AQA GCSE HIGHER PHYSICS - Energy Changes in a System

AQA GCSE Higher Physics - Energy Changes in a System

Introduction:

In this tutorial, we'll explore energy changes in a system, a fundamental concept in physics. We'll focus on the following:

• What is a system?
• Types of energy stores:
• Energy transfers:
• The principle of conservation of energy:
• Sankey diagrams:
• Efficiency:

1. What is a system?

A system is a defined area of space that we are interested in studying. Anything outside the system is considered the surroundings. For example, if we are studying a kettle, the system could be the water inside the kettle, while the surroundings would be the kettle itself, the air, and everything else around it.

2. Types of energy stores:

Energy can be stored in various ways within a system. Here are the common energy stores:

• Thermal energy: The energy stored by particles due to their random motion.
• Kinetic energy: The energy stored by an object due to its motion.
• Gravitational potential energy: The energy stored by an object due to its position in a gravitational field.
• Elastic potential energy: The energy stored by a stretched or compressed object.
• Chemical potential energy: The energy stored in the bonds between atoms and molecules.
• Nuclear energy: The energy stored in the nucleus of an atom.

3. Energy Transfers:

Energy can be transferred between different energy stores within a system or between the system and its surroundings. These transfers can occur through:

• Heating: Energy transferred due to a temperature difference.
• Work done: Energy transferred when a force causes a displacement.
• Radiation: Energy transferred by electromagnetic waves.

4. The principle of conservation of energy:

The principle of conservation of energy states that energy cannot be created or destroyed, only transferred and transformed from one form to another. This means the total energy in a closed system remains constant.

5. Sankey diagrams:

Sankey diagrams are visual representations of energy transfers. They show the flow of energy through a system, with the width of the arrows proportional to the amount of energy transferred.

Example:

          100 J (Chemical potential energy)
                 |
                 |
                 |
           ______|______
           |       |
           |       |
       50 J (Thermal energy)   30 J (Kinetic energy)
           |       |
           |       |
           |______|______
                 |
                 |
                 |
               20 J (Lost as heat) 

This Sankey diagram shows the energy transfers in a car engine. 100 J of chemical potential energy is transferred from the fuel. 50 J is transferred to thermal energy, 30 J to kinetic energy, and 20 J is lost as heat to the surroundings.

6. Efficiency:

Efficiency is a measure of how much useful energy is transferred out of a system compared to the total energy input. It is calculated as:

Efficiency = (Useful energy output / Total energy input) * 100%

Example:

If a light bulb has an efficiency of 20%, it means that only 20% of the electrical energy input is converted to light energy, while the remaining 80% is lost as heat.

Key points to remember:

• Energy is always conserved.
• Energy can be transferred and transformed from one form to another.
• Sankey diagrams are useful for visualizing energy transfers.
• Efficiency is a measure of how effectively energy is converted from one form to another.

Practice problems:

1. A kettle is used to heat 0.5 kg of water from 20°C to 100°C. If the specific heat capacity of water is 4200 J/kg°C, calculate the energy transferred to the water.
2. A car engine has an efficiency of 25%. If the engine produces 100 kJ of kinetic energy, calculate the total energy input to the engine.
3. Draw a Sankey diagram to represent the energy transfers in a hairdryer.

By understanding the concept of energy changes in a system, you can analyze and solve various physics problems related to energy transfer, efficiency, and the conservation of energy.