Question
If an automobile engine is overheated, it is cooled by putting water on it. It is advised that the water should be put slowly with engine running. Explain the reason.
✓
Answer
In a hot engine the hot parts are expanded because of heat, if cold water is poured suddenly then there will be uneven thermal contraction in the parts. This will result in a stress to develop between the various parts of the engine and may let the engine to crack down.
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Calculate the intensity of the electric field due to the helium nucleus at a distance of $1Å$ from it.
→Microwave oven: The spectrum of electromagnetic radiation contains a part known as microwaves. These waves have frequency and energy smaller than visible light and wavelength larger than it. What is the principle of a microwave oven and how does it work? Our objective is to cook food or warm it up. All food items such as fruit, vegetables, meat, cereals, etc., contain water as a constituent. Now, what does it mean when we say that a certain object has become warmer? When the temperature of a body rises, the energy of the random motion of atoms and molecules increases and the molecules travel or vibrate or rotate with higher energies. The frequency of rotation of water molecules is about 2.45 gigahertz (GHz). If water receives microwaves of this frequency, its molecules absorb this radiation, which is equivalent to heating up water. These molecules share this energy with neighbouring food molecules, heating up the food. One should use porcelain vessels and non-metal containers in a microwave oven because of the danger of getting a shock from accumulated electric charges. Metals may also melt from heating. The porcelain container remains unaffected and cool, because its large molecules vibrate and rotate with much smaller frequencies, and thus cannot absorb microwaves. Hence, they do not get eaten up. Thus, the basic principle of a microwave oven is to generate microwave radiation of appropriate frequency in the working space of the oven where we keep food. This way energy is not wasted in heating up the vessel. In the conventional heating method, the vessel on the burner gets heated first and then the food inside gets heated because of transfer of energy from the vessel. In the microwave oven, on the other hand, energy is directly delivered to water molecules which is shared by the entire food.
(i) As compared to visible light microwave has frequency and energy
(a) Frequency is less but energy is more
(b) less than visible light
(c) more than visible light
(d) equal to visible light
(ii) When the temperature of a body rises
(a) the energy of the random motion of atoms and molecules decreases.
(b) the energy of the random motion of atoms and molecules remains same.
(c) the energy of the random motion of atoms and molecules increases
(d) the random motion of atoms and molecules becomes streamlined.
(iii) he frequency of rotation of water molecules is about
(a) 2.45 THz
(b) 2.45 kHz
(c) 2.45 MHz
(d) 2.45 GHz
OR
In the microwave oven
(a) Energy is directly delivered to the food grains.
(b) The vessel gets heated first and then the water molecules collect heat from the body of the vessel
(c) Energy is directly delivered to water molecules which is shared by the entire food
(d) The vessel gets heated first, and then the food grains inside
(iv) Why should one use porcelain vessels and non-metal containers in a microwave oven?
(a) Because it will prevent the food items to become hot
(b) Because it will get too much hot
(c) Because of the danger of getting a shock from accumulated electric charges
(d) Because it may crack due to high frequency
(i) As compared to visible light microwave has frequency and energy
(a) Frequency is less but energy is more
(b) less than visible light
(c) more than visible light
(d) equal to visible light
(ii) When the temperature of a body rises
(a) the energy of the random motion of atoms and molecules decreases.
(b) the energy of the random motion of atoms and molecules remains same.
(c) the energy of the random motion of atoms and molecules increases
(d) the random motion of atoms and molecules becomes streamlined.
(iii) he frequency of rotation of water molecules is about
(a) 2.45 THz
(b) 2.45 kHz
(c) 2.45 MHz
(d) 2.45 GHz
OR
In the microwave oven
(a) Energy is directly delivered to the food grains.
(b) The vessel gets heated first and then the water molecules collect heat from the body of the vessel
(c) Energy is directly delivered to water molecules which is shared by the entire food
(d) The vessel gets heated first, and then the food grains inside
(iv) Why should one use porcelain vessels and non-metal containers in a microwave oven?
(a) Because it will prevent the food items to become hot
(b) Because it will get too much hot
(c) Because of the danger of getting a shock from accumulated electric charges
(d) Because it may crack due to high frequency
Point charge q is placed (i)at the mid-point of the core of a cube of side a meter (ii)centrally on a plane of a cube of side a meter. Calculate the total flux associated with the cube. Also find out how many surfaces of the cube will be the total flux associated?
When tall buildings are constructed on earth, the duration of day-night slightly increases. Is it true?
To convert two atoms into ions, 2 electrons have been removed from each of them. When these ions are placed in a vacuum, they repel each other with a force of $3.7 \times 10^{-9}$ Newton. Find the distance between them.
→A heavy nucleus breaks into comparatively lighter nuclei, which are more stable compared to the original heavy nucleus. When a heavy nucleus like uranium is bombarded by slow moving neutrons, it splits into two parts, releasing large amount of energy. The typical fission reaction of $_{92}\text{U}^{235}$.
$_{92}\text{U}^{235}+\ _0\text{n}^1\rightarrow\ _{56}\text{Ba}^{141}+\ _{36}\text{kr}^{92}+\ 3_0\text{n}^1+\ 200\text{ MeV}$
The fission of $_{92}\text{U}^{235}$ approximately released $200 MeV$ of energy.
→$_{92}\text{U}^{235}+\ _0\text{n}^1\rightarrow\ _{56}\text{Ba}^{141}+\ _{36}\text{kr}^{92}+\ 3_0\text{n}^1+\ 200\text{ MeV}$
The fission of $_{92}\text{U}^{235}$ approximately released $200 MeV$ of energy.
- If $200 MeV$ energy is released in the fission of a single nucleus of $_{92}^{235}\text{U}$, the fissions which are required to produce a power of $1kW$ is.
- $3.125 \times 10^{13}$
- $1.52 \times 10^6$
- $3.125 \times 10^{12}$
- $3.125 \times 10^{14}$
- The release in energy in nuclear fission is consistent with the fact that uranium has
- More mass per nucleon than either of the two fragments.
- More mass per nucleon as the two fragment.
- Exactly the same mass per nucleon as the two fragments.
- Less mass per nucleon than either of two fragments.
- When $_{92}\text{U}^{235}$ undergoes fission, about $0.1\%$ of the original mass is converted into energy. The energy released when $1\ kg$ of $_{92}\text{U}^{235}$ undergoes fission is.
- $9 \times 10^{11}J$
- $9 \times 10^{13}J$
- $9 \times 10^{15}J$
- $9 \times 10^{18}J$
- A nuclear fission is said to be critical when multiplication factor or $K$.
- $K = 1$
- $K > 1$
- $K < 1$
- $K = 0$
- Einstein's mass-energy conversion relation $E = mc^2$ is illustrated by.
- Nuclear fission
- $\beta-$ decay
- Rocket propulsion
- Steam engine
Huygen's principle is the basis of wave theory of light. Each point on a wavefront acts as a fresh source of new disturbance, called secondary waves or wavelets. The secondary wavelets spread out in all directions with the speed light in the given medium.
An initially parallel cylindrical beam travels in a medium of refractive index $\mu(\text{I})=\mu_0+\mu_2\text{I}$, where $\mu_0$ and $\mu_2$ are positive constants and I is the intensity of the light beam. The intensity of the beam is decreasing with increasing radius.
→An initially parallel cylindrical beam travels in a medium of refractive index $\mu(\text{I})=\mu_0+\mu_2\text{I}$, where $\mu_0$ and $\mu_2$ are positive constants and I is the intensity of the light beam. The intensity of the beam is decreasing with increasing radius.
- The initial shape of the wavefront of the beam is:
- Planar.
- Convex.
- Concave.
- Convex near the axis and concave near the periphery.
- According to Huygens Principle, the surface of constant phase is:
- Called an optical ray.
- Called a wave.
- Called a wavefront.
- Always linear in shape.
- As the beam enters the medium, it will:
- Travel as a cylindrical beam.
- Diverge.
- Converge.
- Diverge near the axis and converge near the periphery.
- Two plane wavefronts oflight, one incident on a thin convex lens and another on the refracting face of a thin prism. After refraction at them, the emerging wavefronts respectively become.
- Plane wavefront and plane wavefront.
- Plane wavefront and spherical wavefront.
- Spherical wavefront and plane wavefront.
- Spherical wavefront and spherical wavefront.
- Which of the following phenomena support the wave theory of light?
- Scattering.
- Interference.
- Diffraction.
- Velocity of light in a denser medium is less than the velocity of light in the rarer medium.
- 1, 2, 3
- 1, 2, 4
- 2, 3, 4
- 1, 3, 4
Derive the formula of electric potential at a point due to electric dipole.
The saturation current from a thoriated$-$tungsten cathode at $2000K$ is $100mA.$ What will be the saturation current for a pure$-$tungsten cathode of the same surface area operating at the same temperature? The constant $A$ in the Richardson$-$Dushman equation is $60 \times 10^4Am^{-2}K^{-2}$ for pure tungsten and $3.0 \times 10^4Am^{-2}k^{-2}$ for thoriated tungsten. The work function of pure tungsten is $4.5eV$ and that of thoriated tungsten is $2.6eV.$
→A closed bottle contains some liquid. The bottle is shaken vigorously for 5 minutes. It is found that the temperature of the liquid is increased. Is heat transferred to the liquid? Is work done on the liquid? Neglect expansion on heating.