One mark questions with answers

Q1. Name the types of radiations present in the sunlight.

Ans1. (i) Infrared radiations, (Heat radiation).
(ii) Visible radiations, (Light radiation).
(iii) A small amount of ultraviolet radiations.

Q2. What is the percentage of incident solar radiation that is reflected back into the space before it actually reaches the earth's surface?

Ans2. About 53% is reflected back. Water vapour, Ozone, dust and Carbon dioxide absorb sun's energy.

Q3. The wavelength of three radiations A, B and C are 8 x 10-5 cm, 4 x 10-5 and 2 x 10-5 cm respectively. Which of these three radiations will be visible?

Ans3.The radiation B will be visible because it's wavelength fall in between the range of the visible radiations.

Q4. Light from two different sources A and B has wavelengths 0.7 micron and 0.3 micron, respectively. Which of the two carries lesser energy?

Ans4. Energy of any radiation depends inversely on its wavelength. Therefore, the radiation from the source A (it has longer wavelength) carry lesser energy.

Q5. What type of radiations will a body emit at 4oC?

Ans5. Infra-red radiations. All hot bodies emit Infra-red radiations irrespective of their temperature.

Q6. In a box-type solar cooker, what is the range of temperature that can be achieved in two to three hours?

Ans6. 100 – 140ºC. This is done with the help of sun's radiations.

Q7. Which radiations of sunlight is utilized in solar cookers and solar heaters?

Ans7. Infrared radiations.

Q8. Name the chief constituent of the sun.

Ans8. Hydrogen gas.



Two mark questions with answers

Q1. How can we use Uranium-238 in nuclear reactors?

Ans1. Uranium-238 is fissionable by fast neutrons only. By bombarding Uranium-238 with fast neutrons it is converted into Uranium-235 in the process. (U-238 is not converted into U-235 straight but there are intermediate nuclei also such as U-239, Pu-239 and at last U-235)

Q2. Define the following:
(i) Ultraviolet light.
(ii) Infrared light.

Ans2. (i) Ultraviolet light. It lies on the lower wavelength side of the visible light, (< 400 nm).
(ii) Infrared light. Its wavelength is longer than that of the visible light (> 700 nm). It produces heating effect.

Q3. What happens to the shape of the nucleus of U-235 when a slow neutron hits it?

Ans3. When a slow neutron hits the nucleus of U-235 then the nucleus gets elongated and the distance between neutrons and protons increases. It decreases the nuclear force of attraction between them and the electrostatic force of repulsion becomes more dominant gradually and ultimately the nucleus splits into lighter elements along with emission of three neutrons.

Q4. Calculate the power of nuclear reactor if 6 x 1018 fissions take place in one minute. Given that single fission releases 3.2 x 10-11 J of energy.

Ans4. The energy released in one fission = 3.2 x 10-11 J of energy
Number of fissions in one sec = 6 x 1018/60 = 1 x 1017 fissions
Power of the nuclear reactor = (3.2 x 10-11) x (1 x 1017) W = 3.2 x 106 W = 3.2 MW

Q5. Give one example each of renewable and non-renewable sources of energy.

Ans5. Example of renewable are falling water, tidal energy, sunlight etc. and example of non-renewable sources are coal, natural gas.

Q6. Which of the light energy received from the sun has
(i) the longest wavelength (ii) the shortest wavelength? Give one use of each.

Ans6. (i) Light with longest wavelength: Infrared radiations.
(ii) Radiations with shortest wavelength: Ultraviolet radiations. The most important use of Infra-red radiations is that they heat up the earth and water on the surface of the earth. It is the process of evaporation that takes place and the water of seas and oceans come in the form of rain.
Ultra-violet radiations are used to cause fluorescence on substances coated with fluorescent materials.



Three mark questions with answers

Q1. What are the various functions performed by the sun's energy on the earth?

Ans1. Due to the process of nuclear fusion in the interior of the sun, a huge amount of energy is being radiated out constantly. The sun has been shining for billions of years due to nuclear fusion. So, the sun is taken as the source of energy that feeds every process on the earth. In fact, it is sun's energy that initiated life on earth and it is sustaining life on this planet beautifully. The various functions given below take place due to the sun's energy:
(i) Sun's energy powers the flow of winds.
(ii) Sun's energy powers the flow of water on the earth.
(iii) Sun's energy provides food by the process of photosynthesis and hence sustains all life on the earth.
(iv) Sun's energy created the fossil fuels over a period of millions of years.
(v) Sun's energy provides light and warmth on the earth.

Q2. What is meant by a nuclear chain reaction? Give its types and explain them briefly.

Ans2. Chain reaction: The reaction, in which the bombarding particle is also produced in the reaction so that the reaction is carried on further and more fissions of unstable nucleus take place, is called chain reaction.
For example: In the fission of U-235, neutrons are used as bombarding particles and same are produced in the fission process. The neutrons produced are used to cause fission of more uranium atoms so, such reactions are called chain reactions.
Chain reactions are of two types:

(i) Controlled chain reaction: If the chain reaction goes in a controlled manner and a number of fissions taking place per unit time are fixed. For this reaction to take place the number of neutrons or the bombarding particle is kept constant so that the energy released per unit time is constant.
For example: Chain reaction in nuclear reactors.

(ii) Uncontrolled chain reaction: If the number of fissions goes in an uncontrolled manner then the reaction is called uncontrolled chain reaction. The energy released is also not constant per unit time. A large amount of energy is released in a very short time.
For example: Chain reaction in atom bomb.

Q3. What is a nuclear reactor? State the function of a moderator in a nuclear reactor? Name two substances, which are used as moderator.

Ans3. Nuclear reactor: It is a device used for carrying out nuclear fission reaction in a controlled fashion. It uses fissionable material, which is fissioned by some suitable particles. The energy is produced at a constant rate.
Moderator: Moderator slows down the speed of fast neutrons produced in the fission of Uranium or some other fissionable material. It helps in causing fission of more Uranium atoms. Fast neutrons if not slowed down will not be able to split the heavy nuclei and the chain reaction will stop.
Common moderators: (a) Graphite (b) Heavy water (D2O).

Q4. Give the advantages of nuclear fusion process over nuclear fission to generate electricity.

Ans4. (i) Nuclear fusion process produces approximately seven times more energy per unit mass of the material.
(ii) Products of nuclear fusion are not radioactive. So, there is lesser problem of the waste-disposal.
(iii) The fuel needed in fusion reaction is hydrogen, which is available in plenty. On the other hand, uranium sources are limited.

Q5. Explain why small nuclei are stable whereas big nuclei are unstable?

Ans5. In a small nucleus, the protons and neutrons are very close together. Due to the very small distances between the nucleons, the nuclear force of attraction is very strong in them. Hence, they are very stable. In this case the electrostatic force of repulsion is weaker than the nuclear force of attraction.
In a large nucleus, the distances between nucleons (protons and neutrons) are larger than those in small nuclei. Due to this the nuclear force of attraction decreases greatly. In such cases the nucleus is unstable. Hence, heavy nuclei are unstable.

Q6. Name any two nuclear radiations, and describe their harmful effects on the human body?

Ans6. The radiations emitted during nuclear fission reactions are called nuclear radiations. These include a-, b- and g-rays, X-rays etc. X-rays and g-rays are the most harmful radiations to the human being. These radiations cause irreversible damages in the human body. Tissues, cells and blood corpuscles in human body are destroyed.


Five mark questions with answers

Q1. Explain how are nuclear wastes generated and why do they pose a threat to the environment? What steps are taken for the safe disposal of these wastes?

Ans1. Sources of nuclear wastes : (i) from nuclear reactors.
(ii) during mining of the radioactive materials.
(iii) at nuclear fuel processing complexes.
(iv) from laboratories and hospitals which use radioactive isotopes for research / medical purposes.
Nuclear waste : a threat to environment :
(i) Radioactive wastes pollute the earth to a dangerous level of toxicity.
(ii) Radioactive wastes release high energy radiation which are harmful to the living beings. These can cause diseases like cancer, leukemia etc.
Steps for safe disposal of nuclear wastes. Presently nuclear wastes are being disposed off in thick concrete underground storage silos.

Q2. What do you mean by nuclear energy? How is energy produced in the sun? Give atleast two reactions to show the process of energy production in the sun. What would happen if hydrogen in the sun is exhausted.

Ans2. Nuclear energy is the energy released by the fission of heavier nuclei of an atom into lighter nuclei or by the fusion of lighter nuclei into heavier nuclei. For example:When U-235 is bombarded with slow neutrons, it splits into Barium and Krypton along with the release of three neutrons and tremendous energy. When hydrogen nuclei fuse together, a helium nucleus is formed and in this process also large amount of energy is released.
The energy produced in the sun and other stars is by the process of nuclear fusion. Hydrogen nuclei after combining with each other form a helium nucleus and large amount of energy is released. To be specific, in the sun there are two processes for the fusion called Carbon-Nitrogen cycle and proton-proton cycle responsible for the release of tremendous energy.

1H1 + 1H1 ----> 1H2 + +1e0 + Neutrino + Energy
1H1 + 1H2 ----> 2He3 + 0n1 + Energy

Reactions given above are the fusion reactions taking place in the interior of the sun.
If the hydrogen in the sun is exhausted then the nuclear fusion process will stop and the temperature of the sun would decrease greatly.

Q3. Explain how are nuclear wastes generated and why do they pose a threat to the environment? What steps are taken for the safe disposal of these wastes?

Ans. (Try yourself).

Q4. What do you mean by nuclear energy? How is energy produced in the sun? Give atleast two reactions to show the process of energy production in the sun. What would happen if hydrogen in the sun is exhausted.

Ans. (Try yourself).

Q5. What do you mean by isotopes of an element? Write the name of all the isotopes of hydrogen and explain why these isotopes have almost identical chemical properties? Write two reactions showing the fusion reaction between these nuclei.

Ans. (Try yourself).

Q6. What do you mean by a nuclear reaction? How is it different from a chemical reaction? Give two points of difference between these reactions. Write two nuclear reactions.

Ans. (Try yourself).