Energy transfer and conservation
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Lesson notes
Energy Transfer
- **Energy** is the ability to do work or cause change.
- Energy can be **transferred** from one object to another (e.g., a bat hitting a ball transfers kinetic energy).
- Energy can be **transformed** from one form to another (e.g., chemical energy in food becomes kinetic energy when you run).
- Common forms: **kinetic** (motion), **potential** (stored), **thermal** (heat), **chemical**, **electrical**, **sound**, and **light**.
Law of Conservation of Energy
- The **law of conservation of energy** states that energy cannot be created or destroyed, only transformed or transferred.
- In a **closed system**, the total amount of energy remains constant.
- Energy may change form, but the total energy before and after any process is equal.
- Example: In a swinging pendulum, **potential energy** converts to **kinetic energy** and back, with total mechanical energy conserved (ignoring friction).
Kinetic and Potential Energy
- **Kinetic energy (KE)** = ½mv², where m is mass and v is speed.
- **Gravitational potential energy (GPE)** = mgh, where m is mass, g is gravity, and h is height.
- As an object falls, GPE decreases while KE increases, keeping total mechanical energy constant (if no air resistance).
- **Elastic potential energy** is stored in stretched or compressed objects like springs or rubber bands.
Energy Transfer in Collisions
- In **elastic collisions**, kinetic energy is conserved (e.g., two billiard balls).
- In **inelastic collisions**, some kinetic energy is transformed into other forms (e.g., heat, sound), but total energy is conserved.
- Momentum is always conserved in collisions, but kinetic energy may not be.
- Example: A car crash converts kinetic energy into thermal energy (heat) and sound energy.
Energy Transfer in Everyday Systems
- A **light bulb** transforms electrical energy into light and thermal energy.
- A **toaster** converts electrical energy into thermal energy to toast bread.
- **Photosynthesis** transforms light energy from the Sun into chemical energy stored in glucose.
- In a **roller coaster**, GPE at the top converts to KE as it descends, then back to GPE on the next hill.
Mass-Energy Equivalence
- Einstein's equation **E = mc²** shows that mass and energy are equivalent.
- Mass can be converted into energy (e.g., in nuclear reactions) and vice versa.
- This is important in extreme conditions like the Sun's core or nuclear power plants.
- At the middle school level, we focus on energy conservation in everyday situations where mass changes are negligible.
Perpetual Motion Machines
- A **perpetual motion machine** would run forever without an energy source.
- The law of conservation of energy shows such machines are impossible because energy cannot be created from nothing.
- Friction and air resistance always convert some mechanical energy into thermal energy, causing motion to stop.
- No machine can produce more energy than it consumes.
Particle arrangement in solids, liquids, and gases. Energy transfer affects particle motion and state changes.
A simple series circuit showing energy transfer from the cell to the bulb (electrical → light + thermal).
A hydrogen atom. Energy is stored in the electron's position (potential energy) and motion (kinetic energy).
Slides
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Practice questions
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1.Which of the following best describes the law of conservation of energy?
Easy- AEnergy can be created but not destroyed.
- BEnergy can be destroyed but not created.
- CEnergy cannot be created or destroyed, only transformed or transferred.
- DEnergy is always lost as heat in any process.
2.What happens to the total energy in an isolated system over time?
Easy- AIt increases.
- BIt decreases.
- CIt remains constant.
- DIt fluctuates randomly.
3.In the explosion of dynamite, chemical energy is converted into which forms of energy?
Easy- AOnly kinetic energy
- BOnly heat and sound
- CKinetic energy, potential energy, heat, and sound
- DOnly light and heat
4.What is the main difference between elastic and inelastic collisions regarding energy?
Medium- AIn elastic collisions, kinetic energy is conserved; in inelastic collisions, it is not.
- BIn elastic collisions, kinetic energy is not conserved; in inelastic collisions, it is.
- CBoth types conserve kinetic energy.
- DNeither type conserves total energy.
5.Which of the following is an example of energy transformation?
Medium- AA book sitting on a shelf
- BA battery powering a flashlight
- CA rock at the top of a hill
- DA stretched rubber band held in place
6.What does the equation E = mc² represent?
Medium- AEnergy equals mass times the speed of light.
- BEnergy equals mass times the speed of light squared.
- CMass equals energy times the speed of light squared.
- DEnergy equals mass divided by the speed of light squared.
7.Why is a perpetual motion machine of the first kind impossible?
Hard- ABecause friction always stops it.
- BBecause it would violate the law of conservation of energy.
- CBecause it would require an external energy supply.
- DBecause energy can be created from nothing.
8.In nuclear reactions, the mass defect is observed. What does this imply?
Hard- AMass is destroyed.
- BMass is converted into energy.
- CEnergy is converted into mass.
- DBoth mass and energy are lost.
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