Conservation of energy Revision Notes

Energy Conservation vs. Dissipation

The law of conservation of energy states that energy cannot be created or destroyed, only transferred or transformed. However, in any real-world system change, not all of the transferred or transformed energy remains in a 'useful' store. Some energy always spreads out, becoming less concentrated and harder to harness for work. This process is known as energy dissipation.

What is 'Less Useful' Energy?

Energy is considered 'less useful' when it is dissipated, typically as low-grade thermal energy (heat) or sound, into the surroundings. Once dissipated, this energy is spread out among a vast number of particles in the environment, making it effectively impossible to gather it back up and use it to do the original intended work. For instance, the heat generated by a constantly running computer or a refrigerator warming the kitchen is dissipated energy.

Examples of Energy Dissipation

When a portable music player operates, chemical energy from its battery is converted into electrical energy, which then powers the components. Some of this electrical energy is converted into useful sound energy, but a portion is dissipated as heat, warming the device. Another example is a bouncing ball: With each bounce, some of its kinetic and gravitational potential energy is dissipated as heat (due to air resistance and deformation of the ball and surface) and sound, which is why it eventually stops bouncing.

Increasing Disorder (Entropy)

Energy dissipation is linked to the concept of increasing disorder, or entropy. When useful energy transforms into dispersed thermal energy, it increases the overall randomness and disorder of the system and its surroundings. This is a fundamental principle of physics and explains why systems naturally tend towards states of higher entropy, meaning 'less useful' energy stores are the inevitable outcome of energy transfers.

What makes mechanical processes wasteful?

Mechanical processes become wasteful when useful energy transforms into less useful forms, primarily thermal energy. This often happens due to friction between moving parts or air resistance. This increase in thermal energy causes a rise in temperature of the process components and their surroundings, which is a key indicator of energy dissipation.

Energy Dissipation and Efficiency

Energy dissipation refers to the spreading out of thermal energy into the environment, making it harder to recover and use. This 'wasted' energy contributes to heating the surroundings. While the total energy in a closed system remains constant (conservation of energy), the useful output energy decreases, thereby reducing the efficiency of the mechanical process.

Examples of Wasteful Mechanical Processes

Common examples include the heating of bicycle chains due to friction, the warmth generated by a car engine, or the heat produced by an electric motor. In all these cases, a portion of the input energy is converted into thermal energy, which then dissipates into the environment, contributing to the inefficiency of the system.

Understanding Efficiency

Efficiency is the ratio of useful energy output to the total energy input, expressed as a decimal or percentage. All energy conversions result in some wasted energy, usually in forms that are not useful for the system's purpose, such as heat or sound. The goal of increasing efficiency is to minimise these energy losses.

Methods to Increase Efficiency

Key strategies for improving efficiency involve reducing energy transfer to undesirable forms. This includes:

1. Reducing friction: Using lubricants (e.g., oil, grease) in moving parts, or choosing materials with smooth surfaces.
2. Reducing air resistance (drag): Streamlining the shape of objects moving through fluids (air or water).
3. Reducing electrical resistance: Using thicker, shorter wires, or materials with higher conductivity.
4. Insulation: Preventing unwanted heat transfer into or out of a system (e.g., double-glazing, loft insulation, vacuum flasks).
5. Maintaining equipment: Regular maintenance ensures components operate optimally and prevents efficiency degradation over time.

Impact of Increased Efficiency

Improving efficiency directly leads to several benefits. It means less energy is required to perform the same amount of useful work, which conserves energy resources and reduces fuel consumption. This often translates to lower running costs and a reduced environmental impact due to decreased emissions associated with energy generation.

Understanding Kinetic Energy (KE)

Kinetic energy is the energy an object possesses strictly because it is in motion. If an object is not moving, its kinetic energy is zero. This energy is a scalar quantity, meaning it has magnitude but no direction.

The Kinetic Energy Formula

The kinetic energy of an object can be calculated using the formula: KE = ½ × m × v², where KE is kinetic energy in joules (J), m is mass in kilograms (kg), and v is speed in metres per second (m/s). It's crucial to use the correct SI units for all quantities before calculation.

Impact of Mass and Speed

Both mass and speed influence kinetic energy. Doubling the mass of an object will double its kinetic energy. However, doubling the speed of an object will quadruple its kinetic energy because speed is squared in the formula, making it a much more significant factor.

Units in Kinetic Energy Calculations

Always ensure mass is in kilograms (kg) and speed is in metres per second (m/s) before applying the formula. If given in grams or kilometres per hour, convert them first. The resulting kinetic energy will be in joules (J).