# JKBOSE Physics: Solved Previous Year’s Question Paper for NCERT/CBSE Class 12th Physics Paper Code (RKDO18—X)

## JKBOSE Physics: Solved Previous Year’s Question Paper for NCERT/CBSE Class 12th Physics Paper Code (RKDO18—X)

Question: Explain the theory and working of a cyclotron. Give its limitations

Answer: A cyclotron is a type of particle accelerator used to accelerate charged particles to very high speeds. It is used in engineering, and medicine for a variety of purposes, including particle therapy for cancer treatment, radioisotope production for medical diagnostics, and materials science research.

The cyclotron works by using a series of metal plates, called dees, arranged in a circular path. A magnetic field is created between these plates, and an electric field is created between the dees and an inner electrode. The charged particles are injected into the centre of the cyclotron and accelerated by the alternating electric and magnetic fields. As the particles accelerate, they spiral outward and gain energy. When the particles reach a certain energy, they are ejected from the cyclotron and can be used for various applications.

Limitations of Cyclotron

1. Due to their lack of charge, which prevents them from colliding with magnetic or electric fields, neutrons are not propelled by the use of cyclotrons.

2. Because of their small mass, which causes a rapid rise in speed, electrons cannot be accelerated using cyclotrons. The atom is lost in motion as a result of the sudden acceleration.

3. The main limitation of a cyclotron is that it can only accelerate charged particles and cannot accelerate neutral particles. Additionally, the cyclotron can only accelerate particles up to a certain energy level before the particles leave the cyclotron, meaning that higher energies cannot be achieved. This limits the types of particles that can be used in the cyclotron and the types of experiments that can be conducted.

Question: Define Magnetic dipole moment. Derive an expression for Magnetic field intensity at a point on an axial line of a bar magnet.

Answer: Magnetic dipole moment is a measure of the strength and orientation of a magnet’s magnetic field. It is the product of the magnitude of the magnetic moment and the unit vector along the direction of the magnet’s magnetic field.

The magnetic dipole moment is defined by the vector, μ = m.r, where m is the magnitude of the magnetic moment and r is the unit vector along the direction of the magnet’s magnetic field.

The magnetic field intensity at a point along an axial line of a bar magnet is given by:

B = μ0/4π(r3) x (3cos2θ -1)

where μ0 is the permeability of free space, r is the distance from the magnet to the point and θ is the angle between the magnet’s axis and the line from the magnet to the point.

The magnetic dipole moment, often simply called the magnetic moment, may be defined then as the maximum amount of torque caused by magnetic force on a dipole that arises per unit value of the surrounding magnetic field in a vacuum.

Solution

Let NS be a bar magnet of magnetic length 2l and have each pole of magnetic strength m. O is the centre of the magnet and P is a point on the axial line at a distance r from the centre O of the magnet, at which the magnetic field has to be measured.

The magnetic field B1​ at P due to the N pole of the magnet,

B1​=4πμ0​​.NP2m​

or   B1​=4πμ0​​.(r−l)2m​   (along PX) …………………..eq1

And, the magnetic field B2​ at P due to the S pole of the magnet,

B2​=4πμ0​​.SP2m​

or   B2​=4πμ0​​.(r+l)2m​   (along PS) ………………….eq2

Therefore, the resultant magnetic field at point P,

B=B1​−B2​  (-ive sign is due to opposite directionS of B1​ and B2​)

It is clear from eq1 and eq2 that B1​>B2​ , therefore the direction of  B will be along PX.

or    B=4πμ0​​.(r−l)2m​−4πμ0​​.(r+l)2m​     (alongPX)

or    B=4πμ0​​.(r2−l2)2m(4rl)​     (alongPX)

Now, m(2l)=M (magnetic dipole moment of the magnet)

Hence , B=4πμ0​​.(r2−l2)22Mr​     (alongPX)

Question: Discuss the effect of temperature on the resistance of conductors.

Answer: Temperature has a direct effect on the resistance of conductors. As temperature increases, the resistance of the conductor increases as well. This effect is known as the temperature coefficient of resistance.

When a conductor is heated, its atoms vibrate more rapidly and collide with each other more often. These collisions create resistance to the flow of electrons, increasing resistance. At the same time, the increased temperature causes the atoms of the conductor to spread out, increasing the distance between the atoms and reducing their ability to conduct electricity. This further increases the resistance of the conductor.

The temperature coefficient of resistance is different for different materials. For example, the temperature coefficient of resistance for copper is 0.00393/°C, while for silver it is 0.00382/°C. Generally, metals with higher melting points have higher temperature coefficients of resistance. The temperature coefficient of resistance can be used to calculate the resistance of a conductor at a given temperature. The resistance of a conductor at a temperature of 20°C can be calculated by multiplying the temperature coefficient of resistance by the change in temperature (20°C – 0°C = 20). The resistance of the conductor at 20°C can then be determined by adding the result to the

Question: What do you understand by Self-induction

Answer: Self-induction is a process in which an electric current in a circuit produces a changing magnetic field that induces an electromotive force (EMF) in the same circuit. This EMF opposes the original current, causing a decrease in the current strength. This phenomenon is known as Lenz’s Law, and it is the basis for the operation of electrical inductors, transformers, and certain types of motors.

Question: What is a transistor? How can we use it as an amplifier (common emitter configuration)?

Answer: A transistor is a semiconductor device used to amplify or switch electrical signals and power. It is made up of three layers of semiconductor material, with each layer having a different electrical charge.

Transistors can be used as amplifiers in a common emitter configuration by allowing a small input signal to control a larger output signal. This is done by applying a small current to the base of the transistor which then controls the current flowing from the collector to the emitter. The larger current from the collector is amplified and the output signal is greater than the input signal. This is useful for amplifying audio signals, for example.

Question: Derive an expression for energy stored in a capacitor.

Answer: The energy stored in a capacitor is equal to the product of the capacitance and the square of the voltage across the capacitor: Energy = C * V²

Solution

Let us consider a capacitor of capacitance C and the potential difference V between the plates.

Let the charge on one plate be +q and -q on the other.

Suppose the capacitor is being charged gradually.

Now, at any stage the charge on the capacitor is q.

Therefore, the potential difference = Cq​

A small amount of work doe in giving n additional charge dq to the capacitor is
dW= Cq​∗dq

The total work done in giving a charge Q to the capacitor  is
W=∫dW

W= ∫0Q​CQ​dq

W = CQ2​

Energy = E
E= 2CQ2​ = 2CV2​ = 2QV​
The energy is stored in the form of potential energy.

Question: Give any six properties of e.m.waves.

1. Electromagnetic waves can travel through a vacuum.

2. Electromagnetic waves travel at the speed of light.

3. Electromagnetic waves can be reflected, refracted, and diffracted.

4. Electromagnetic waves can be polarized.

5. Electromagnetic waves have a frequency and an amplitude.

6. Electromagnetic waves are made up of electric and magnetic fields that are perpendicular to each other and the direction of propagation.

Question: What is total internal reflection? Give the condition for this phenomenon

Answer: Total internal reflection is a phenomenon that occurs when light passes from a medium with a higher refractive index, such as glass, to a medium with a lower refractive index, such as air. When light hits the boundary between the two media at an angle greater than the critical angle, the light is reflected into the medium with the higher refractive index. The critical angle is determined by the ratio of the refractive indices of the two media.

Question: State and Explain Kirchhoff’s loop law of the electric circuit.

Answer: Kirchhoff’s Loop Law, also known as Kirchhoff’s First Law, states that the sum of the voltages around a closed loop must be zero. In other words, if you add up the voltages in any closed loop in an electric circuit, the sum of the voltages must equal zero. This law is based on the conservation of energy, which states that energy must be conserved in a circuit; it cannot be created or destroyed. This law is often used to solve complex electric circuit problems as it helps to reduce the number of unknowns in a circuit.

Question: What is Nuclear fusion? Give one representative reaction.

Answer: Nuclear fusion is a process in which two or more atomic nuclei join together to form a single, more massive nucleus. It is the process that powers stars, including our sun. A representative reaction in physics is the deuterium-tritium reaction:

1H + 2H → 3He + n + 17.6 MeV

Question: Explain Sky wave propagation.

Answer: Sky wave propagation is a type of radio wave propagation that occurs when the direct wave (also known as the ground wave) is reflected off of the ionosphere, allowing communication between two points that are beyond the horizon.

Sky wave propagation is used for long-distance communication because it can allow signals to travel much further than they could with just the ground wave. The atmosphere acts as a reflector, and the signal is bounced off the ionosphere and back down to Earth, allowing the signal to travel much farther than it would without the reflected wave.

Sky wave propagation is used in radio communications, satellite communications, and other forms of long-distance communication.

Question: What do you mean by conservation of electric charge?

Answer: Conservation of electric charge is a fundamental law that states that the total electric charge of an isolated system remains constant. This means that electric charge can neither be created nor destroyed, but can only be transferred from one object to another. This law is a consequence of the symmetry of nature and is one of the four fundamental conservation laws (along with conservation of energy, conservation of linear momentum, and conservation of angular momentum).

Question: Why ammeters are connected in series in electric circuits?

Answer: Ammeters are connected in series in electric circuits in physics because they measure the current passing through the circuit. When an ammeter is connected in series, it becomes part of the circuit and the current passes through it, allowing the ammeter to accurately measure the current. This arrangement is important for measuring the exact amount of current passing through the circuit.

Question: State laws of Photoelectric emission?

Answer: The photoelectric effect is the release of photoelectrons from the surface of some photosensitive materials when they are exposed to ultraviolet light. Photoelectric discharge is the term for this action.

Laws of photoelectric emission:
(i) There is a set frequency cut-off number below which no substance can expel electrons.

(ii) The number of electrons released is inversely related to the brightness of the incoming light.

(iii) The frequency of the light that strikes the substance affects the kinetic energy of the electrons that are released.

(iv) There is no lag period between the incident light and the electron emission.

Question: Define wave front and name its various sources.

Answer: A wave front is a surface of constant phase, in a wave field such as a sound wave, light wave, or water wave. It is the locus of points of constant phase in a wave field.

Sources of Wave Fronts:

1. Light waves 2. Sound waves 3. Electromagnetic waves 4. Water waves 5. Seismic waves 6. Gravity waves 7. Shock waves

Question: Give any four properties of beta—particles.

Answer: 1. Beta particles are high-energy, high-speed electrons or positrons emitted by certain types of radioactive nuclei.

2. They have a charge of -1 (electrons) or +1 (positrons).

3. They travel at speeds close to the speed of light.

4. They have a relatively low mass compared to other subatomic particles.

Question: What is Rectifier?

Answer: A rectifier is an electrical device that converts alternating current (AC) to direct current (DC), a process is known as rectification.

Rectifiers have many uses, including power supplies for computers and other electronic equipment.

Objective Type Questions

Question: State Lenz’s law

Answer: Lenz’s law states that the direction of an induced current is such that it will create a magnetic field that opposes the change in magnetic flux that created it. This law is a consequence of the law of conservation of energy.

Question: What is Radioactive Decay Constant

Answer: The radioactive decay constant, usually denoted by the symbol λ (lambda), is the probability that a given atom will decay per unit of time. It is related to the decay rate, which is a measure of how quickly a given amount of material will decay. The decay constant is also known as the disintegration constant or the decay coefficient.

Question: Define Amplitude modulation.

Answer: Amplitude modulation (AM) is a method of radio communication in which the amplitude of the signal is varied in proportion to the information being sent. This is accomplished by modulating the signal with an information signal, typically a voice or music. The strength of the signal is determined by the amplitude of the information signal, and the frequency of the signal remains constant.

Question: The resistance of a conductor depends upon its length and area of cross-section. True/False

Question: The dimensional formula for magnetic field strength is ————

Answer: Mₐₙ = [Mₐₙ] = [M L⁻¹ T⁻¹]

Question: Transformer works on the principle of

Answer: A transformer works on the principle of electromagnetic induction.

Question: The phenomenon of polarization of light indicates that

Answer: when a beam of light is passed through certain substances or certain optical devices, it separates into two beams of light with perpendicular polarization.

Question: UHF range can propagate by means of—————-

Answer: Ground wave and sky wave.