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
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.