ICSE 7 Physics Energy Basic Notes

1.0 Work and Energy

In common language, "work" refers to any mental or physical activity. However, in Physics, Work has a very precise definition involving force and movement. Closely related to work is Energy, which provides the "capacity" to perform that work.

Definition of Work

Work is said to be done only when a force applied on a body makes the body move through a certain distance in the direction of the force.

SI Unit: Joule ($J$)
CGS Unit: erg ($1\,J = 10^7\,erg$)

Mathematical Expression

$$W = F \times s$$

Where: $W$ = Work done, $F$ = Force applied, $s$ = Displacement

1.1 The Concept of Energy

A body capable of doing work is said to possess energy. Thus, Energy is the capacity to do work. The amount of energy possessed by a body is equal to the total work it can perform. Because of this direct link, the units of Energy are the same as the units of Work (Joules).

Fig: Work being done by applying Force

⚠️ No Displacement = No Work:

If you push against a wall with all your strength but the wall doesn't move, in the language of Physics, the Work Done is ZERO. This is because $s = 0$, making $W = F \times 0 = 0$.

📝 Solved Example:

A girl applies a force of 50 N to push a trolley through a distance of 10 m. Calculate the work done by the girl.

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Solution:
1. Given Force (F): $50\,N$
2. Given Displacement (s): $10\,m$
3. Formula: $W = F \times s$
4. Calculation: $W = 50 \times 10 = 500\,J$
Final Answer: The work done is $500\,Joules$.

💡 Did You Know?

The unit Joule is named after the English physicist James Prescott Joule, who proved that heat is a form of energy and established the relationship between mechanical work and heat.

2.0 Kinetic Energy (K.E.)

Mechanical energy is divided into two primary forms. The first is Kinetic Energy, which is the energy possessed by a body by virtue of its state of motion. Every moving object, from a speeding bullet to a walking person, carries kinetic energy.

Factors Affecting K.E.

The amount of kinetic energy in a moving body depends on two factors:

• Mass ($m$): Heavier objects have more K.E. than lighter ones moving at the same speed.
• Velocity ($v$): Faster objects have significantly more K.E. than slower ones of the same mass.

Kinetic Energy Formula

$$K.E. = \frac{1}{2}mv^2$$

Where: $m$ = mass of the body (in kg), $v$ = velocity of the body (in m/s)

Fig: Kinetic Energy in Moving Objects

⚠️ The "Square" Effect:

Notice that velocity is squared in the formula. This means if you double the speed of a car, its kinetic energy doesn't just double—it becomes four times ($2^2$) greater! This is why high-speed accidents are so destructive.

📝 Solved Example:

A ball of mass 0.5 kg is moving with a velocity of 4 m/s. Calculate its kinetic energy.

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Solution:
1. Given Mass ($m$): $0.5\,kg$
2. Given Velocity ($v$): $4\,m/s$
3. Formula: $K.E. = \frac{1}{2}mv^2$
4. Calculation:
$K.E. = \frac{1}{2} \times 0.5 \times (4)^2$
$K.E. = 0.25 \times 16$
$K.E. = 4\,J$
Final Answer: The kinetic energy of the ball is $4\,Joules$.

💡 Did You Know?

Windmills take the kinetic energy of moving air (wind) and convert it into mechanical energy to grind grain or pump water, or into electrical energy using a generator.

3.0 Potential Energy (P.E.)

Unlike kinetic energy, which requires motion, Potential Energy is "stored" energy. It is the energy possessed by a body by virtue of its position or configuration (shape). When you lift an object or stretch a rubber band, you are doing work that gets stored as potential energy.

3.1 Gravitational Potential Energy

This is the energy stored in an object due to its height above the ground. The higher you lift an object, the more work you do against gravity, and the more gravitational P.E. it acquires.

Gravitational P.E. Formula

$$P.E. = mgh$$

Where: $m$ = mass (kg), $g$ = acceleration due to gravity ($9.8\,m/s^2$), $h$ = height (m)

3.2 Elastic Potential Energy

This is the energy stored in a body when its shape is changed by stretching, compressing, or bending. When the deforming force is removed, this stored energy helps the body return to its original shape.

• Example 1: A stretched bow string possesses elastic P.E. that is transferred to the arrow.
• Example 2: A wound-up spring in a toy car.

Fig: Examples of Gravitational and Elastic Potential Energy

⚠️ Reference Level:

Gravitational P.E. is usually measured from the ground level, where $h = 0$ and $P.E. = 0$. However, it is relative. If you move an object from a table to a shelf, you only calculate the change in height between the two surfaces.

📝 Solved Example:

A brick of mass 2 kg is lifted to the top of a wall which is 5 m high. Calculate its potential energy. (Take $g = 10\,m/s^2$ for ease of calculation).

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Solution:
1. Given Mass ($m$): $2\,kg$
2. Given Height ($h$): $5\,m$
3. Gravity ($g$): $10\,m/s^2$
4. Formula: $P.E. = mgh$
5. Calculation: $P.E. = 2 \times 10 \times 5 = 100\,J$
Final Answer: The potential energy is $100\,Joules$.

💡 Did You Know?

Hydroelectric dams use gravitational potential energy! Water stored at a great height behind the dam is released to fall on turbines, converting its potential energy into kinetic energy, and eventually into electricity.

4.0 Transformation of Energy

Energy is a dynamic quantity. It doesn't just sit still; it constantly changes from one form to another. This process is called Transformation of Energy. While the form changes, the total amount of energy remains constant.

4.1 Law of Conservation of Energy

This is one of the most fundamental laws in Physics. It states that:

"Energy can neither be created nor destroyed; it can only be transformed from one form to another. The total energy of an isolated system remains constant."

4.2 Common Energy Interconversions

In our daily lives, we use devices that act as energy converters:

• Electric Bulb: Electrical Energy $\rightarrow$ Light and Heat Energy.
• Electric Motor: Electrical Energy $\rightarrow$ Mechanical Energy.
• Photosynthesis: Light Energy $\rightarrow$ Chemical Energy.
• Battery/Cell: Chemical Energy $\rightarrow$ Electrical Energy.
• Microphone: Sound Energy $\rightarrow$ Electrical Energy.

Fig: Conservation of Mechanical Energy in a Pendulum

Total Mechanical Energy

$$E_{total} = K.E. + P.E.$$

In the absence of friction, $E_{total}$ remains constant at every point of motion.

⚠️ Where does the "lost" energy go?

When a ball stops rolling, its kinetic energy isn't destroyed. It is transformed into Heat Energy and Sound Energy due to friction against the ground and air. We call this "dissipation" of energy.

📝 Solved Example:

A body of mass 5 kg is dropped from a height of 10 m. Calculate its kinetic energy just before it hits the ground. (Take $g = 10\,m/s^2$)

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Solution:
1. According to the Law of Conservation of Energy:
   Loss in Potential Energy = Gain in Kinetic Energy.
2. Initial P.E. at height (10m): $mgh = 5 \times 10 \times 10 = 500\,J$.
3. At the top: $K.E. = 0$ (since body is at rest).
4. Just before hitting ground: All P.E. converts to K.E.
Final Answer: The kinetic energy just before hitting the ground is $500\,Joules$.

💡 Did You Know?

The Sun is the ultimate source of almost all energy on Earth. Fossil fuels like coal and petroleum are actually stored solar energy from plants and animals that lived millions of years ago!

5.0 Sources of Energy

As the global demand for energy increases, we must distinguish between the different sources available to us. These are broadly classified into Renewable and Non-renewable sources based on their availability and environmental impact.

5.1 Non-Renewable Sources

These are sources that have accumulated over millions of years and cannot be replaced once they are used up. They are often referred to as Conventional Sources.

• Fossil Fuels: Coal, petroleum, and natural gas. They release greenhouse gases when burnt.
• Nuclear Energy: Energy released during nuclear fission of elements like Uranium. While it doesn't emit smoke, it produces radioactive waste.

5.2 Renewable Sources

These are inexhaustible sources that are naturally replenished at a rate faster than they are consumed. They are also known as Non-conventional or Green Energy.

• Solar Energy: Energy from the Sun captured using solar cells or heaters.
• Wind Energy: Kinetic energy of wind used to rotate turbines.
• Hydro Energy: Energy of falling or flowing water.
• Biomass: Energy obtained from organic matter like wood and agricultural waste.

Fig: Classification of Energy Sources

⚠️ Global Concern:

Burning fossil fuels increases the concentration of Carbon Dioxide ($CO_2$) in the atmosphere, leading to the Greenhouse Effect and Global Warming. This is why the world is shifting toward renewable energy.

5.3 Energy Degradation

While the total energy remains constant (as per the Law of Conservation), in every transformation, some energy is converted into a non-useful form (mostly heat due to friction). This "waste" energy cannot be easily converted back into useful work. This process is called Degradation of Energy.

📝 Solved Example:

Why is solar energy considered superior to energy from coal, even though coal provides more energy per kg?

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Solution:
1. Sustainability: Solar energy is renewable and will never run out, whereas coal is non-renewable.
2. Environment: Solar energy is "clean" as it produces no harmful gases ($CO_2$, $SO_2$) or ash, whereas burning coal causes significant air pollution and global warming.

💡 Did You Know?

Geothermal Energy is the heat energy stored inside the Earth. In some places, this heat escapes as steam through hot springs, which can be used to generate electricity!