4.2 - Conservation of Energy

The principle of conservation of energy

The principle of conservation of energy states:

Energy can be changed from one form to another, but the total amount of energy does not change. It can never be created or destroyed.

The total amount of energy never changes: the total energy before a process always equals the total energy after it.

Dissipation and wasted energy

• Whenever energy changes form, some energy is always dissipated: it spreads out into the surroundings as a less useful form.
• Dissipated energy is often called wasted energy, because it's no longer in a form you can usefully use.
• It almost always ends up as heat, warming the device and the surrounding air.
• No energy change is perfectly efficient - Some energy is always wasted.
• Wasted energy isn't destroyed - It has simply spread out into the surroundings, making it very difficult to recover and use again.

Example: when a motor lifts a load, most of the input energy changes usefully into the load's gravitational potential energy, but some changes into heat in the motor and surrounding air as dissipated energy.

Dissipation in open and closed systems

A mobile phone (an open system)

• As a phone is used, chemical energy in the battery changes usefully to run the screen and processor.
• Some energy is dissipated as heat to the phone and the surrounding air.
• Because energy leaves the phone into the surroundings, the phone alone is an open system and its total energy decreases.

A sealed flask of soup (a closed system)

• A cold spoon is placed inside an insulated flask of hot soup, and the flask is sealed.
• Heat transfers from the hot soup to the cold spoon, cooling the soup slightly and warming the spoon.
• Because no energy enters or leaves the insulated flask, it is a closed system and the net change in total energy is zero.