Mix Examples - Sources of Energy Questions in English

(A) In our homes,energy is mainly utilised in two forms:
$1$. Electrical energy: Used for operating appliances like lights,fans,refrigerators,and televisions.
$2$. Thermal energy (Heat energy): Used for cooking food,heating water,and room heating.

(N/A) Renewable sources of energy are those that can be replenished naturally over a short period of time. Two examples of renewable sources of energy are:
$1$. Solar energy: Energy derived from the sun.
$2$. Wind energy: Energy derived from the movement of air.

(N/A) Two activities from our daily life in which solar energy is utilized are:
$1$. Drying of food grains and clothes.
$2$. Obtaining salt from sea water through the process of evaporation.

(N/A) The two materials commonly used to make solar cells are $Silicon$ and $Selenium$. $Silicon$ is the most widely used semiconductor material for manufacturing photovoltaic cells due to its abundance and efficiency. $Selenium$ is also used in specific types of solar cells.

(B) To generate useful energy from a windmill,the wind must blow at a minimum velocity of $15\,km\,h^{-1}$.
If the wind speed is lower than this threshold,the windmill blades will not rotate with sufficient force to drive the turbine effectively for power generation.

(N/A) Solar energy manifests itself in oceans primarily in the following two forms:
$1$. Ocean Thermal Energy: This is caused by the temperature difference between the warm surface water and the colder deep water of the ocean.
$2$. Tidal Energy: This is caused by the gravitational pull of the Moon and the Sun on the Earth's oceans,which creates the rise and fall of sea levels.

(B) Biogas is a mixture of gases produced by the breakdown of organic matter in the absence of oxygen. The primary component of biogas is $CH_4$ (Methane),which typically accounts for $50-75\%$ of its composition.

(N/A) The two examples of fossil fuels are coal and petroleum. These are formed from the remains of ancient plants and animals that were buried under the Earth's crust for millions of years.

(N/A) $LPG$: Liquefied Petroleum Gas
$CNG$: Compressed Natural Gas

(N/A) Solar cells are used in various fields due to their ability to convert solar energy into electricity. Four such areas are:
$(i)$ Artificial satellites and space probes.
$(ii)$ Radio or wireless transmission systems in remote areas.
$(iii)$ Traffic signals and street lighting.
$(iv)$ Research centres and calculators.

(B) sailing boat utilizes the kinetic energy of the wind to move across the water. The wind exerts force on the sails,which is converted into mechanical work to propel the boat.

(B) In a semiconductor,the energy gap between the valence band and the conduction band is small. When light (photons) falls on the semiconductor,the electrons in the valence band absorb energy and jump to the conduction band. This process increases the number of free charge carriers (electrons and holes),which leads to an increase in the electrical conductivity of the semiconductor.

(B) Geothermal energy is the heat energy generated and stored within the Earth.
This heat originates from the original formation of the planet and the radioactive decay of materials.
It is harnessed by tapping into steam or hot water reservoirs found beneath the Earth's surface to generate electricity or for direct heating purposes.

(B) Black surfaces are excellent absorbers of heat radiation.
By painting the inside of solar heating devices black,they can absorb the maximum amount of solar heat energy,which increases the efficiency of the device in heating water or air.

(B) The glass sheet acts as a transparent cover that allows short-wavelength infrared radiation from the sun to enter the solar heating device. Once inside,this radiation is absorbed by the internal components,which then re-emit it as long-wavelength infrared radiation. The glass sheet is opaque to these long-wavelength rays,preventing them from escaping. This phenomenon is known as the greenhouse effect,which helps in trapping heat inside the device.

(B) The greenhouse effect is a natural process that warms the Earth's surface.
When the Sun's energy reaches the Earth's atmosphere,some of it is reflected back to space and the rest is absorbed and re-radiated by greenhouse gases.
The confinement of these heat radiations by the Earth's atmosphere,which prevents them from escaping into space,thereby raising the global temperature,is called the greenhouse effect.

(N/A) spherical reflector (such as a concave mirror) can focus parallel rays of light onto a single point (the focus). This concentration of solar energy allows it to generate high temperatures,reaching up to $1500 ^{\circ}C$,which is not possible with a plane reflector.

(A) In a solar water heater,copper tubes are used because copper is an excellent conductor of heat. By increasing the number of coils,the total surface area of the copper tube exposed to solar radiation increases. This allows the water flowing through the tubes to absorb more heat energy from the sun,resulting in hotter water.

(N/A) solar cell is a device that converts incident solar light energy directly into electrical energy in $DC$ form.
It is also known as a solar photovoltaic cell ($PV$ cell).

(A) solar cell is a device that converts solar energy directly into electricity. The most common material used for manufacturing solar cells is $Silicon$ $(Si)$. Sometimes,other semiconductor materials like $Gallium$ $(Ga)$ (often in the form of $Gallium$ $Arsenide$) are also used to increase efficiency.

(B) modern solar cell typically converts about $15\%$ to $20\%$ of the incident solar energy into electricity. While some high-efficiency research cells can reach higher values, the standard commercial efficiency range is generally considered to be around $15-20\%$.

(B) box-type solar cooker uses the greenhouse effect to trap solar radiation.
When placed in direct sunlight for $2$ to $3$ hours,the internal temperature of the cooker typically reaches a range of $100^{\circ}C$ to $140^{\circ}C$,which is sufficient for cooking most food items.

(B) solar panel is a device that consists of a large number of solar cells connected in a specific arrangement (series or parallel) to generate a higher amount of electricity. These cells are mounted on a frame to capture sunlight and convert it into electrical energy.

(D) Solar cells are semiconductor devices that convert sunlight directly into electricity.
They are widely used in artificial satellites and space probes because they provide a reliable and long-lasting source of power in space where other conventional energy sources are unavailable.
Additionally,they are also used in calculators,watches,and street lighting.

(N/A) The two main merits of solar cells are:
$1$. They provide a clean and pollution-free source of energy as they do not emit any harmful gases.
$2$. They are noiseless in operation and require very low maintenance,making them a safe and sustainable energy option.

(N/A) $(i)$ Hydroelectricity is a clean and renewable source of energy that does not cause pollution.
$(ii)$ This energy can be stored in high-level reservoirs and utilized as per requirement,unlike wind or solar energy which are intermittent.

(B) Tidal energy is a form of hydropower that converts the energy obtained from tides into useful forms of power,mainly electricity.
It is generated due to the gravitational pull of the Moon and the Sun on the Earth's oceans,which causes a periodic rise and fall in the water level.
The difference in the levels of ocean water between high-tide and low-tide is harnessed to drive turbines.

(B) Ocean thermal energy is the energy available due to the difference in temperature between the water at the surface of the ocean and the water at deeper levels.
In this process,the warm surface water is used to boil a volatile liquid like ammonia,and the resulting vapor is used to run a turbine to generate electricity.
The cold water from the deeper layers is then pumped up to condense the vapor back into a liquid,completing the cycle.

(B) Geothermal energy is the heat energy originating from the Earth's core.
It is stored in the form of hot water springs (geysers) and molten rock,known as magma,located deep beneath the Earth's surface.
When this magma reaches the surface,it is called lava,which can cause volcanic eruptions.

(B) The fuels used in atomic power plants are known as nuclear fuels. These materials,such as $U^{235}$ (Uranium-$235$) or $Pu^{239}$ (Plutonium-$239$),undergo nuclear fission reactions to release a large amount of energy,which is then converted into electricity.

(N/A) The two examples of fossil fuels are:
$(i)$ Coal
$(ii)$ Petroleum
$(iii)$ Natural gas

(A) Natural gas is primarily composed of $CH_4$ (Methane), typically ranging from $70\%$ to $90\%$. Biogas is produced by the anaerobic decomposition of organic matter and also consists mainly of $CH_4$ (Methane), usually between $50\%$ and $75\%$. Therefore, $CH_4$ (Methane) is the common gas present in both.

(N/A) Gaseous fuels are preferred because:
$(i)$ They can be transported easily through pipelines.
$(ii)$ They leave no ash on burning.
$(iii)$ They burn completely and cause less pollution than solid fuels.

(A) Coal is the primary fossil fuel that currently provides the largest share of the world's energy needs,particularly for electricity generation and industrial processes.

(N/A) $(i)$ Coke is used as a reducing agent in metallurgical processes for the extraction of metals.
$(ii)$ Coke is used in the manufacture of fuel gases such as water gas $(CO + H_2)$ and producer gas $(CO + N_2)$.

(B) In a biogas plant,the slurry that remains after the anaerobic digestion of biomass is rich in nitrogenous and phosphorus compounds. Because of this high nutrient content,it serves as an excellent organic manure for crops and improves soil fertility.

(A) The largest component of biogas is methane $(CH_4)$.
It constitutes approximately $50 \%$ to $75 \%$ of the total volume of biogas produced.

(A) In a nuclear reactor,energy is produced by the process of nuclear fission.
In this process,a heavy nucleus,such as $U-235$,is bombarded with neutrons,causing it to split into smaller nuclei and releasing a large amount of energy.

(B) Currently,nuclear power plants generate electricity using the process of nuclear fission. In this process,the nucleus of a heavy atom (such as uranium-$235$ or plutonium-$239$) is bombarded with low-energy neutrons,causing it to split into lighter nuclei. This reaction releases a significant amount of energy in the form of heat,which is used to produce steam to drive turbines for electricity generation.

(B) $CNG$ (Compressed Natural Gas) is considered an environmentally friendly fuel because it burns more efficiently than traditional fossil fuels like petrol and diesel.
It produces significantly lower amounts of carbon monoxide,nitrogen oxides,and particulate matter during combustion.
Furthermore,it does not produce smoke or toxic gases,making it a cleaner alternative for reducing air pollution.

(B) The process responsible for the massive energy production in the sun is $Nuclear \ fusion$. In this process,lighter nuclei,primarily hydrogen atoms,fuse together at extremely high temperatures and pressures to form heavier nuclei,such as helium,releasing a tremendous amount of energy in the form of heat and light.

(C) good fuel is defined by several characteristics,but the most practical and important characteristic for widespread use is that it should be economical and easily available. Other essential properties include having a high calorific value,moderate ignition temperature,and producing minimal pollution.

(B) The glass plate allows short-wavelength solar radiation to enter the solar cooker but prevents long-wavelength infrared radiation (heat) from escaping.
This phenomenon is known as the greenhouse effect.
Consequently,the temperature inside the solar cooker increases significantly,which helps in cooking the food efficiently.

(N/A) The chief component of solar cells is $Silicon$.
Solar cells convert solar energy (light energy) into electric energy.

(N/A) When large areas of forests are submerged due to the construction of dams,the vegetation undergoes anaerobic decomposition under water. This process releases large amounts of methane $(CH_4)$,which is a potent greenhouse gas,thereby contributing to the greenhouse effect and global warming.

(B) Solar cookers are painted black because black surfaces are excellent absorbers of solar radiation. By absorbing the maximum amount of heat energy from the sun,the internal temperature of the cooker increases efficiently,which helps in cooking the food.

(N/A) The primary reason is that the transportation of electricity is significantly more efficient and cost-effective than transporting bulky raw materials like coal or petroleum over the same distance. By locating power plants near the fuel source,the energy loss and logistical costs associated with moving heavy fuel are minimized.

(B) The reaction responsible for the large energy production in the sun is nuclear fusion.
In this process,lighter atomic nuclei,primarily hydrogen,combine to form heavier nuclei,such as helium,releasing a tremendous amount of energy in the form of heat and light.

(N/A) The student may have made the following mistakes in the construction or operation of the solar cooker:
$(i)$ Not using a black-painted surface inside the box to absorb heat radiation.
$(ii)$ Using a plastic sheet instead of a glass sheet,which fails to trap heat effectively due to the greenhouse effect.
$(iii)$ Not using a plane mirror reflector to direct additional sunlight into the box.
$(iv)$ Using a non-black or reflective cooking vessel,which reduces the absorption of heat radiation.

(N/A) Hydrogen is considered a cleaner fuel than $CNG$ because its combustion produces only water vapour $(H_2O)$,which is non-polluting and does not contribute to greenhouse gas emissions or particulate matter. In contrast,$CNG$ (Compressed Natural Gas) contains hydrocarbons,and its combustion releases carbon dioxide $(CO_2)$ and other pollutants. However,hydrogen is not widely used in daily life due to its highly explosive nature,which makes its storage and transportation extremely difficult and hazardous.