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CIE IGCSE Physics (0625) Energy Study Notes

CIE IGCSE Physics (0625) Energy Study Notes - New Syllabus

CIE IGCSE Physics (0625) Energy   Study Notes

LEARNING OBJECTIVE

  • Understanding the concepts of Energy  

Key Concepts: 

  • Forms of Stored Energy &Energy Transfers Between Stores
  • Kinetic Energy & Potential Energy
  • Principle of Conservation of Energy
  • Sankey diagrams

CIE iGCSE Physics (0625)  Study Notes – All topics

Forms of Stored Energy &Energy Transfers Between Stores

Forms of Stored Energy:

Kinetic Energy: Energy of a moving object.

    • Example: A moving car, a thrown ball, or a spinning fan.

Gravitational Potential Energy: Energy stored due to an object’s height in a gravitational field.

    • Example: Water held behind a dam, or a book on a shelf.

Chemical Energy: Energy stored in chemical bonds, released during chemical reactions.

    • Example: Batteries, fuel, and food.

Elastic (Strain) Energy: Energy stored when an object is stretched or compressed.

    • Example: A stretched spring or compressed rubber band.

Nuclear Energy: Energy stored in the nucleus of atoms, released in nuclear reactions (fission or fusion).

    • Example: Nuclear power plants and atomic bombs.

Electrostatic Energy: Energy stored due to the position of charged objects in an electric field.

    • Example: Energy between two opposite charges or in a charged capacitor.

Internal (Thermal) Energy: The total kinetic and potential energy of the particles in a substance.

    • Example: Boiling water, heated metal, or steam in an engine.

Energy Transfers Between Stores:

Energy is not created or destroyed – it is transferred from one store to another through different processes.

Types of Energy Transfers:

1. Mechanical Work (Forces): When a force moves an object, energy is transferred.

    • Example: A person pushing a trolley – chemical energy from muscles is transferred to kinetic energy of the trolley.
    • Example: A falling object – gravitational potential energy is transferred into kinetic energy.

2. Electrical Work (Electric Currents): When a current flows, electrical energy is transferred between stores.

    • Example: A kettle – electrical energy is transferred to thermal energy in the water.
    • Example: A bulb – electrical energy is transferred to light and thermal energy.

3. Heating: Energy transferred from a hotter object to a colder one due to temperature difference.

    • Example: Heating a metal rod – thermal energy flows from the hot end to the cold end.
    • Example: A hot spoon placed in water – thermal energy transfers to the water by conduction.

4. Radiation (Electromagnetic Waves): Energy can be transferred through empty space by radiation.

    • Example: The Sun transferring thermal and light energy to Earth by infrared and visible light.
    • Example: A microwave oven – electromagnetic waves transfer energy to the food.

5. Sound Waves: Energy transferred by vibrating particles through a medium.

    • Example: A speaker cone vibrates – electrical energy is converted into sound energy through air vibrations.
    • Example: A drum being hit – kinetic energy is transferred into sound energy and some thermal energy.

Example:

An electric kettle is used to boil water. Describe the energy transfers taking place from the moment it is switched on.

▶️ Answer/Explanation

Step 1: Initial energy store

Energy starts in the chemical store of fuel (in a power station or battery) or the kinetic store of turbines if renewable.

Step 2: Electrical transfer

Energy is transferred by an electric current (electrical work) to the kettle’s heating element.

Step 3: Heating

The electrical energy is transferred to the thermal store of the heating element, and then by heating to the internal (thermal) store of the water.

Step 4: Final store

The water’s temperature rises – its internal energy increases, storing more thermal energy.

Waste energy:

Some energy is also lost to the surroundings as thermal energy (heating the air) and a small amount as sound energy.

Chemical store → (electric current) → thermal store (element) → (heating) → thermal store (water)

Kinetic Energy & Potential Energy

Kinetic Energy:

  • Kinetic energy is the energy possessed by a moving object due to its motion.
  • It depends on both the mass of the object and its speed.

 

Equation for Kinetic Energy:

\( KE = \dfrac{1}{2}mv^2 \)

  • \( KE \) = kinetic energy (joules, J)
  • \( m \) = mass of the object (kg)
  • \( v \) = speed of the object (m/s)

Key Point: Since velocity is squared, even small increases in speed lead to large increases in kinetic energy.

Example:

Calculate the kinetic energy of a 3 kg ball moving at 4 m/s.

▶️ Answer/Explanation

Step 1: Write the formula

\( KE = \dfrac{1}{2}mv^2 \)

Step 2: Substitute the values

\( KE = \dfrac{1}{2} \times 3 \times 4^2 = \dfrac{1}{2} \times 3 \times 16 = 24 \, \text{J} \)

Final Answer: \(\boxed{24 \, \text{J}}\)

Gravitational Potential Energy (GPE):

Gravitational potential energy is the energy stored in an object due to its height in a gravitational field.

  • Raising an object increases its GPE; lowering it decreases the GPE.

Equation for Change in Gravitational Potential Energy:

\( \Delta GPE = mgh \)

  • \( \Delta GPE \) = change in gravitational potential energy (Joules, J)
  • \( m \) = mass (kg)
  • \( g \) = gravitational field strength (usually \( 9.8 \, \text{m/s}^2 \) on Earth)
  • \( h \) = change in height (m)

Key Point: GPE increases when an object is lifted and decreases when it falls (assuming no energy lost to friction or air resistance).

Example:

A 2 kg object is lifted to a height of 5 meters above the ground. Calculate the increase in gravitational potential energy.

▶️ Answer/Explanation

Step 1: Use the formula

\( \Delta GPE = mgh \)

Step 2: Substitute the values

\( \Delta GPE = 2 \times 9.8 \times 5 = \boxed{98 \, \text{J}} \)

The object’s GPE increases by 98 joules.

Principle of Conservation of Energy

Principle of Conservation of Energy:

  • Energy cannot be created or destroyed; it can only be transferred from one store to another, or transformed from one form to another.
  • Total energy input = total energy output (assuming no loss to surroundings in an ideal case).

Real-world Consideration:

  • In practical systems, some energy is always “lost” or dissipated – usually as thermal energy (heat) or sound.

Useful Energy Output + Wasted Energy = Total Energy Input

Example:

A filament bulb is powered by a battery. 100 J of electrical energy is supplied. Only 10 J is converted into light.

▶️ Energy Flow Diagram

Electrical energy input = 100 J

Light (useful): 10 J

Heat (wasted): 90 J

 Total Output = 10 J + 90 J = 100 J

Example :

A ball of mass 1 kg is dropped from a height of 5 m. Calculate its speed just before it hits the ground, assuming no energy is lost.

▶️ Answer/Explanation

Step 1: Find initial GPE

\( \text{GPE} = mgh = 1 \times 9.8 \times 5 = 49 \, \text{J} \)

Step 2: Use energy conservation

All GPE converts to KE: \( KE = 49 \, \text{J} \)

Step 3: Use KE formula to find speed

\( \dfrac{1}{2}mv^2 = 49 \Rightarrow \dfrac{1}{2} \times 1 \times v^2 = 49 \)

\( v^2 = 98 \Rightarrow v = \sqrt{98} = \boxed{9.9 \, \text{m/s}} \)

Conclusion: Energy transferred from GPE to KE with no loss, demonstrating conservation of energy.

Conservation of Energy – Extended Principle:

  • In a complex process, energy may transfer through multiple stores and pathways, but the total energy is always conserved.
  • At each stage, energy may be:
    • usefully transferred to another store
    • dissipated (usually as heat, sound, etc.)
  • Sankey diagrams are used to visually represent energy flow, including both useful and wasted energy.

Key Principle:

Total Input Energy = Total Useful Output Energy + Total Wasted Energy

Example:

A petrol engine receives 2000 J of chemical energy from fuel combustion. It transfers energy through several stages:

  • 700 J to kinetic energy of the car (useful)
  • 500 J lost as heat from the engine
  • 300 J lost due to friction and vibration (sound)
  • 500 J lost through exhaust gases
▶️ Answer/Explanation

Total Input: 2000 J (Chemical energy)

Total Output:

  • Useful: 700 J (Kinetic)
  • Wasted: 500 + 300 + 500 = 1300 J

Check: 700 + 1300 = 2000 J 

Efficiency of the engine:

\( \text{Efficiency} = \dfrac{700}{2000} \times 100 = \boxed{35\%} \)

Sankey Diagram :

This shows how energy is distributed and where inefficiencies occur.

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