CBSE Class 10 Science Advanced Physics Source of Energy Nuclear Fission and Fusion Short Answer Questions Set 01

Short Answer Type

Question 1. What are the disadvantages of fossil fuels?
Answer:
1. They are non-renewable and will be exhausted.
2. Burning them produces carbon dioxide (global warming) and oxides of nitrogen and sulphur that cause acid rain.
3. They cause air pollution and smoke, harming health.
In simple words: They run out and they pollute.

Teacher's Note:
1. Mention both exhaustion and pollution.
2. Two points are enough for two marks.

Question 2. Why are we looking at alternate sources of energy?
Answer:
Fossil fuels are limited and will be exhausted, and burning them pollutes the air and adds to global warming. The demand for energy is also rising. So we need renewable, cleaner sources such as solar, wind, hydro and biogas.
In simple words: Because fossil fuels are limited and polluting.

Teacher's Note:
1. Link demand, shortage and pollution.
2. Name one alternative.

Question 3. What kind of mirror – concave, convex or plane – would be best suited for use in a solar cooker? Why?
Answer:
A concave mirror is best, because it converges the parallel rays of the Sun at its focus, where the vessel is kept, and so produces a very high temperature. (A plane mirror reflects the rays without concentrating them; a convex mirror spreads them.)
In simple words: A concave mirror focuses sunlight on the cooking vessel.

Teacher's Note:
1. Draw the parallel rays meeting at the focus.
2. Convex mirrors diverge light.

Question 4. What is geothermal energy?
Answer:
Geothermal energy is the heat energy of the hot rocks deep inside the earth. Underground water heated by these rocks becomes high-pressure steam, which can turn the turbine of a generator to produce electricity. Places with very hot rocks near the surface are called hot spots.
In simple words: It is the heat stored inside the earth.

Teacher's Note:
1. It does not come from the Sun.
2. Mention hot spots and steam.

Question 5. What are the advantages of nuclear energy.
Answer:
1. A tiny mass of fuel gives an enormous amount of energy (1 kg of uranium-235 gives about 8 × 10¹³ J).
2. It does not produce smoke or carbon dioxide.
3. It gives a steady supply, unlike wind or solar energy.
In simple words: Huge energy from a small mass of fuel, with no smoke.

Teacher's Note:
1. Quote the energy from 1 kg of uranium to impress.
2. Also mention the problem of waste in a separate answer.

Question 6. Describe the principle of solar cell.
Answer:
A solar cell works on the photovoltaic effect. It is made of thin wafers of doped semiconductors (such as silicon). When sunlight falls on it, electrons are set free and a potential difference (about 0.4 V to 0.5 V per cell) is produced between two regions of the semiconductor, so a current flows in an external circuit.
In simple words: Light on a semiconductor produces a voltage.

Teacher's Note:
1. Name the photovoltaic effect.
2. Many cells make a panel.

Question 7. State three advantages of solar cooker?
Answer:
1. It uses free, renewable solar energy.
2. It causes no pollution and needs no fuel.
3. Food cooks slowly without needing attention and nutrients are preserved; it needs no maintenance.
In simple words: It saves fuel and is clean.

Teacher's Note:
1. Any three advantages are acceptable.
2. Cost savings is a strong point.

Question 8. What are the forms of ocean energy that can be harnessed? Give some places in our country where it can be harnessed ?
Answer:
Forms: tidal energy, sea-wave (ocean-wave) energy and ocean thermal energy.
Places in India: the Gulf of Kutch (Gujarat) and the Sundarbans (West Bengal) for tidal energy; coastal areas such as Tamil Nadu and Kerala are possible sites for wave and thermal plants.
In simple words: Tides, waves and ocean heat; for example Kutch and the Sundarbans.

Teacher's Note:
1. The chapter names two tidal sites.
2. Other sites are suggestions.

Question 9. A concentrator type solar heater having a surface area of 4 m² can absorb only 80% of the solar radiations incident on it, while the rest is reflected. Calculate the energy concentrated by the heater at its focus in 3 hrs, if the rate of solar energy incident on it is 0.5 KJ/m²s.
Answer:
Time = 3 h = 3 × 3600 = 10800 s.
Energy incident = 0.5 × 4 × 10800 = 21600 kJ.
The reflected 20% is concentrated at the focus: 21600 × 0.2 = 4320 kJ.
In simple words: Multiply rate, area, time and the 20% that is reflected to the focus.

Teacher's Note:
1. Convert hours to seconds first.
2. The concentrator focuses the reflected part.

Teacher's Note:
1. Write at least four differences in a table.
2. Examples: U-235 for fission, hydrogen for fusion.

Question 11. Define nuclear fusion reaction. Describe the conditions for the occurrence of a nuclear fusion reaction.
Answer:
Definition: a nuclear fusion reaction is one in which two light nuclei combine to form a heavier nucleus, releasing energy (for example d + d → ³He + n + 3.3 MeV).
Conditions: the nuclei must come extremely close and overcome the strong electrostatic repulsion between them. This needs a very high temperature (millions of kelvin, about 10⁶ K or more), at which atoms are completely ionised and matter exists as a plasma. High pressure/density keeps the nuclei close together.
In simple words: Light nuclei fuse at millions of kelvin.

Teacher's Note:
1. Mention plasma and thermonuclear reaction.
2. Quote the temperature.

Question 12. What are the advantages of nuclear energy ?
Answer:
1. Very large amount of energy from a small mass of fuel.
2. No carbon dioxide or smoke is produced, so it does not add to global warming.
3. Nuclear power plants give continuous power.
In simple words: Huge, clean and continuous energy.

Teacher's Note:
1. This question also appears in the board-level exercise.
2. Give at least two points.

Question 13. What measures we can take to minimize hazards caused by nuclear reactors?
Answer:
1. Use thick concrete and lead shielding to stop radiations.
2. Store and dispose of radioactive waste safely in deep, sealed underground sites.
3. Use control rods and strict safety systems to avoid accidents and leaks.
4. Regular monitoring and protective clothing for workers, and careful site selection.
In simple words: Shielding, safe waste disposal and strict control.

Teacher's Note:
1. Safety measures are for radiation and waste.
2. Control rods absorb extra neutrons.

Question 14. It is said that energy from nuclear fusion would create fewer pollution problems than energy from fission. Can you give reason, why?
Answer:
Fission of uranium produces many radioactive fission products (nuclear waste) that stay dangerous for thousands of years. Fusion of hydrogen isotopes gives helium, which is harmless, and leaves no long-lived radioactive waste. So fusion would cause fewer pollution problems.
In simple words: Fission leaves radioactive waste; fusion gives harmless helium.

Teacher's Note:
1. Mention radioactive waste.
2. Fusion fuel (hydrogen) is plentiful.

Question 15. 48 kJ of energy is produced in 60 s in a nuclear reactor. Find the number of fissions which would be taking place per second if the energy released per fission is 3.2 x 10^–11 J.
Answer:
Energy per second = 48000 J ÷ 60 s = 800 J/s.
Number of fissions per second = 800 ÷ (3.2 × 10^–11) = 2.5 × 10¹³.
In simple words: Divide the power by the energy per fission.

Teacher's Note:
1. Convert kJ to J first.
2. Check the unit: J/s ÷ J = per second.