Energy stores and systems

In a nutshell

Energy is always conserved. It can be transferred between energy stores via different methods. A system is a term used to describe the object, or objects, being looked at.

Energy stores and transfers

This lesson will go further into detail about the content detailed in Energy Stores and Transfers from the Conservation of Energy topic. Here's a quick recap.

Energy is stored in different types of energy stores.

Energy stores

Thermal
Kinetic
Chemical
Nuclear
Magnetic
Electrostatic
Gravitational potential
Elastic potential

The conservation of energy states that energy cannot be created or destroyed, only transferred. Energy can be transferred from the different stores by methods of energy transfer.

Energy transfer methods

Mechanical work
Electrical work
Heating
Radiation

Systems

Definition

Describes the object, or objects, being looked at.

Example:

The system for a bungee jumper would be the jumper and the bungee cord.
The system for a moving vehicle would be the vehicle.

When energy is transferred to or from a system, the system changes. It also changes when energy is transferred within the system, between the different stores.

A closed system doesn't allow energy to be transferred in or out. Instead all transfers take place between the energy stores in the system. The total energy of a closed system stays the same. The overall change in the energy of a closed system is always zero.

Energy transfers

Energy is transferred either by doing work, or by heating.

Doing work

Energy transfer by doing work is when energy is transferred due to forces. This can be done mechanically, via a push or pull force. It can also be done electrically, when a current flows against a resistance. The resistance acts as a force that the current has to do work against to overcome.

Work done is another way of saying energy transferred. Work is done against forces.

Example

*Dropping a box from a height above the ground.*	Energy is transferred mechanically between the gravitational potential energy store of the box to its kinetic energy store. The force of gravity does work on the box.
*Car starting from rest.*	Energy is transferred electrically between the chemical energy of the fuel and the kinetic energy of the wheels. The current does work against the resistance of the car's electrical circuit.
*Spring being stretched by a pulling force on either end.*	Energy is transferred mechanically from the kinetic energy store of the spring to its elastic potential store. The spring does work against its own resistive force.
*Wind turbines turning.*	Energy is transferred mechanically from the kinetic energy store of the wind to the kinetic energy store of the wind turbine's blades. The wind does work against the weight of the windmill's blades.

Heating

Heating transfers energy into the thermal store of an object. Heating often involves a heating element. Energy is transferred electrically to the element's thermal store. The heating element then transfers energy by heating from its thermal store to the object's thermal store.

Example

A kettle heating water. The current of the kettle's circuit does work against the resistance in the kettle's heating element. This transfers energy to the heating element's thermal store by electrical work. The energy of this thermal store is transferred by heating to the water's thermal energy store.

An energy transfer diagram can be drawn to represent this process. One diagram can be drawn for the first transfer and another for the second.

$\boxed{electrostatic\space energy \space of\space current} \xrightarrow[electrically] {energy \space transferred} \boxed{thermal\space energy \space of\space heating \space element} \xrightarrow[by\space heating] {energy \space transferred} \boxed {thermal\space energy \space of \space water}$