2.1 - Energy Stores & Systems

Energy is held in energy stores

• Energy is never used up. Instead it's transferred between objects and held in different energy stores.
• An energy store describes where energy is at a given moment, before or after it's transferred.

The energy stores to know

• Kinetic - Anything moving has energy in its kinetic energy store.
• Thermal (internal) - Any object has energy in this store; the hotter it is, the more energy it has.
• Chemical - Anything that can release energy through a chemical reaction, such as food and fuels.
• Gravitational potential - Anything raised up in a gravitational field, so anything that can fall.
• Elastic potential - Anything stretched or squashed, such as springs and rubber bands.
• Electrostatic - Two charges that attract or repel each other.
• Magnetic (or electromagnetic) - Two objects that exert a magnetic force on each other, such as two magnets.
• Nuclear - Atomic nuclei release energy from this store in nuclear reactions.

Systems

• A system is a single object or a group of objects being studied (such as the air in a piston, or two colliding vehicles).
• When a system changes, energy is transferred:
  • Into or away from the system
  • Between different objects in the system
  • Between different stores

Closed systems

• A closed system is one where no matter and no energy can enter or leave.
• Because nothing crosses the boundary, the net change in the total energy of a closed system is always zero.

Energy can be transferred in several ways

Energy can be transferred between objects and stores along four main pathways:

1. Mechanically - A force acting on an object and moving it, such as pushing, pulling, stretching or squashing.
2. Electrically - A charge moving through a circuit, such as charges driven round by a power supply.
3. By heating - Energy passing from a hotter object to a colder one.
4. By radiation - Energy carried by waves, such as light and other electromagnetic waves, and sound.

Doing work is transferring energy

• Work done means energy transferred.
• Work is done when a force moves an object (mechanical work) or when a charge flows through a circuit (electrical work).