ans: According to de Broglie's hypothesis, an electron moving around the nucleus is pictured as a kind of wave packet (standing wave). An electron can circle a nucleus indefinitely without radiating energy provided that its orbit contains an integral number of de Broglie wavelengths.

Q.7: What is rest mass of a body?
Ans: Rest mass ( $m_{0}$ ) is the mass of a body when it is at rest with respect to an observer. The relativistic mass of a body moving with certain velocity $v$ is given by

$m = \frac{m_{0}}{\sqrt{1 - \frac{v^{2}}{c^{2}}}}$

Q.8: If we keep applying a force on a material object, can the object gain the speed of light?
Ans: If we keep applying a force which can produce a velocity equal to the velocity of light ( $v = c$ ), then the mass of the material body would become infinite. This is not possible.

Q.9: A block of polished metal having a black spot in the middle is heated above 3000 K and then placed in a dark room. Write your observations?
Ans: If a metal block is heated to incandescence at about 1000K, the metal has a dull red glow. A further high temperature changes into orange, then yellow and finally to white (3000 K) the black spot behaves as a black body. It absorbs maximum energy and appears as black. When seen in a dark room, the black spot radiates more energy (since a good emitter is a good emitter) than the rest of the block. The black spot appears brighter than the rest of the surface.

Q.10: Does the fact that an atom can emit a photon violate the law of conservation of energy? Explain?
Ans: No. An atom in an excited state can emit a photon. The energy received in jumping up is released in the emission.

Q.11: Can matter (e.g., electron) be created or destroyed?
Ans: Matter can be created from energy (photon) in pair production; and can be destroyed as photons in annihilation of matter process.

Q.12: Can pair production take place in vacuum? Explain
Ans: No, because this process requires a heavy nucleus to conserve momentum and energy of the system. The heavy nucleus takes the recoil after stopping the photon.

Q.13: Can an intense beam of television waves focused on a metal cause photoemission?
Ans: No, because TV waves are of low frequency, while a metal requires high threshold frequency for photoemission.

Q.14: Both photoelectric emission and Compton scattering are processes that involve interaction of radiation with matter. How do they differ?
Ans: In photoelectric effect, a low energy photon (e.g., ultraviolet light) can lose all its energy on striking an electron, and the photon vanishes. But in Compton Effect, a high energy photon (e.g., X-ray) loses part of energy and a photon is scattered with the remaining energy (and hence frequency decreases).

Q.15: The speed limit of our highways is 65 km/h. If the speed of light were the same, would you be able to drive at the speed limit?
Ans: No, because mass would become infinite.

Q.16: A ball is dropped from a helicopter flying at constant speed horizontally. Describe its motion relative to the pilot and an observer on earth’s surface.
Ans: According to the observer on the earth, it will fall forward towards the ground following a projectile path.

According to a pilot, it will to a point on the earth vertically.

Q.17: Why does the casing of a large electric transformer have metal blades fastened to it perpendicular to the surface and painted black?
Ans: The blades transfer heat by radiation to the atmosphere by increasing the surface area. They are painted black because black body radiates energy at a faster rate.

Electromagnetic Waves and Electronics - Question Answers - Physics XII

Chapter - 16

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

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.

Q.3: A radio transmitter has a vertical antenna. Does it matter whether the receiving antenna is vertical or horizontal?
Ans: A receiving antenna should be vertical just like a transmitting antenna. A horizontal receiving antenna will intercept much less radio frequency signals.

Q.4: Explain why are light waves able to travel through a vacuum, whereas sound waves cannot?
Ans: Light waves are electromagnetic waves (of wavelength 400 nm to 760 nm). Sound waves are produced due to the vibration of the molecules of a medium. Hence, sound waves require a material medium, whereas light waves do not require a medium for their propagation.

Q.5: Explain the condition under which radiation of electromagnetic waves takes place from a certain source?
Ans: When a transmitting antenna is coupled with an alternating source of potential (known as oscillator), charges (electrons) are accelerated up and down the antenna. This creates a fluctuating electric flux, which generates a magnetic flux. Hence the waves propagated from an antenna are electromagnetic waves.

Q.6: Can a diode be used for amplifying a weak signal?
Ans: Normally, a diode cannot be used for amplifying a weak signal. But specially constructed diode (e.g., tunnel diode) can be used as an amplifier and oscillator for microwave frequencies.

Q.7: Are radio waves form of light?
Ans: Since both radio waves and visible light are electromagnetic in nature, hence we can say that radio waves are a form of light (of frequency 4 x $10^{14}$ Hz), having frequencies much lower (30 kHz to 300 MHz) than light.

Q.8: Can e.m. waves be propagated through a piped vacuum?
Ans: Yes.

Q.9: Why does a semiconductor act as an insulator at 0K and why does its conductivity increase with an increase in temperature? OR - Discuss the effect of temperature on semiconductors?
Ans: In a semiconductor, at 0K, the valence band is completely filled and the conduction band is totally empty. The semiconductor, therefore, behaves like a perfect insulator. At room temperature, some of the electrons in the valence band gain energy from thermal agitation of the lattice atoms and move up into the conduction band, leaving holes in the valence band. If the temperature is increased, due to further thermal agitation, more electrons occupy the conduction band. Thus, the conductivity of the semiconductor increases with an increase in temperature.

Q.10: Explain the role of forbidden band in solids?
Ans:

In conductors, the conduction band and valence bands are overlapping and hence no forbidden band exists.
In insulators, the conduction and valence bands are separated by a large forbidden band.
In semiconductors, the conduction and valence bands are separated by a small forbidden energy gap.

Q.11: Why is light not seen in an ordinary diode but an LED emits light?
Ans: Silicon is opaque to light. So, an ordinary silicon diode does not emit light, but an LED is a junction diode made from gallium arsenide phosphate crystal. When it is forward biased, electron-hole recombination takes place, which results in the release of light.

ELECTRICAL MEASURING INSTRUMENTS

Chapter - 15

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).

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).

Q.3: What is meant by the sensitivity of a galvanometer? On what a factor does it depend? How can we have large sensitivity of a moving coil galvanometer?
Ans: A galvanometer is sensitive if it gives large deflection for a very small current. The sensitivity of a galvanometer is the current in microamperes required to cause a deflection of 1mm or 1 division.

S = \frac{I (\text{in } \mu A)}{\theta (\text{in div})}

Since $I = K \cdot \theta$ , hence the galvanometer is sensitive if $K (= C/BAN)$ is small. Sensitivity depends on $C$ (couple per unit twist), $N$ (number of turns), $A$ (area of coil) and $B$ (strength of magnetic field).

For large sensitivity a soft iron core (sphere or cylinder) is placed inside the coil and the poles are made circular or cylindrical. This makes $4B$ stronger and radial.

Q.4: Which galvanometer usually has greater sensitivity, aluminum pointer or lamp and scale type? Why?
Ans: Lamp and scale type galvanometer has greater sensitivity ( $10^{-15}$ A/div), because it gives large deflection for a very small current.

Q.5: We want to convert a galvanometer into (a) an ammeter (b) a voltmeter. What do we need to do in each case?
Ans:

To convert a galvanometer into an ammeter, we connect a low resistance in parallel (called shunt).
To convert a galvanometer into a voltmeter, we connect a high resistance in a series (called multiplier).

Q.6: Why is it necessary for an ammeter to have zero or negligibly small resistance?
Ans: An ammeter must have negligibly small resistance so that it may not alter the current being measured.

Q.7: What necessary condition must a voltage measuring device satisfy?
Ans: A voltage measuring device must contain a very high (in fact, infinite) resistance, so that it will, practically, draw no current from the circuit across which it is connected.

Q.8: Why must an ammeter be connected to a circuit in series and a voltmeter in parallel?
Ans: An ammeter must be connected in series to a circuit because its resistance is very small as compared to the total resistance of the circuit. Hence it does not alter the current being measured. But a voltmeter has very high resistance, so it must be connected in parallel to a circuit.

Q.9: An ammeter and voltmeter of suitable ranges are to be used in a circuit. What might happen if by their mistake positions are interchanged?
Ans:

If, by a mistake, an ammeter is connected in parallel to a circuit, its coil will be burnt out to heavy current (because of its extremely low resistance).
When a voltmeter, by mistake, is connected in series to a circuit it will give reading but will not record correct p.d. because it will decrease the current (due to its very high resistance). It will not cause damage.

Q.10: The terminals of ammeters are usually made of thick and bare metal while those of voltmeters are quite thin and well insulated. Explain why?
Ans:

An ammeter must have very low resistance. So its terminals should have almost zero resistance. Hence terminals must be made of thick, bare metal.
A voltmeter must have very high resistance. So its terminals should be thin and well insulated to avoid sparking between the terminals.

Q.11: Why is a potentiometer considered one of the most accurate voltage measuring devices?
Ans: The principle of a potentiometer is that the potential drop across any length of wire of uniform cross-section is directly proportional to the length of the wire. At the balance point, the two terminals of the galvanometer are at the same potential, and no current will flow through it. Hence, a potentiometer is an instrument that can be used to measure the emf of a source and compare potentials without drawing any current from the source. Essentially, it balances an unknown p.d. against an adjustable, measurable p.d.

Q.12: How is a Wheatstone bridge used for measuring an unknown resistance?
Ans: If we connect three resistances $R_1$ , $R_2$ , and $R_3$ of precisely known adjustable values and a fourth unknown resistance $R_4$ , and these are so adjusted that the galvanometer shows no deflection; then in this balanced condition, $R$ square root $R_2$ is equal to $R_3$ square root $R_4$ . Hence $R_4$ can be calculated.

MAGNETISM & ELECTROMAGNETISM

Chapter – 14

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.

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).

Q.3: Beams of electrons and protons are made to move with the same velocity at right angles to a uniform magnetic field of induction. Which of them will suffer a greater deflection? What will be the effect on the beam of electrons if their velocity is doubled?
Ans: The radius of the circular path of a particle moving in a magnetic field is $r = \frac{mv}{Bq}$ . Thus $r$ is proportional to $m$ , but deflection is proportional to $1/m$ . Thus, the electron, being lighter, will be deflected more than the proton.

Since $r \propto v$ , if velocity is doubled, the radius will also be doubled; but deflection is halved.

Q.4: A circular loop of wire hangs by a thread in a vertical plane. An electric current is maintained in the loop anti-clockwise when looking at the front face. To what direction will the front face or the coil turn in the earth’s magnetic field?
Ans: Toward the geographic north pole.

Q.5: Imagine that the room in which you are seated is filled with a uniform magnetic field pointing vertically upward. A loop of wire, which is free to rotate about the horizontal axis is placed through its center parallel to its length, has its plane horizontal. For what direction of current in the loop, as viewed from above, will the loop be in a stable equilibrium with respect to forces and torque of magnetic origin?
Ans: Anti-clockwise.

Q.6: Two identical loops, one of copper and the other of aluminum, are similarly rotated in a magnetic field of induction. Explain the reason for their different behavior. Is an electric generator a ‘generator of electricity’? Where is the electricity before it is generated? What do such machines generate?
Ans: Since the conductivities of copper and aluminum are different, they show different behavior with the induced e.m.f. As the conductivity of copper is higher than that of aluminum, a copper loop will have a greater induced current than an identical aluminum loop moving with the same speed in the same magnetic field. An electric generator is not a generator of electricity (i.e., quantity of charge). Electricity is present in the conducting coil of the generator before it was driven in an electrical circuit. A generator provides e.m.f. to drift the haphazardly moving electrons in the conducting coil. In fact, a generator converts mechanical energy into energy of moving charges.

Q.7: A loosely wound helical spring of a stiff wire is mounted vertically with the lower end just touching mercury in a dish. When a current is started in the spring, it excretes a vibratory motion with its lower end jumping out and into mercury. Explain the reason for this behavior?
Ans: When a current is passed through a helical spring, whose one end is just above a mercury pool, a magnetic field is produced. The current through all the loops is in the same direction. This produces attraction between them, so its length decreases. The dipping and moving out of the mercury causes the circuit to break. Due to elasticity, the helix regains its original length. The electrical contact is established again. The process is repeated, so the helix vibrates up and down.

Q.8: What is the mechanism of transfer of energy between the primary and secondary windings of a transformer? A certain amount of power is to be transferred over a long distance. If the voltage is stepped up 10 times, how is the transmission line loss reduced?
Ans: Electromagnetic induction is the phenomenon responsible for the transfer of energy between the primary windings (one circuit) to the secondary windings by means of a changing magnetic field which links the two coils. The mutual induction transforms the voltage or e.m.f. of large or similar value due to a different number of turns in the primary and secondary coils.

Suppose a power line has input power $P$ . The same power can be carried at low current if the voltage is made high. Input current $I_1 \propto \frac{P}{V_1}$ . If voltage is stepped up 10 times:

V_2 = 10V_1, \quad I_2 = \frac{P}{V_2} = \frac{P}{10V_1}, \quad \text{Thus } \frac{I_2}{I_1} = \frac{P / 10V_1}{P / V_1} = \frac{1}{10}

When the current is 10 times smaller, the power loss as heat in the wires $(I^2R)$ is $(10)^2$ , i.e., 100 times smaller.

Q.9: What is the difference between magnetic field a.c. generators? What is meant by the frequency of alternating current?
Ans: An alternating current generator that uses a permanent magnet to provide the magnetic flux rather than an electromagnet is called ‘magnetic’. It is used in systems like petrol engines, motorbikes, and motorboats. The a.c. generator that employs electromagnets is called “alternate.” It has a rotating field magnet (called rotor) and a stationary armature (called stator) or vice versa.

Alternating current (a.c.) is produced by a voltage source whose terminal polarity reverses with time. The number of cycles per second made by an a.c. is called its frequency $f$ . Its unit is hertz (Hz). We have $f = 1/T$ , and an a.c. reverses its polarity $2f$ times per second. An a.c. with a frequency of 50 Hz has a time period of $\frac{1}{50} = 0.02$ seconds. This a.c. reaches zero every 0.01 seconds.

Q.10: In what direction are the magnetic field lines surrounding a straight wire carrying current that is flowing directly towards you?
Ans: Anti-clockwise (using the right-hand rule).

Q.11: What kind of field or fields does or do surround a moving electric charge?
Ans: When an electric charge is in motion, it is surrounded by an electrostatic field as well as a magnetic field.

Q.12: Can an electron at rest be set in motion with a magnet?
Ans: No. When an electron is at rest, it has no magnetic field $(F = qvB = 0 \text{ if } v = 0)$ . So, in the absence of any magnetic field of its own, it cannot interact with a magnet.

Q.13: A beam of electron is directed towards a horizontal wire in which the current flows from left or right. In what direction is the beam deflected?
Ans: If the beam is parallel to the wire, it will follow a spiral path; and if it is perpendicular to the wire, it will adopt a circular path.

Q.14: A charged particle is moving in a circle under the influence of a uniform magnetic field. If an electric field is turned on at that point along the same direction as the magnetic field, what path will the charged particle take?
Ans: When a charged particle is moving in a circle under the influence of a uniform magnetic field; and if an electric field is applied along the same direction, it will exert lateral force on the charged particle. Consequently, the charged particle will move in a cyclic path in the form of a spiral (called helix).

Q.15: A loop of wire is suspended between the poles of a magnet with its plane parallel to the pole faces. What happens if direct current is passed through the coil? What will happen if an alternating current is passed instead?
Ans: When d.c. passes through the loop such that its magnetic field is: (i) opposite the direction of the field of the magnet, the coil will turn round through 180° and then will stay in equilibrium. (ii) Along the field of the magnet, the coil will stay in equilibrium. However, when an a.c. is passed through the loop, it will remain in its initial position (with slight vibration).

Q.16: A current-carrying wire is placed in a magnetic field. How must it be oriented so that the force acting on it is zero or is maximum?
Ans:

Force will be zero if theta is equal to zero (parallel to B);
Force will be maximum if theta is equal to 90° (perpendicular to B).

Q.17: Why is the magnetic field strength greater inside a current-carrying loop of wire?
Ans: In a loop of wire, the direction of current in the opposite sides of the loop is opposite to each other. This is analogous to two parallel conductors carrying current in opposite directions. The directions of both the magnetic fields are along the same direction in the loop. This increases the strength of the field.

Q.18: What exactly does a transformer transform?
Ans: A transformer transforms the magnitude of alternating voltage and current.

Q.19: Can an efficient transformer step up energy? Explain.
Ans: Transformers cannot charge energy. In an ideal transformer, the power remains constant, i.e., power input equals power output $(V_p I_p = V_s I_s)$ . Thus it cannot step up energy.

Q.20: In what three ways can a voltage be induced in a wire?
Ans:

By moving a wire in a magnetic field.
By moving a magnet near it.
By changing current through a circuit near it.

Q.21: Does the voltage output of a generator change if its speed of rotation is increased?
Ans: Yes, because induced e.m.f. = $BNA \, \text{omega} \sin \, (\text{omega} \, \text{time})$ . Thus, an e.m.f. increases if the speed of rotation “Omega” is increased.

Q.22: When a beam of electrons is shot into a certain region of space, the electrons travel a straight line through the region. Can we conclude that in the region there is no electric field? No magnetic field?
Ans: There are two possibilities:

No electric or magnetic field is present.
The electric and magnetic fields are at right angles to each other, and their strengths are exerting equal but opposite forces on the electron beam.

Q.23: A copper ring is placed above a solenoid with an iron core to increase its field. When the current is turned on in the solenoid, the copper ring moves upward. Why?
Ans: When current in a solenoid (with an iron core) increases, an induced current is produced in a copper ring (held above it) in the opposite direction. This is analogous to opposite currents in two parallel wires. Thus they develop similar poles and repel each other. Consequently, the ring moves up.

Q.24: A very long copper pipe is held vertically. Describe the motion of a bar magnet dropped lengthwise down the pipe?
Ans: Suppose a bar magnet falls through a very big copper pipe (under gravity). When the magnet is well inside the pipe, the configuration of the magnetic field remains the same. So it will fall freely with acceleration of gravity only.

Q.25: A solenoid is viewed in such a way that the solenoid current appears clockwise to the viewer. What is the direction of the field within the solenoid?
Ans: The end viewed will develop south polarity. So the direction of the magnetic field will be away from the viewer inside the solenoid.

Q.26: A hollow copper tube carries. Why is $B = 0$ inside the tube? Is $B$ non-zero outside the tube?
Ans: The charges always reside or move on the outer of a conductor. Since inside the tube, current is zero, hence $B = 0$ (according to Ampere’s law). The outer surface of the tube behaves like a set of parallel wires carrying current down their length. The magnetic field outside the tube exists, and its direction is given by the right-hand grip rule.

Q.27: Can a current-carrying coil be used as a compass?
Ans: A current-carrying coil behaves like a bar magnet (magnet dipole). Thus when it is suspended freely, it can be used as a compass.

Q.28: When a charged particle enters a magnetic field, it is deflected by the magnetic force? Can the magnetic force do work on the moving charged particle?
Ans: No, magnetic force can do no work on a moving charged particle because it is always perpendicular to the velocity of the particle.

Q.29: If both electric field (E) and magnetic field (B) act on a charged particle, what is the total force on it?
Ans: The total force is $F = qE + q (v \times B)$ . This force is called the Lorenz force.

Q.30: Can an isolated magnetic pole (monopole) exist? What is the source of the magnetic fields?
Ans: No, magnetic monopoles cannot exist. The only known source of magnetic fields are magnetic dipoles (current loops), even in magnetic materials.

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).

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)

Q.3: Can you construct two wires of the same length, one of copper and one of iron, that would have the same resistance at the same temperature?
Ans: Yes, since resistibility is proportional to cross-sectional area. The resistivity of iron is about 7 times higher, than that of copper. Hence the iron wire must be 7 times thicker than a copper of the same Length to have the same resistance at the same temperature.

Q.4: Why does the resistance of a conductor rise with the rise in temperature?
Ans: As the temperature of a conductor rises, the amplitude of the vibration of the atoms in the lattice increases. This, in turn, increases the probity of their collision with free-electron. This impedes the drift of the electron. Hence the resistance of the conductor increases.

Q.5: Why is heat produced in a conductor due to the flow of electric current?
Ans: As electric charge flows due to certain p.d. through a conductor, it suffers loss of electrical potential energy. The energy is delivered to the lattice atoms. This energy is utilized in increasing their vibration kinetic energy which appears as heat. Consequently, the temperature of the conductor rises.

Q.6: When a metallic object is heated both its dimensions and its resistivity increase. Is the increase in resistivity likely to be a consequence of the increase in length?
Ans: The receptivity is given by $\rho = \frac{RA}{L}$ . The increase in receptivity of a conductor due to heat is a consequence of the increase in resistance, and not a consequence of the increase in length.

Q.7: It is sometimes said that an electrical appliance “uses up” electricity. What does such an appliance actually use in its operation?
Ans: An electrical appliance, in its operation, uses the kinetic energy carried by the moving electrons, and not their quantity of charge.

Q.8: Do bends in a wire affect its resistance?
Ans: No, bends in a wire do not affect its resistance. However, it depends upon length, cross-sectional area, temperature, and nature of the material.

Q.9: Resistances of 10Ω, 30Ω and 40Ω are connected in series. If the current in 10Ω resistance is 0.1A, what is the current through the other?
Ans: When resistors are connected in series, then the same current flows through each (as there is only one path). Hence the current in this case will be 0.1A through all three resistors in series.

Q.10: The resistances of different values are connected in parallel. If the p.d. across one of them is 5V, what is the p.d. across the remaining at ne resistors?
Ans: When resistors are connected in parallel, then the same p.d. exists across each of them as they all are connected to two common points. Hence the p.d. in this case will be 5V across all the nine resistors in parallel.

Q.11: For a given potential difference V, how will the heat developed in a resistor depend on its resistance R? Will the beat be developed at a higher rate in a larger or smaller R.
Ans: The heat developed $H = V^2 / R \times t$ . For a constant p.d. the resistance R should be small to develop heat at a higher rate.

Q.12: Is there any electric field inside a conductor carrying an electric current? Explain motion of charges here?
Ans: When a p.d. is applied across a conductor by connecting it to a battery and electric field E is established inside a conductor (parallel to the conductor, directed from the positive toward the negative terminal). The field exists here because the battery keeps the charges moving and prevents them coming to equilibrium on the outer surface of the conductor (in contrast to the situation in electrostatic), where they would cause the net electric field on the interior to be zero.

Q.13: Can the terminal voltage of battery be zero?
Ans: When a battery is short-circuited, the existence of negligibly small resistance in the circuit makes terminal voltage zero but current to a maximum value.

[ \text{Since} R = 0, V = 0 \text{ and} I = E/r']

Q.14: Why is internal resistance of a cell not constant?
Ans: The internal resistance of a cell depends upon the resistance of electrolyte, terminals and electrons (and also on their area and separation) of the cell. Due to chemical changes (e.g. absorption of hydrogen and sulphate ions) in the electrolyte during the process of discharging the resistance of the electrolyte increase. Thus the internal resistance of cell does not remain constant.

Q.15: What is resistance? What is its mechanical analogue?
Ans: Resistance is a property of a given conductor which limits the current flow. It is due to the collisions of the drifted electrons with the crystal lattice which causes frequent scattering of the electrons under an electric field.

This property is analogous to mechanical friction or moving bodies.

Q.16: Often, you might have noticed crows sitting safely on high tension wires. Why are they not electrocuted, even when sitting on a part of the wire where the insulation has worn off?
Ans: For electrocution, the current should pass through the bodies of crows. When a crow sits on a signal wire, a p.d. is not developed for the flow of current because his both claws are the same potential. Hence they are not electrocuted.

Q.17: Why is it dangerous to touch a live wire standing on the earth barefooted?
Ans: We may be electrocuted if one touches a live wire while standing on barefooted; because we provide lower potential of the earth through our barefoot (conductor). [The effective resistance of the body is 50 kΩ /cm³ which reduces to 0.7 kΩ/cm when it is wet].

Q.18: A heavy duty battery of a truck maintains a current for 3.0A for 24 hours. How charge flows from the battery during this time.
Ans: The charge, $q = I \times t = 3 \times 24 \times 60 \times 60 = 2.6 \times 10^5 C$ .

Q.19: What a short circuit and open circuit mean to you?
Ans: A short circuit is a closed when no load is present i.e. external resistance (R) is zero. But an open circuit implies an infinite resistance (or gap) along its conduction path (i.e. wires).

Q.20: Is it possible to have a situation in which the terminal voltage will be greater than the e.m.f. of a battery?
Ans: In general, $V = E - Ir, V > E$ in the case when a battery is being charged. $[V = E + Ir]$

Q.21: Why is resistance of a conductor inversely proportional to the area of cross sectional a conductor?
Ans: The larger the area of cross section of a conductor, the wider path is provided by it for the flow of charges through it $(R \text{ proportional to } 1/A)$ . Hence, the resistance decreases.

Pages

Advent of Modern Physics - Question Answers - Physics XII

Electromagnetic Waves and Electronics - Question Answers - Physics XII

ELECTRICAL MEASURING INSTRUMENTS

MAGNETISM & ELECTROMAGNETISM

CURRENT ELECTRICITY