Year 10 Higher Physics: Energy Stores and Systems Study Notes

Lesson Overview: Energy Stores and Systems

This unit covers the fundamental concepts of energy within the Year 10 Combined Higher Physics curriculum. The primary focus of this lesson is identifying how energy is stored and how it moves within a system according to physical laws.

Lesson Objectives

By the end of this lesson, a student should be able to:

• Recall the eight different energy stores.

• Identify specific energy stores within a given system.

• Describe the Law of Conservation of Energy.

• Apply the Law of Conservation of Energy to various energy transfer examples.

Defining Systems and Energy

In physics, a system is defined as an object or a group of objects. Systems are dynamic and constantly undergoing changes. Because we live in a world of change, the energy stored within these systems changes as well. Energy is not a single "thing" but is stored in various ways and can be transferred between different stores when a system changes.

The Eight Energy Stores

There are eight primary ways that energy can be stored in a system. Each store is defined by the physical state or position of the objects involved.

1. Gravitational Potential Energy

This energy is stored by objects that are raised above the ground. The amount of energy stored depends on the height of the object relative to the ground.

2. Elastic Potential Energy

This energy is stored when objects are physically deformed, specifically when they are squashed or stretched.

3. Electrostatic Energy

This energy is stored when charged particles build up on an object.

4. Kinetic Energy

All moving objects possess kinetic energy. This applies regardless of the size of the object, ranging from the motion of microscopic atoms to the movement of entire planets.

5. Magnetic Energy

Any object that possesses a magnetic field stores energy in this manner.

6. Nuclear Energy

This is the energy stored within the nucleus of atoms. This energy is released during nuclear reactions.

7. Chemical Energy

This energy is stored in the chemical bonds between particles. It is released when particles undergo a chemical reaction and those bonds are broken.

8. Heat (Thermal) Energy

All hot objects store this energy. The hotter an object is, the more energy it stores in its thermal store.

The Law of Conservation of Energy

The fundamental principle governing all energy transfers is the Law of Conservation of Energy.

Definition: Energy cannot be created or destroyed; it can only be transferred from one store to another.

This means that the total amount of energy in a closed system remains constant throughout any process.

$\text{Total Energy Before Transfer} = \text{Total Energy After Transfer}$

Transfer Visualization

When a system changes, energy moves from Energy Store 1 to Energy Store 2.

• Example 1 (Falling Apple): As an apple falls from a tree, its store of Gravitational Potential energy decreases while its Kinetic energy store increases. If the apple has $100\,J$ of Gravitational Potential energy at the top, it will have transferred $100\,J$ to other stores (primarily kinetic) by the time it hits the ground.

• Example 2 (Burning Fuel): When fuel is burnt, Chemical energy is transferred into Heat energy. If the chemical store decreases by $100\,J$, the thermal energy of the surroundings/system will increase by a total of $100\,J$ (this may be split across multiple objects, such as $50\,J$ to the air and $50\,J$ to a container).

Practical Examples of Energy Stores

To identify energy stores, consider the state of the object in the following examples:

• Petrol/Fuel: Chemical energy.

• A speeding bullet: Kinetic energy.

• A climber stood on a mountain: Gravitational Potential energy.

• Food (e.g., a chocolate bar): Chemical energy.

• A bar magnet: Magnetic energy.

• A Bunsen burner: Heat energy (Thermal).

• A compressed or stretched spring: Elastic Potential energy.

• Uranium (Nuclear fuel): Nuclear energy.

• Hair attracted to a charged balloon: Electrostatic energy.

Energy Transfer Scenarios

Burning a Candle

• Decreasing Store: Chemical (as the bonds in the wax react).

• Increasing Store: Heat (Thermal energy released into the surroundings).

Battery Powered Fan

If a battery starts with $200\,J$ of energy:

• Decreasing Store: Chemical energy in the battery.

• Increasing Store: Kinetic energy as the fan blades spin.

• End State: When the battery "dies," it stores $0\,J$. The full $200\,J$ has been transferred from the battery to the fan and its surroundings.

Catapult or Sling Shot

• Action: Stretching the band.

• Transfer: As the sling shot is released, the Elastic Potential energy store decreases and the Kinetic energy store of the projectile increases.

Throwing a Rocket Upwards

• Action: A toy rocket is thrown into the air.

• Explanation: As the rocket moves upwards, it slows down. This means the Kinetic energy store is decreasing. Simultaneously, because the rocket is gaining height, its Gravitational Potential energy store is increasing. Because energy cannot be created or destroyed, the total energy remains constant: all Kinetic energy lost is transferred into Gravitational Potential energy (ignoring air resistance).

Quantitative Energy Calculations

Using the Law of Conservation of Energy, we can calculate energy values based on the principle that the total energy remains the same.

1. Falling Ball: If a ball has $30\,J$ of Gravitational Potential energy at the top, it will have exactly $30\,J$ of Kinetic energy just before it hits the ground (assuming no energy is lost to heat/sound).

2. Camp Stove: A stove stores $35,000\,J$ of Chemical energy. When used to boil water, it will transfer $35,000\,J$ of Thermal energy to the water and surroundings.

3. Bungee Jumper Calculation:

   • At the platform, the jumper has $46,400\,J$ of Gravitational Potential energy.

   • Halfway down the cliff, the jumper has $34,400\,J$ of Kinetic energy.

   • To find the Elastic Potential energy in the cord:     $\text{Total Energy} = \text{Kinetic} + \text{Elastic Potential}$     $46,400\,J - 34,400\,J = 12,000\,J$

   • Therefore, the Elastic Potential energy stored is $12,000\,J$.

Summary of Transfers in Different Systems