Energy Sources and Transfer of Energy - Physics For Class IX (Science Group) - Question Answers

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Physics For Class IX (Science Group)
UNIT 8 ENERGY SOURCES AND TRANSFER OF ENERGY
Questions Answers

Q.1: Define force?
Ans: FORCES:
Force is a push or pull. It moves the objects. It stops the objects. It gives shape to the objects.
Definition:

"Force is an agent which tends to change the state of an object."
OR
"Force is the agent that changes the state of rest or uniform motion of a body."

Nature: It is a vector quantity. Therefore, it has both magnitude (size) and specific direction. It is denoted by 'F'
Formula: 

F=ma

Where,
F is force applied on a body
m is mass of a body and
a is acceleration of a body

Q.2(a): Define work and its SI unit and write down its formula Or Calculate work done using equation Work = force x distance moved in the direction of force. Also write down the factors on which work depends?
Ans: WORK:
In physics work has a proper meaning i.e.
“work is done only when a force makes something to move."
Definition:
Thus work can be define as:

"The amount of work is the product of force and the distance moved in the direction of force."

Nature:
It is a scalar quantity and it is denoted by 'W'.

Units of Work:
The S.I unit of work is Joule other units of work can be Foot, Pound, Erg.

1 Joule = 1Nm

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

W = Fd cosθ ................ (i)

The force “F” however 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, i.e. Suppose a constant force “F” acts on a body.
Work = (component of force)· (distance)

W = (F cosθ) d

or W = (F cosθ) d

If θ = 0 ⇒ cosθ = 1

then Work = W= Fd ....... (ii)

Factors On Which Works Depends:

1. Work is directly proportion to the force applied to the body.
2. Work is directly proportion to the displacement of the body in the direction of the force.

Q.2(b): Write down some examples of work in daily life?
Ans: Generally, work refers to perform some task or job.
For example:

• A tailor stitching a suit.
• A shopkeeper selling fruits at his shop.
• A women cooking in her kitchen are all considered as “doing work” in daily life.

Q.3: Define energy? What is the SI unit of energy?
Ans: ENERGY:
Energy is define as:

"The ability to do work."

UNIT:
The S.I unit of energy is joule (J).

Q.4: Define kinetic energy? Write down its formula and units? Also write down the factors on which kinetic energy depends?
Ans: KINETIC ENERGY:
Kinetic energy of a body defined as:

"Energy possessed by an object due to its motion is called kinetic energy."

OR

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.

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

Expression For Kinetic Energy:
Mathematically kinetic energy is given as:

K.E = ½ mv² ........ (i)

Factors On Which Kinetic Energy Depends:
As we know that kinetic energy is due to the motion of object. Therefore for an object of mass 'm' moving with speed 'v' kinetic energy depends upon:

• The mass 'm' of the object:- the greater the mass, the greater its K.E.
• The speed 'v' of the object:- the greater the speed, the greater the K.E.

Examples Of Kinetic Energy:

• Sailing boat
• Moving air
• Driving car
• Running and
• Walking are example of kinetic

Q.5: Derive the relation K.E = ½ mv²
Ans: Derivation of the Equation, K.E = ½ mv²:
To obtain an expression for K.E 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:

W = F . S ...... (i)

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

F = ma

And by using the third equation of motion i.e.:

v_f² - v_i² = 2aS

When

v_i = 0,

v_f = V and

S = ?

therefore,

v² - 0 = 2aS

Q.6: Define potential energy? Write down its formula (expression) and units? Also define types of potential energy?
Ans: POTENTIAL ENERGY:
Definition:
Potential energy of a body is defined as:

"The energy that a body possesses by virtue of its position, shape or state of a system."

OR

It is also defined as:

"The work done stored in a body in lifting it to a height “h”."

The potential energy changes only when its position relative to ground changes; otherwise it remains same.

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

Expression For Potential Energy:
Mathematically potential energy is given as:

P∙E = mgh ........ (i)

Where,
'm' is the mass of an object
'g' is acceleration due to gravity
'h' is height

Examples Of Potential Energy:

1. A book lying on the table have potential energies.
2. The water stored in a dam have potential energies.

Types Of Potential Energy:
There are different types of potential energy, Like:

• gravitational potential energy,
• elastic potential energy and
• chemical potential energy:

For Example:

• A body raised to a height “h” above the ground has gravitational potential energy.
• A stretched spring has elastic potential energy due to its stretched position (condition).
• The energy stored in the plants that we eat is chemical potential energy.

Q.7: 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 up through height “h” from the ground. The work done in lifting it to height “h” is stored in it as its gravitational potential energy “P∙E”, i.e.

P∙E = Work done

P∙E = W

We know that W = F.d, therefore

P∙E = F ∙ d

We also know that F = mg (weight), therefore

P.E = (mg) . d

Here d = h (height), therefore

P∙E = (mg) ∙ h

Hence, equation becomes:

P∙E = mgh ........... (i)

Q.8: Name the different forms of energy?
Ans: Forms Of Energy:
There are many forms of energy. Some of them are:

• Kinetic energy
• Potential energy
• Gravitational energy
• Electrical energy
• Sound Energy
• Fossil Fuel energy
• Hydroelectric energy
• Chemical energy
• Heat energy
• Nuclear energy
• Geothermal energy
• Wind energy
• Biomass energy
• Tidal energy etc.

Q.9: State the law of conservation of energy?
Ans: CONVERSION OF ENERGY:
Energy neither be created nor it can be destroyed but it can be converted from one form to other form. This is called law of conservation of energy.

Q.10: Describe the different forms of energy? OR Describe the processes by which energy is converted from one form to another with reference to fossil fuel energy, Hydroelectric generation, solar energy, nuclear energy, geothermal energy, wind energy, biomass energy and tidal energy.
Ans: Forms Of Energy:
i. Fossil Fuel Energy:
Fossil fuel energy is formed from decayed plants and animals that have been converted to crud oil, coal, natural gases or heavy oils by exposure to heat and pressure in the Earth's crust over hundreds of millions of years.
Fossils fuels have stored chemical energy. This energy is converted by oxidation through burning. Thus on burning a fossil fuel like charcoal, produce heat energy and light energy.

ii. Hydroelectric Energy:
Hydro electricity is the term referring to electricity generated by hydro power by using gravitational force of falling or flowing water.
Most common type of hydro electric power plants uses a dam on a river to store water in a reservoir. Water releases 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. There are many devices which 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 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:
The energy released during a nuclear reaction such as 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 utilize 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 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.
Geothermal well can be built by drilling deep near hot rocks at different places, where hot molten or 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 purpose or to generate electricity.

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

• It is being used as source of energy for sailing ships in oceans.
• It is being used by wind mills to pump water.
• It is being used by wind mills to grind grain.
• 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 Sun, garbage, wastes, sugarcane etc. Solid biomass, such as wood, organic material and garbage, can be burned directly to produce heat.
Biomass can also converted into gas called biogas and into liquid biofuels such as ethanol and biodiesel.

viii. Tidal Energy:
It is a form of hydro power that converts the energy obtained from tides into 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.11: What are renewable energy source and non-renewable energy source?
Ans: RENEWABLE ENERGY SOURCE:

• The renewable sources can be consumed and used again and again.
• E.g.: Solar energy, wind energy, tidal energy and geothermal energy are renewable sources.
• Since very earlier 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 ENERGY SOURCE:

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

Q.12: What is the difference between renewable and non-renewable energy sources?
Ans: Difference Between Renewable And Non-renewable Energy Sources
Following are major differences between renewable and non-renewable resources:

S.NO.	Renewable Energy Sources	Non-renewable Energy Sources
Depletion	Renewable resources cannot be depleted over time.	Non-renewable resources deplete over time.
Sources	Renewable resources include sunlight, water, wind and also geothermal sources such as hot springs and fumaroles.	Non-renewable energy includes fossil fuels such as coal and petroleum.
Environmental Impact	Most renewable resources have low carbon emissions and a low carbon footprint.	Non-renewable energy has a comparatively higher carbon footprint and carbon emissions.
Cost	The upfront cost of renewable energy is high. For instance, generating electricity using technologies running on renewable energy is costlier than generating it with fossil fuels.	Non-renewable energy has a comparatively lower upfront cost.
Infrastructure Requirements	Infrastructure for harvesting renewable energy is prohibitively expensive and not easily accessible in most countries.	Cost-effective and accessible infrastructure is available for non-renewable energy across most countries.
Area Requirements	Requires a large land/ offshore area, especially for wind farms and solar farms.	Comparatively lower area requirements.

Q.13: Define input and output?
Ans: Every machine needs some energy to perform work.
Input: Whatever energy given to a machine is called input.
Output: The work done by the machine is called output.
For example: We give electric energy as input to the electric motor in washing machines and in drilling machines.

Q.14: Define efficiency? Write down its expression. OR Define efficiency of a working system and calculate the efficiency of an energy.
Ans: EFFICIENCY:
A system in which some energy ‘E₁’ is supplied to it as ‘input’ and the system returns back some energy ‘E₂’ 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”.

Unit:
As it is the ratio of two energies therefore it has no unit.

Expression:
No machine is 100% efficient because some energy is always wasted in the form of heat, sound or light etc. Therefore efficiency of any machine is can be calculated as:

Q.15: Define power? write down its expression and units?
Ans: POWER:
When we run up and cover distance in 5 seconds or take slow walk up the same distance in 20 seconds. We are doing the same amount of work, However, we are doing it at different rate. When we run up, we are working much faster and we have a higher power then 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:

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

Unit of Power
In SI system unit of power is:

Thus SI 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.

Larger Units Of Power Are:
Kilo watt (kW), Mega watt (MW), Horse Power (hp) etc.

• 1 kW = 1000 W = 10³ watt
• 1 MW=1000000 W=10⁶Watt
• 1 hp = 746 Watt