ENERGY SOURCES AND TRANSFER OF ENERGY CHAPTER # 08 PHYSICS 9TH - Short / Detailed Question Answers

 PHYSICS 9TH

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PHYSICS 9TH - Short / Detailed Question Answers
ENERGY SOURCES AND TRANSFER OF ENERGY
CHAPTER # 08

Q.1: Define force.

Ans:
Force is an agent which tends to change the state of an object.

Q.2: Define work. Write its formula and unit. Write down the factors on which work depends.

Ans: Work:
Work is done only when a force makes something move.

Illustration:
Demonstration of work done (shows a person pushing a box with force applied over a distance)

Thus, work can be defined as:
The amount of work is the product of force and the distance moved in the direction of the force.

Unit of Work:
The S.I. unit of work is Joule. Other units of work can be Foot, Pound, Erg.
1 Joule = 1 Nm

Formula:
Suppose a constant force “F” acts on a body, and motion takes place in a straight line in the direction of the force. Then, work done is equal to the product of the magnitude of force “F” and the distance “d” through which the body moves.

$$\text{Work done} = \text{Force} \times \text{Distance} W = F \cdot d$$

The force “F” may not act in the direction of motion of the body; instead, it makes some angle “θ” with it. In that case, we define the work by the force as the product of the component of the force along the line of motion and the distance “d” the body moves along that line.

Suppose a constant force “F” acts on a body:

$$\text{Work} = (\text{component of the force}) \times (\text{distance})$$ $$W = (F \cos \theta) d$$

If $\theta = 0 \Rightarrow \cos \theta = 1$
then $\text{Work} = W = Fd$

Illustrations:

• Force making some angle θ with the direction of motion.

Factors on which work done depends:

There are two factors on which work depends:

1. The work is directly proportional to the force applied to the body.
2. The work is directly proportional to the displacement of the body in the direction of the force.

Q.3: Write down the names of any three units of work.

Ans:
(i) Joule
(ii) Erg
(iii) Horsepower-hour
(iv) Foot-pound
(v) Kilowatt-hour

Q.4: At what angle between force and displacement will the work done by a body be maximum?

Ans:
The work done by a body will be maximum at an angle of $0^\circ$.

Q.5: Define energy. What is the S.I. unit of energy?

Ans:
Energy:
Energy is defined as the ability to do work.

S.I. Unit of Energy:
The S.I unit of energy is the joule (J).

Q.6: Name the different forms of energy.

Ans:
Forms of Energy:
There are many forms of energy. Some of them are:

1. Kinetic energy
2. Potential energy
3. Electrical energy
4. Sound energy
5. Chemical energy
6. Heat energy
7. Nuclear energy, etc.

Q.7: Define kinetic energy. Write its formula and unit. Write down the factors on which kinetic energy depends.

Ans:
Kinetic Energy:
The kinetic energy of a body is defined as the energy possessed by an object due to its motion. It is also defined as "The work required to accelerate a body of a given mass from rest to its stated velocity." A moving body maintains its kinetic energy unless its speed changes.

S.I. Unit of Kinetic Energy:
The S.I. unit of kinetic energy is the joule.

Expression for Kinetic Energy:
Mathematically, kinetic energy is given as:

$$\text{K.E.} = \frac{1}{2} m v^2$$

The factors on which kinetic energy depends:
As we know that kinetic energy is due to the motion of an object. Therefore for an object of mass $m$ moving with speed $v$, kinetic energy depends upon:

1. The mass $m$ of the object – the greater the mass, the greater its kinetic energy.
2. The speed $v$ of the object – the greater the speed, the greater the kinetic energy.

Q.8: Derive the relation, $\text{K.E.} = \frac{1}{2} mv^2$.

Ans:
Derivation of the Equation, $\text{K.E.} = \frac{1}{2} mv^2$:
To obtain an expression for kinetic energy, we have to determine the work done by the body in motion. This work is equal to the kinetic energy of the body. Consider a body of mass ‘m’ placed on a horizontal surface initially at rest. When a force ‘F’ is applied, it covers a distance ‘S’ and its final velocity becomes ‘v’. Then work done is:

But by the second law of motion, when a force acts on a body, it produces acceleration in the direction of the force.

And by using the third equation of motion, we know:

When $v_i = 0$, $v_f = v$, and $S = ?$

Putting the values of ‘F’ and ‘S’ in equation (1), we get:

Q.9: Define potential energy and its types. Write its expression and unit.

Ans:
Potential Energy:
The energy that a body possesses by virtue of its position, shape, or state of a system is called potential energy.

It is also defined as the work done stored in a body by lifting it to a height “h”. The potential energy changes only when its position relative to the ground changes; otherwise, it remains the same.

Examples:
A book lying on the table and the water stored in a dam have potential energies.

Types of Potential Energy:
There are different types of potential energy, such as gravitational potential energy, elastic potential energy, and chemical potential energy.

Illustrations:

1. Gravitational Potential Energy: A body raised to a height "h" above the ground has gravitational potential energy.
2. Elastic Potential Energy: A stretched spring has elastic potential energy due to its stretched position (condition).
3. Chemical Potential Energy: The energy stored in plants that we eat is chemical potential energy.

Unit of Potential Energy:
The S.I. unit of potential energy is Joule (J).

Expression of Potential Energy:
Mathematically, potential energy is given as:

Q.10: Define Gravitational Potential Energy. Derive its equation, P.E = mgh.

Ans:
Gravitational Potential Energy:
The potential energy possessed by a body in the gravitational field is called the gravitational potential energy.

Derivation of Gravitational Potential Energy, P.E = mgh:
To derive the expression for gravitational potential energy, let us consider an object of mass “m” which is raised through height “h” from the ground. The work done in lifting it to height “h” is stored in it as its gravitational potential energy “RE”, i.e.,

We know that $W = F \cdot d$, therefore

We also know that $F = mg$, hence

Here $d = h$ (height), therefore

Therefore, the equation becomes

Q.11: Give the energy changes when a ball is dropped from a height of 7m to the ground.

Ans:
Potential Energy of an Object:
The potential energy of an object at some height with respect to gravity is:

where

• $\text{P.E}$ is the initial potential energy in joules (J),
• $m$ is the mass of the object in kg-mass,
• $g$ is the acceleration due to gravity (9.8 m/s²),
• $h$ is the height above the ground in meters.

When the object reaches the ground, $h = 0$, and thus the final potential energy is:

Note: In reality, there is still a gravitational force on the object at the surface of the Earth, so the object has gravitational potential energy at that point. But since the object cannot go anywhere, we say its P.E. from gravity is zero.

Kinetic Energy of Falling Object:
Kinetic energy (K.E) is the energy of motion. Since the object is not moving at the initial position, the initial K.E is:

Once the object is released, it accelerates downward. When the object reaches the ground, its kinetic energy is:

K.Ef​=21​mvf2​

where $\text{K.E}_f$ is the kinetic energy at the ground in joules (J) and $v_f$ is the downward velocity of the object at the ground in m/s.

Total Energy for Falling Object:
The total energy of the object is:

$$\text{T.E} = \text{P.E} + \text{K.E}$$

The total energy is a constant value, provided no external forces besides gravity act on the object. Thus, the initial total energy equals the final total energy:

$$\text{T.E}_i = \text{T.E}_f$$ $$\text{P.E}_i + \text{K.E}_i = \text{P.E}_f + \text{K.E}_f$$

When the object is simply dropped,

$$mgh + 0 = 0 + \frac{1}{2} m v_f^2$$ $$mgh = \frac{1}{2} m v_f^2$$

Final Velocity for Falling Object:
From that equivalence, we can determine the final velocity of the dropped object. Divide by $m$ and multiply by 2:

$$v_f^2 = 2gh$$ $$v_f = \sqrt{2gh}$$

Summary: Potential energy with respect to gravity is $\text{P.E} = mgh$. When the object is dropped, thrown downward, or projected upward, its kinetic energy becomes $\text{K.E} = \frac{1}{2} mv^2$, along with a factor of the initial velocity. The sum of the potential energy (P.E.) and kinetic energy (K.E.) is the total energy (T.E.), which is a constant. Equating the initial total energy with the final total energy, we can determine the final velocity of the object.

Q.12: State the law of conservation of energy.

Ans:
Law of Conservation of Energy:
Energy can neither be created nor destroyed, but it can be converted from one form to another form. This is called the law of conservation of energy.

Q.13: Describe the different forms of energy.

Ans:

(i) Fossil Fuel Energy:
Fossil fuel energy is formed from decayed plants and animals that have been converted to crude oil, coal, natural gases, or heavy oils by exposure to heat and pressure in the Earth’s crust over hundreds of millions of years.
Fossil fuels have stored chemical energy. This energy is converted by oxidation through burning. Thus, burning a fossil fuel like charcoal produces heat energy and light energy.

(ii) Hydroelectric Energy:
Hydroelectricity is the term referring to electricity generated by hydropower by using the gravitational force of falling or flowing water.
The most common type of hydroelectric power plant uses a dam on a river to store water in a reservoir. Water released from the reservoir flows through a turbine, spinning it, which in turn runs a generator to produce electricity.

(iii) Solar Energy:
The energy radiated from the sun is known as solar energy. This is the most available source of energy throughout Pakistan. Many devices are capable of absorbing solar energy, which is then converted into electrical energy or heat energy. These devices may be photovoltaic solar panels and solar cells which convert the sun’s rays into electricity for different uses. Also, solar heaters are used to convert solar energy “sun rays” into heat energy to heat water tanks and indoor spaces.

(iv) Nuclear Energy:
This energy is released during a nuclear reaction, such as a fission or fusion reaction. All radioactive materials store nuclear energy, for example, Uranium, Radium, etc. It is released from the nucleus in the form of radiation in addition to heat and light. A nuclear power plant utilizes nuclear energy to produce steam to turn a turbine and generate electricity.

(v) Geothermal Energy:
Geothermal energy is stored in the Earth as its natural heat. Deep in the Earth, there is a hot molten part called magma. Water, close to magma, changes to steam due to high temperature. This thermal energy is conducted to the surface of the Earth. This energy is called geothermal energy.

A geothermal power plant utilizes geothermal energy to drive an electrical generator.
A geothermal well can be built by drilling deep near hot rocks at different places, where hot molten magma is very close; water is then pushed down into the well. The rocks quickly heat the water and change it into steam. The steam is used for heating purposes or to generate electricity.

(vi) Wind Energy:
The energy obtained by the wind is called wind energy. It is generated by windmills. A windmill consists of a turbine that rotates due to wind. Kinetic energy is produced due to the motion of the turbine. Wind turbines convert this kinetic energy into mechanical power. A generator converts that mechanical power into electricity.

Application:
(a) It is being used as a source of energy for salting ships in oceans.
(b) It is being used by windmills to pump water.
(c) It is being used by windmills to grind grain.
(d) It is used to turn wind turbines to produce electricity.

(vii) Biomass Energy:
Biomass is the organic material that comes from plants and animals. Biomass consists of stored energy from the Sun, garbage, wastes, sugarcane, etc. Solid biomass, such as wood, organic material, and garbage, can be burned directly to produce heat. Biomass can also be converted into a gas called biogas and into liquid biofuels such as ethanol and biodiesel.

(viii) Tidal Energy:
It is a form of hydropower that converts the energy obtained from tides into a useful form of power, mainly electricity, as the Earth uses the gravitational forces of both the moon and the sun every day to move vast quantities of water around the oceans and seas producing tides, and in this way, energy is produced called tidal energy.

Q.14: What is biomass?

Ans:
Biomass:
Biomass is the organic material that carries from plants and animals. Biomass consists of stored energy from the Sun, garbage, wastes, sugarcane, etc. Solid biomass, such as wood, organic material, and garbage, can be burned directly to produce heat. Biomass can also be converted into a gas called biogas and into liquid biofuels such as ethanol and biodiesel.

Q.15: Write down the name of fossil fuel.

Ans:
Names of fossil fuels are:

1. Coal
2. Natural gas
3. Oil
4. Charcoal

Q.16: Which type of energy is stored deep in the Earth?

Ans:
Geothermal Energy Stored Deep in Earth:
Geothermal energy is stored in the Earth as its natural heat. Deep in the Earth, there is a hot molten part called magma. Water, close to magma, changes to steam due to high temperature. This thermal energy is conducted to the surface of Earth. This energy is called geothermal energy.

A geothermal power plant utilizes geothermal energy to drive an electrical generator. A geothermal well can be built by drilling deep near hot rocks at different places, where hot molten magma is very close; water is then pushed down into the well. The rocks quickly heat the water and change it into steam. The steam is used for heating purposes or to generate electricity.

Q.17: What are renewable energy sources and non-renewable energy sources?

Ans:
Renewable Energy Source:
Renewable sources can be consumed and used again and again. Solar energy, wind energy, tidal energy, and geothermal energy are renewable sources. Since a very early age, people have tried to consume renewable sources of energy for their survival, such as wind and water for milling grain and solar for lighting.

Non-Renewable Sources:
Non-renewable resources are limited and will finish once used. Coal, petroleum, and natural gases are nonrenewable sources. About 150 years ago, scientists invented a new technology to extract energy from the ancient fossilized remains of plants and animals. This super-rich but limited source of energy (coal, oil, and natural gas) replaced wood, wind, and water as the main source of fuel. They are being used at a faster rate than they can be restored and, therefore, cannot be renewed.

Q.18: Write down the names of any three renewable energy sources.

Ans:

1. Solar Energy
2. Wind Energy
3. Tidal Energy
4. Geothermal Energy

Q.19: Write down the name of any three non-renewable energy sources.

Ans:

1. Coal
2. Natural Gas
3. Petroleum

Q.20: Define input and output.

Ans:
Every machine needs some energy to perform work.

• Input: Whatever energy is given to a machine is called input.
• Output: And the work done by the machine is called output.

For example, when we supply electric energy as input to the electric motor in washing machines and drilling machines.

Q.21: Define efficiency. Write down its expression.

Ans:
Efficiency:
A system in which some energy ‘$E_1$’ is supplied to it as ‘input’ and the system returns some energy ‘$E_2$’ as output has some efficiency. This efficiency is defined as:

The ratio of output to the input is called Efficiency. Efficiency is denoted by “η”.

As it is the ratio of two energies, therefore it has no unit. No machine is 100% efficient because some energy is always wasted in the form of heat, sound, or light, etc.

$$\text{Efficiency} = \frac{\text{Energy as output } (E_2)}{\text{Energy as input } (E_1)}$$ $$\eta = \frac{E_2}{E_1} \times 100$$ $$\text{Efficiency} = \frac{\text{Output}}{\text{Input}} \times 100$$

Q.22: Define power. Write its expression and unit.

Ans:
Power:
When we run up and cover the distance in 5 seconds or take a slow walk up the same distance in 20 seconds, we are doing the same amount of work; however, we are doing it at a different rate. When we run up, we are working much faster, and we have higher power than when we walk up.

This quantity that tells us the rate of doing work is called power. Thus, power is defined as:

• "The rate of doing work"
• or "The amount of energy transferred per unit time"

Mathematically:

$$\text{Power} = P = \frac{\text{work done}}{\text{time taken}}$$ $$P = \frac{W}{t}$$

Since work and time are scalar quantities, therefore, power is also a scalar quantity.

Unit of Power:
In S.I. unit of power is:

Thus, S.I. unit of power is watt which is defined as:
The power of a body is said to be one watt if it does work at the rate of one joule per second. Large units of power are Kilowatt (kW), Megawatt (MW), Horse Power (hp), etc.

Q.23: Name the physical quantity which gives the rate of doing work.

Ans:
This quantity that tells us the rate of doing work is called power.

Q.24: Is energy a vector quantity?

Ans:
No, energy can be specified by magnitude alone, and it has nothing to do with the direction. Energy is, therefore, a scalar quantity.

Q.25: Describe the importance of solar energy in Pakistan.

Ans:
Importance of Solar Energy in Pakistan:
Energy triggers economic prosperity. Energy generation is heavily dependent on fossil fuels in Pakistan. Due to the huge population and current progress in industrialization, these sources are not fulfilling the existing energy needs of the country. Meanwhile, they have adverse environmental impacts and are economically unsuitable to electrify remote areas. Consequently, there is a need to look for alternate energy sources. Pakistan is looking for alternate energy.

Solar energy is the best renewable energy option for Pakistan in terms of price, lifespan, operation, and maintenance cost. Fortunately, Pakistan is among those countries in which the sun warms the surface throughout the year and therefore has a strong potential for solar power generation.

Energy plays a key role in the development of modern economies. All human activities i.e., education, health care, agriculture, and employment require energy for proper functioning. A country cannot succeed without proper utilization of energy.

considered the main component of a country’s economy. Pakistan is a developing country. Due to recent development and to support its large population and industry, the country needs a huge amount of energy to keep all things on track. However, there is a shortage of energy supply and the country is in its worst energy crisis. The gap between electricity demand and supply has been increased in the past few years and is highly obvious during the summer season which has resulted in the complete shutdown of power for 10-12 hours in urban areas and 16 to 18 hours in rural areas.

Energy shortage problems not only affect the lives of individuals but also hinder the economic development of the country. All sectors including agricultural, industrial, transport, domestic, and energy generation have been affected severely due to long power outages and caused huge economic loss to the country. The current share of renewable energy is insufficient in the total energy mix of Pakistan. The country fulfills its energy needs by utilizing fossil fuels. Huge dependency on fossil fuels not only has a burden on the national economy but also has led to different environmental hazards like the greenhouse effect, $CO_2$ emissions, global warming, and irregular weather patterns. Moreover, natural resources are being exhausted due to the overuse of fossil fuels. Hence, there is a need to develop a new energy mix with renewable sources of energy. In this new economy, renewable energy sources (wind, solar, and biomass) will be utilized to produce energy, which can decrease the fossil import bill on one hand, and lessen the climatic challenges on the other hand.

Energy demand is increasing by more than 9% annually in Pakistan. It is expected that energy demand will increase 8-fold by 2030 and 20-fold by 2050 in Pakistan.

Advantages of Solar Energy:

1. Solar is an inexhaustible source.
2. It generates ample electricity.
3. It reduces pollution.
4. Sun generates energy. As a result, climate damage reduces.
5. The use of solar energy ensures that fossil fuel prices are lower.