# +2 Physics Unit 6 - Atomic Physics

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Prepared by P. ILAIYARAJA M. Sc.,B. Ed., PG ASST IN PHYSICS , GHSS, PANCHANATHIKKULAM EAST, NAGAI DT.
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• 1.

### The cathode rays are

• A.

A stream of electrons

• B.

A stream of positive ions

• C.

A stream of uncharged particles

• D.

The same as canal rays

A. A stream of electrons
Explanation
Cathode rays refer to a stream of electrons. These rays were discovered in cathode ray tubes, where a high voltage is applied between two electrodes, causing a beam of electrons to travel from the negatively charged cathode to the positively charged anode. This stream of electrons is responsible for producing the glow or beam of light observed in the tube. Therefore, the correct answer is "a stream of electrons."

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• 2.

### A narrow electron beam passes undeviated through an electric field and an overlapping magnetic field . The electron motion, electric field and magnetic field are mutually perpendicular. The speed of the electron is

• A.

(3)

• B.

(1)

• C.

(2)

• D.

(4)

A. (3)
Explanation
The correct answer is (3) because when a narrow electron beam passes undeviated through mutually perpendicular electric and magnetic fields, it indicates that the velocity of the electron is perpendicular to both fields. This implies that the electron is moving in a circular path due to the magnetic field and the electric field is not affecting its motion. Therefore, the speed of the electron remains constant.

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• 3.

### According to Bohr’s postulates, which of the following quantities take discrete values?

• A.

Angular momentum

• B.

Kinetic energy

• C.

Potential energy

• D.

Option 4

A. Angular momentum
Explanation
According to Bohr's postulates, angular momentum takes discrete values. This means that the angular momentum of an electron in an atom can only have certain specific values and cannot take on any arbitrary value. This is one of the fundamental principles of Bohr's model of the atom, which helped explain the quantized nature of electron orbits and the stability of atoms.

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• 4.

### The ratio of the radii of the first three Bohr orbit is,

• A.

1 : 4 : 9

• B.

1 : 8 : 27

• C.

1 : 1/2 : 1/3

• D.

1 : 2 : 3

A. 1 : 4 : 9
Explanation
The correct answer is 1:4:9. This ratio represents the relationship between the radii of the first three Bohr orbits. According to the Bohr model of the atom, electrons orbit the nucleus in specific energy levels or orbits. The radii of these orbits increase as the energy level increases. The ratio 1:4:9 indicates that the radius of the second orbit is four times greater than the radius of the first orbit, and the radius of the third orbit is nine times greater than the radius of the first orbit. This ratio follows the pattern of the square of the energy level.

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• 5.

### The first excitation potential energy or the minimum energy required to excite the atom from ground state of hydrogen atom is,

• A.

10.2 eV

• B.

13.6 eV

• C.

3.4 eV

• D.

1.89 eV

A. 10.2 eV
Explanation
The first excitation potential energy is the minimum amount of energy required to excite an atom from its ground state. In the case of a hydrogen atom, the first excitation potential energy is 10.2 eV. This means that in order to move the electron from the ground state to the first excited state, at least 10.2 eV of energy needs to be supplied to the atom.

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• 6.

### According to Rutherford atom model, the spectral lines emitted by an atom is,

• A.

Continuous spectrum

• B.

Continuous absorption specturm

• C.

line spectrum

• D.

Band spectrum

A. Continuous spectrum
Explanation
According to Rutherford's atom model, the spectral lines emitted by an atom are not continuous but rather discrete, which means they appear as separate lines in the spectrum. Therefore, the correct answer is line spectrum.

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• 7.

### Energy levels A, B, C of a certain atom correspond to increasing values of energy (i.e.,) . If  are the wavelengths of radiations corresponding to the transitions C to B, B to A and C to A respectively, which of the following statements is correct.

• A.

(2)

• B.

(1)

• C.

(3)

• D.

(4)

A. (2)
Explanation
The correct answer is (2) because in a certain atom, the energy levels A, B, and C correspond to increasing values of energy. Therefore, the wavelength of radiation corresponding to the transition from C to B will be shorter than the wavelength of radiation corresponding to the transition from B to A. Similarly, the wavelength of radiation corresponding to the transition from C to A will be shorter than the wavelength of radiation corresponding to the transition from B to A.

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• 8.

### The elliptical orbits of electron in the atom were proposed by

• A.

Sommerfeld

• B.

J.J.Thomson

• C.

Bohr

• D.

De Broglie

A. Sommerfeld
Explanation
Sommerfeld proposed the concept of elliptical orbits of electrons in the atom. This was an extension of Bohr's model of the atom, which suggested that electrons move in circular orbits around the nucleus. Sommerfeld's contribution was to introduce the idea that these orbits could also be elliptical, allowing for more complex electron motion within the atom. This concept helped to explain certain experimental observations that could not be accounted for by the simple circular orbit model. Therefore, Sommerfeld is credited with proposing the elliptical orbits of electrons in the atom.

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• 9.

### X−ray is

• A.

Phenomenon of conversion of kinetic energy into radiation.

• B.

Conversion of momentum

• C.

Conversion of energy into mass

• D.

Principle of conservation of charge

A. Phenomenon of conversion of kinetic energy into radiation.
Explanation
X-ray is a phenomenon of conversion of kinetic energy into radiation. When high-energy electrons collide with a metal target, their kinetic energy is converted into X-ray radiation. This process is known as X-ray production. X-rays are a form of electromagnetic radiation with high energy and short wavelength. They can penetrate through materials and are commonly used in medical imaging and industrial applications.

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• 10.

### In an X-ray tube, the intensity of the emitted X−ray beam is increased by

• A.

Increasing the filament current

• B.

Decreasing the filament current

• C.

increasing the target potential

• D.

Decreasing the target potential

A. Increasing the filament current
Explanation
Increasing the filament current in an X-ray tube increases the intensity of the emitted X-ray beam. The filament current heats up the filament, causing it to emit more electrons. These electrons are accelerated towards the target, resulting in a greater number of X-ray photons being produced. Therefore, increasing the filament current increases the intensity of the X-ray beam.

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• 11.

### The energy of a photon of characteristic X-ray from a Coolidge tube comes from

• A.

An atomic transition in the target.

• B.

The kinetic energy of the striking electron

• C.

The kinetic energy of ions of the target

• D.

The kinetic energy of the free electrons of the target

A. An atomic transition in the target.
Explanation
When a Coolidge tube generates characteristic X-rays, the energy of a photon is derived from an atomic transition in the target material. This means that an electron in an atom of the target material undergoes a transition from a higher energy state to a lower energy state, emitting a photon in the process. The energy of this photon corresponds to the energy difference between the two states involved in the atomic transition. Therefore, the correct answer is "an atomic transition in the target."

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• 12.

• A.

(1)

• B.

(2)

• C.

(3)

• D.

(4)

A. (1)
• 13.

### In hydrogen atom, which of the following transitions produce a spectral line of maximum wavelength

• A.

6 → 5

• B.

4 → 1

• C.

2 → 1

• D.

5 → 2

A. 6 → 5
Explanation
The transition from energy level 6 to energy level 5 in a hydrogen atom produces a spectral line of maximum wavelength. This is because as the electron moves from a higher energy level to a lower energy level, it releases energy in the form of light. The energy of the emitted light is inversely proportional to its wavelength, so a transition from a higher energy level to a lower energy level results in a longer wavelength and thus a spectral line of maximum wavelength.

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• 14.

### In hydrogen atom, which of the following transitions produce a spectral line of maximum frequency

• A.

2 → 1

• B.

6 → 2

• C.

4 → 3

• D.

5 → 2

A. 2 → 1
Explanation
The transition from 2 → 1 in a hydrogen atom produces a spectral line of maximum frequency because it involves the electron moving from a higher energy level (n=2) to a lower energy level (n=1). As the electron moves closer to the nucleus, it loses energy, and this energy loss is emitted as a photon with a higher frequency. Therefore, this transition results in a spectral line with the maximum frequency.

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• 15.

### After pumping process in laser,

• A.

The number of atoms in the excited state is greater than the number of atoms in the ground state.

• B.

The number of atoms in the ground state is equal to the number atoms in the excited state.

• C.

No atoms are available in the excited state.

• D.

The number of atoms in the ground state is greater than the number of atoms in the excited state.

A. The number of atoms in the excited state is greater than the number of atoms in the ground state.
Explanation
In the pumping process of a laser, energy is supplied to the atoms to excite them from the ground state to the excited state. This causes more atoms to be in the excited state compared to the ground state. This is necessary for the laser to work as the transition from the excited state to the ground state releases energy in the form of light. Therefore, the correct answer is that the number of atoms in the excited state is greater than the number of atoms in the ground state.

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