Physics: CUET Mock Test - 10 - CUET MCQ

A P-type semiconductor is formed when a trivalent electron deficient impurities such as boron group elements are doped with intrinsic semiconductor. As the impurities are electron deficient, they take electrons from the valence band creating a number of holes. Due this reason conductivity in P-type semiconductor is mainly due to holes rather than electrons.

Concept:

The electron in the nth orbit is given as,

Bohr radius is symbolized by a

Where, ϵ₀ = 8.854 × 10^-12 Fm is the permittivity in air.

m = 9.109 × 10^-31 kg is the mass of the electron

e = 1.6 × 10^-19 C is the charge of the electron

Z = atomic number

​Explanation:

The electron in the nth orbit is given as,

For Hydrogen as, Z = 1

r_n = a₀ n²

CONCEPT:

Hydrogenic atoms:

• Hydrogenic atoms are atoms consisting of a nucleus with positive charge +Ze and a single electron, where Z is the proton number.
• Examples are a hydrogen atom, singly ionized helium, doubly ionized lithium, and so forth.

De Broglie's Explanation of Bohr's Second Postulate of Quantisation:

• Bohr's second postulate states that the electron revolves around the nucleus only in those orbits for which the angular momentum is some integral multiple of h/2π where h is the Planck’s constant (6.6×10-34 J-sec).
• Thus the angular momentum (L) of the orbiting electron is quantized.
• So the angular momentum of the electron orbiting in the nth orbit around the nucleus is given as,

• Louis de Broglie argued that the electron in its circular orbit, as proposed by Bohr, must be seen as a particle wave.
• For an electron moving in the nth circular orbit of radius rn, the total distance is the circumference of the orbit.
• If the wavelength of the electron orbiting in the nth circular orbit is λ, then,

⇒ 2πrn = nλ

• We know that the De Broglie wavelength of the electron moving with speed vn in the nth orbit is given as,

CALCULATION:

Given v₁ = v and v₂ = 2v

Where v1 = initial speed of the electron and v2 = final speed of the electron

• We know that the De Broglie wavelength of the electron moving with speed vn in the nth orbit is given as,

-----(1)

• When the speed of the electron is v₁, the De Broglie wavelength of the electron is given as,

-----(2)

• When the speed of the electron is v2, the De Broglie wavelength of the electron is given as,

-----(3)

By equation 2 and equation 3,

• Hence, option 2 is correct.

Additional Information

Limitations of the Bohr model:

1. The Bohr model is applicable to hydrogenic atoms. It cannot be extended even to mere two-electron atoms such as helium. The analysis of atoms with more than one electron was attempted on the lines of Bohr’s model for hydrogenic atoms but did not meet with any success. The formulation of the Bohr model involves electrical force between the positively charged nucleus and electron. It does not include the electrical forces between electrons which necessarily appear in multi-electron atoms.
2. Bohr’s model correctly predicts the frequencies of the light emitted by hydrogenic atoms, the model is unable to explain the relative intensities of the frequencies in the spectrum.

Concept:

The kinetic energy is defined as the energy stored in an object because of its motion.

When subatomic particles (electron, proton, etc.) behave like a wave then the wavelength is called de Broglie wavelength

• 
• p = mv
• λ = de Broglie wavelength, h = Planck constant, p = momentum of the subatomic particles, m = mass of subatomic particles, v = velocity of subatomic particles
• Mass of electron = 9.1093837015 × 10⁻³¹ kg
• Speed of light, c = 3 × 10⁸ m/s

Kinetic energy of photon,

• Speed of light, c = 3 × 10⁸ m/s

Calculation:

Given, de-Broglie wavelength, λ = 10^-10 m

Let the kinetic energy of electron is E,

Also,

---- (1)

For the case of a photon,

---- (2)

Dividing equation (2) by (1), we get

Substituting value,

Clearly, E' is greater than E, so the photon has greater kinetic energy than electron

A diode can have two different capacitances:

Diffusion Capacitance:

It is dominant when the diode is forward and is the result of storage charges when forward biased.

The diffusion capacitance is given by the formula:

---(1)

IDQ = Quiescent current of the diode.

τ = Minority carrier lifetime

VT = Thermal voltage

Since the diode current Equation is given as:

---(2)

From Equation (1) and (2), we conclude that the diffusion capacitance increases exponentially with forward bias voltage (Vf).

Junction Capacitance:

It is dominant when the diode is reverse biased and is the result of the charge stored in the Depletion layer.

The junction capacitance is given by the formula:

W = depletion region width

• A p-n junction is formed when a p-type and an n-type semiconductor is joined together.
• It is the basic building block of many devices.
• At the junction the electrons diffuse from n side to p side because of the process of diffusion which states that an entity will flow from an area of higher concentration to that of a lower one.
• Since electrons are the majority charge carriers on the n-side, therefore electrons move from high density area of n-side to low density area of p-side.

In the case of metals, the valence and conduction bands have an overlap, and the energy gap between them is zero. This overlap allows electrons to move freely between the valence band and the conduction band, making metals good conductors of electricity

In P-type doping, boron or gallium is the dopant. Boron and gallium each have only three outer electrons. When mixed into the silicon lattice, they form "holes" in the lattice where a silicon electron has nothing to bond to. The absence of an electron creates the effect of a positive charge, hence the name P-type.Holes can conduct current. A hole happily accepts an electron from a neighbor, moving the hole over a space. P-type silicon is a good conductor.

Materials that have the resistance levels between those of a conductor and an insulator are referred to as semiconductors.They are quite common, found in almost all electronic devices. Good examples of semiconductor materials are germanium, selenium, and silicon.Radium is a chemical element with symbol Ra and atomic number 88. It is the sixth element in group 2 of the periodic table, also known as the alkaline earth metals.

An intrinsic semiconductor, also called an undoped semiconductor or i-type semiconductor, is a pure semiconductor without any significant dopant species present. The number of charge carriers is therefore determined by the properties of the material itself instead of the amount of impurities. In intrinsic semiconductors the number of excited electrons and the number of holes are equal: n = p.

Doping is the process of adding impurities to intrinsic semiconductors to alter their properties. Normally Trivalent and Pentavalent elements are used to dope Silicon and Germanium. When an intrinsic semiconductor is doped with Trivalent impurity it becomes a P-Type semiconductor.

The electron theory states that all matter is composed of atoms and the atoms are composed of smaller particles called protons, electrons, and neutrons. The electrons orbit the nucleus which contains the protons and neutrons. It is the valence electrons that we are most concerned with in electricity. These are the electrons which are easiest to break loose from their parent atom. Normally, conductors have three or less valence electrons; insulators have five or more valence electrons; and semiconductors usually have four valence electrons.

A pure semiconductor is called intrinsic semiconductor, e.g., silicon, germanium. The presence of the mobile charge carriers is the intrinsic property of the material. At room temperature, some covalent bonds are broken and electrons are made free. The absence of electron in the covalent bond form hole.The electrical conduction is by means of mobile electrons and holes. Hole act as positive charge, because it can attract an electron. If some other bond is broken and the electron thus freed fills this hole(vacancy), it seems as though the hole is moving.Actually an electron is travelling in opposite direction. In a pure(intrinsic) semiconductor, the number of holes is equal to the number of free electrons.

Magnetic poles exist in pairs. It is not possible to isolate a north pole or a south pole. Magnetic field lines start from the north pole and go to the south pole and return to the north pole. They form continuous closed loops unlike electric lines of force which do not as an electric monopole, a single charge does exist.

For paramagnetic materials orbital and spin magnetic moments of the electrons are of the order of Bohr Magneton.
Paramagnetic materials have some unpaired electrons due to these unpaired electrons the net magnetic moment of all electrons in an atom is not added up to zero. Hence atomic dipole exists in this case. On applying external magnetic field the atomic dipole aligns in the direction of the applied external magnetic field. In this way, paramagnetic materials are feebly magnetized in the direction of the magnetizing field.

Spin and Orbital angular momentum arise due to the presence of unpaired electrons. If there are unpaired electrons, these momenta will be there otherwise not.

The magnitude of induced emf is independent of the resistance of the coil.
∴ e = −dϕ/dt = −N (d/dt) [(BA)/dt]
Thus, e depends upon N, B and A.
But induced current depends upon R.

Induced emf = Blv = 12V. It is induced in the northward direction by right hand rule (emf=)
therefore if the south end of the pole has potential of 0V, the north end will have a potential of 12V.

By using the phenomenon of mutual induction, transformers allow us to easily change voltage of AC. This is necessary to cut down power losses while supplying electricity to our homes

The standard domestic AC supply voltage in India is 230 V. However, it is commonly referred to as 220 V.The supply operates at a frequency of 50 Hz.

By convention phasor diagrams are made to show the rotation of phasor in counterclockwise direction with constant speed

The resistance is R=200ohm the inductor is L = 0.4 H, the capacitance is C = 5  µF and the amplitude voltage is V = CV
The frequency depends on the inductance and the capacitance and it is given by,
f₀ = 1/ (2π√(LC))  
So, plug the values of L and C into equation 1 to get f₀
f₀ = 1/ (2π√(LC)) = 2/(2π√[0.4H)(5 x 10^-6)] = 113Hz
for the current, we use ohm’s law to get the current,
I = V/R
Now, plug the values for V and R to get I
I = V/R = 3V/200Ω = 0.015A = 15mA

In an inductance, the current

• In an inductive circuit, the current lags behind the voltage.

• This means that the current reaches its maximum value after the voltage does.

• The reason for this lag is due to the effect of the inductor, which resists changes in current.

• Thus, the correct answer is that the current lags the voltage.

We know that in capacitor Current leads Voltage by 90degree. Over one complete cycle, in first quarter cycle Capacitor charges and next quarter cycle it's discharge. This will continue in next negative half cycle. So the NET POWER ABSORB IS ZERO.

The frequency of radiation arising from the two close energy levels in hydrogen known as Lamb shift i.e. 1057 MHz is radio waves as it belongs to the short wavelength end of the electromagnetic spectrum.

• Infrared waves are emitted by hot bodies. They are produced due to the deexcitation of atoms.
• They are called Heat waves as they produce heat when they fall on matter. This is because water molecules present in most materials readily absorb infrared waves. After absorption, their thermal motion increases, that is, they heat up and heat their surroundings.

Uses: Infra red lamps; play an important role in maintaining warmth through the greenhouse effect.

E is the electric field vector, and B is the magnetic field vector of the EM wave. For electromagnetic waves E and B are always perpendicular to each other and perpendicular to the direction of propagation. Electromagnetic waves are transverse waves. The wave number is k = 2π/λ, where λ is the wavelength of the wave.

Answer: C) 400 nm to 700 nm

The radiation of infrared rays of the sun keeps the earth's surface warm even at night due to the greenhouse effect of the atmosphere. Heavy gases likely CO_2​ present in the earth's atmosphere reflect infra-red radiation back towards the earth's surface. Due to which earth's atmosphere becomes richer in infrared radiation. If the atmosphere is absent then earth temperature falls off.

When white light enters the prism it has actually entered our atmosphere and so the parts of the EM spectrum like UV don't appear. After the light comes out of the prism all the parts separate but we can only see the visible part i.e. 400-700nm range. So we look at only a part of the EM spectrum. Hence option B is correct.