CIE IGCSE Physics (0625) Energy Study Notes - New Syllabus
CIE IGCSE Physics (0625) Topic 1.7.1 Energy Study Notes
Key Concepts:
Core
- State that energy may be stored as kinetic, gravitational potential, chemical, elastic (strain), nuclear, electrostatic and internal (thermal)
- Describe how energy is transferred between stores during events and processes, including examples of transfer by forces (mechanical work done), electrical currents (electrical work done), heating, and by electromagnetic, sound and other waves
- Know the principle of the conservation of energy and apply this principle to simple examples including the interpretation of simple flow diagrams
Supplement
- Recall and use the equation for kinetic energy Ek = ½mv²
- Recall and use the equation for the change in gravitational potential energy ΔEp = mgΔh
- Know the principle of the conservation of energy and apply this principle to complex examples involving multiple stages, including the interpretation of 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.