ICSE 7 Physics Energy Short Notes

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Table of Contents

    ⚡ Fast Revision: Energy Concepts

    Work and Energy
    • Work: Done only when a force produces motion in its own direction.
    • Energy: The internal capacity or ability of a body to do work.
    • Relationship: Work done on a body is stored as energy; energy spent equals work done.
    Unit Alert

    Work/Energy: W / E | SI Unit: Joule (J)

    CGS Unit: erg | $1\ J = 10^7\ ergs$

    The Work Formula:

    $Work = Force \times Displacement$

    ($W = F \times s$)

    ❌ Common Error:

    Thinking work is done while standing still with a heavy load. Fix: Since displacement ($s$) is zero, the work done is zero.

    Physics Definition of Work

    ⚡ Fast Revision: Mechanical Energy

    Types of Mechanical Energy
    • Potential Energy (P.E.): Energy possessed by a body due to its position or state of rest.
    • Kinetic Energy (K.E.): Energy possessed by a body due to its motion.
    • Mechanical Energy: The total sum of P.E. and K.E. in a system.
    Type Dependent Factors Example
    Gravitational P.E. Mass and Height ($h$) Water stored in a dam
    Elastic P.E. Deformation/Stretch Compressed spring
    Kinetic Energy Mass and Velocity ($v$) A flying bird
    Energy Formulas:

    $P.E. = m \times g \times h$

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

    ❌ Common Error:

    Thinking K.E. doubles if velocity doubles. Fix: Since $K.E. \propto v^2$, doubling the velocity makes the K.E. four times greater.

    Potential vs Kinetic Energy

    ⚡ Fast Revision: Conservation & Transformation

    Law of Conservation of Energy
    • Definition: Energy can neither be created nor destroyed; it only changes from one form to another.
    • Total Energy: The sum of all forms of energy in an isolated system remains constant.
    • Dissipation: In real systems, some energy is "lost" as heat due to friction, but the total remains the same.
    Device Energy Transformation
    Electric Bulb Electrical $\rightarrow$ Light + Heat
    Solar Cell Light $\rightarrow$ Electrical
    Microphone Sound $\rightarrow$ Electrical
    Loudspeaker Electrical $\rightarrow$ Sound
    The Pendulum Case

    Extreme Positions: P.E. is Max | K.E. is 0

    Mean Position: K.E. is Max | P.E. is Min

    ❌ Common Error:

    Thinking energy is "used up" by a machine. Fix: Energy is never used up; it is simply converted into less useful forms like sound or heat.

    Energy Conservation in a Pendulum

    ⚡ Fast Revision: Energy Sources & Power

    Classification of Sources
    • Renewable: Inexhaustible sources that replenish naturally (e.g., Solar, Wind, Hydro).
    • Non-Renewable: Exhaustible sources available in limited quantities (e.g., Coal, Petroleum, Natural Gas).
    • Power: The rate at which work is done or energy is transferred.
    Unit Alert

    Power: P | SI Unit: Watt (W)

    Commercial Unit: Horsepower (hp) | $1\ hp = 746\ W$

    The Power Formula:

    $Power = \frac{Work}{Time}$

    ($P = \frac{W}{t}$)

    Renewable Non-Renewable
    Environment friendly (Low pollution). Major source of pollution and $CO_2$.
    Initial cost is high. Relatively cheaper and easier to use.
    E.g., Biomass, Geothermal. E.g., Nuclear, Fossil fuels.
    ❌ Common Error:

    Confusing Power with Energy. Fix: Energy is how much work you can do; Power is how fast you do it.

    End of Energy Revision
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