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

Long Answer Type

Question 1. Can any source of energy be pollution-free? Why or why not?
Answer:
No source of energy can be completely pollution-free. Even clean sources have some effect: solar cells need materials made in factories, dams disturb the ecology and silt deposit, windmills need land and cause noise, nuclear plants create radioactive waste, and biogas plants need dung handling. Some sources, such as solar and wind, produce no gaseous pollutants while working, but their manufacture and installation have an environmental cost. So we can only reduce pollution, not remove it.
In simple words: Every source has some cost to the environment, though some are much cleaner.

Teacher's Note:
1. Give examples for at least three sources.
2. Separate pollution while using from pollution while making.

Question 2. Hydrogen has been used as a rocket fuel. Would you consider it a cleaner fuel than CNG? Why or why not?
Answer:
Yes. Hydrogen burns to give only water, whereas CNG (mostly methane) gives carbon dioxide and water, and a little carbon monoxide if burning is incomplete. Hydrogen also has the highest calorific value (150 kJ/g, against about 55 kJ/g for methane). So hydrogen is cleaner, but it is costly and difficult to store and handle safely.
In simple words: Hydrogen gives only water; CNG gives carbon dioxide.

Teacher's Note:
1. Write the products of burning.
2. Mention the practical drawbacks.

Question 3. Name two energy sources that you would consider to be renewable. Give reasons for your choices.
Answer:
Solar energy: the Sun will keep supplying energy for billions of years by fusion of hydrogen, so it cannot be exhausted.
Wind energy: the Sun keeps heating the earth unequally and so wind blows again and again; the supply is replenished continuously.
In simple words: They are replaced by nature all the time.

Teacher's Note:
1. Give a separate reason for each source.
2. Hydro or biomass are alternatives.

Question 4. Give the names of two energy sources that you would consider to be exhaustible. Give reasons for your choices
Answer:
Coal and petroleum are exhaustible. They formed from the remains of plants and animals buried for millions of years, and we are using them much faster than they form, so the stock will be finished one day.
In simple words: They take millions of years to form and are being used up quickly.

Teacher's Note:
1. Fossil fuels are the standard answer.
2. Uranium is also limited.

Teacher's Note:
1. Write a table with similarities and differences.
2. State at least one similarity.

Teacher's Note:
1. Make at least three comparisons.
2. State the common feature too.

Question 7. What are the limitations of extracting energy from the wind?
Answer:
1. Wind is not available at all places and at all times, and no place has wind all the time.
2. Wind speed changes with time and season and cannot be predicted.
3. Windmills and sailboats do not work unless the wind is fast enough (at least about 15 km/h for windmills).
4. The kinetic energy of the wind can be used only at the site, and large wind farms need a lot of land.
5. Setting up wind turbines is costly.
In simple words: Wind is irregular, unpredictable and location-bound.

Teacher's Note:
1. Give four or five clear points.
2. Wind speed minimum is 15 km/h.

Question 8. What are the quantities of an ideal source of energy?
Answer:
An ideal source of energy should:
1. give a large amount of useful energy,
2. be easy to store in a small space and easy to transport,
3. supply energy at a steady rate,
4. be cheap and easily available,
5. be safe to handle and cause little pollution, and be renewable.
In simple words: It should be plentiful, convenient, steady, cheap and clean.

Teacher's Note:
1. Write the answer as a numbered list.
2. The word quantities here means qualities.

Question 9. What are the advantages and disadvantages of using a solar cooker? Are there places where solar cookers would have limited utility?
Answer:
Advantages: it uses free and renewable energy; no smoke or pollution; saves fuel and money; food keeps its nutrients and does not burn.
Disadvantages: it cannot be used at night or in cloudy weather; cooking is slow in a box type cooker; the mirror must be turned to face the Sun; food cannot be fried or chapatis made in the box type.
Limited utility: in places that are cloudy or foggy for long periods, in polar regions or high latitudes with weak sunlight, and in very windy or dusty places.
In simple words: A solar cooker is free and clean but depends on sunshine.

Teacher's Note:
1. Cover all three parts of the question.
2. Give a reason for the limited-use places.

Question 10. What are the environmental consequences of the increasing demand for energy? What steps would you suggest to reduce energy consumption?
Answer:
Consequences: burning more fossil fuels releases more carbon dioxide (global warming, climate change), oxides of sulphur and nitrogen (acid rain) and smoke (air pollution); coal mining and oil drilling damage land and sea; dams and nuclear plants disturb ecology and create waste; the fossil fuel stock is depleted faster.
Steps to reduce consumption: 1. switch off lights and appliances when not in use; 2. use energy-efficient appliances (CFL, LED, star-rated); 3. use public transport or car-pool and walk or cycle for short distances; 4. use solar heaters, cookers and other renewable sources; 5. insulate buildings and use natural light and air; 6. use pressure cookers and cover pans while cooking; 7. spread awareness.
In simple words: More energy use means more pollution and faster exhaustion; saving energy and using renewables helps.

Teacher's Note:
1. Write two clear parts: effects and steps.
2. A five-mark answer needs at least five points.

Question 11. You are given two solar cookers, one with a plane mirror as reflector and the other with concave mirror as reflector. Which one is more efficient? Give reason for your answer. State one more use of concave mirror. [3 Marks/CBSE 10TH BOARD: 2013]
Answer:
The solar cooker with the concave mirror is more efficient. A concave mirror converges the parallel rays of the Sun at its focus, where the vessel is placed, so the temperature rises much higher and the cooking is faster. A plane mirror only reflects the rays without concentrating them, so the temperature is lower.
Another use of a concave mirror: as a shaving mirror (or a dentist's mirror, or headlight reflector, or in a telescope).
In simple words: A concave mirror concentrates the heat at one point.

Teacher's Note:
1. Mention the focus.
2. Give one other use for the third mark.

Question 12. Why is it not possible to make use of solar cells to meet all our energy needs? State at least three reasons to support your answer. [3 Marks/CBSE 10TH BOARD: 2014]
Answer:
1. Solar cells work only in the daytime and in sunshine; they give little or no electricity at night or on cloudy days.
2. They are very costly because very pure semiconductors such as silicon are needed and silver is used for connecting wires.
3. Their efficiency is low (only about 10% to 15% of solar energy is converted), so very large areas of panels are needed.
4. They give d.c., so for a.c. devices it must be converted, which lowers the efficiency; batteries to store the electricity are also costly.
In simple words: Solar cells are costly, inefficient and depend on sunshine.

Teacher's Note:
1. Give three clear, separate reasons.
2. Cost and low efficiency are the most important.

Question 13. Explain the principle and process of converting ocean thermal energy into electricity [3 Mark/CBSE 10TH BOARD: 2016]
Answer:
Principle: the surface water of the ocean is warmer than the deep water. A plant (OTEC) uses this temperature difference, which should be at least 20 °C (the deep water about 1000 m down), to run a heat engine.
Process: 1. Warm surface water boils a liquid with a low boiling point, such as ammonia, in the evaporator (heat exchanger). 2. The high-pressure vapour turns a turbine connected to a generator, which produces electricity. 3. Cold water from the depths cools the vapour in the condenser, changing it back to liquid. 4. A pump sends the liquid back to the evaporator and the cycle repeats.
In simple words: A liquid is boiled by warm surface water, the vapour runs a turbine, and cold deep water condenses it.

Teacher's Note:
1. Draw the ammonia cycle diagram if time permits.
2. Mention the 20 °C difference.