Energy Stores, Systems, and Conservation

This set covers the fundamental definitions of energy stores, the conservation of energy principle, specific heat capacity calculations, and the concept of power from the lecture transcript.

System

A single object (e.g. the air in a piston) or a group of objects (e.g. colliding vehicles).

Closed system

A system where neither matter nor energy can enter or leave, meaning the net change in total energy is always zero.

Work done

Another way of saying energy transferred; it occurs when current flows against resistance or when a force moves an object.

Kinetic energy store

The energy an object has due to its movement, depending on its mass and speed, calculated as $E_k = \frac{1}{2}mv^2$.

Gravitational potential energy (g.p.e.) store

The energy stored in an object due to its height in a gravitational field, calculated as $E_p = mgh$.

Elastic potential energy store

The energy stored in a stretched or squashed object, such as a spring, calculated as $E_e = \frac{1}{2}ke^2$ providing the limit of proportionality is not exceeded.

Specific heat capacity

The amount of energy needed to raise the temperature of $1\,kg$ of a substance by $1^{\circ}C$.

Change in thermal energy formula

The equation linking energy transferred to specific heat capacity: $\Delta E = mc\Delta\theta$, where $m$ is mass, $c$ is specific heat capacity, and $\Delta\theta$ is temperature change.

Conservation of energy principle

The principle that energy can be transferred usefully, stored, or dissipated, but it can never be created or destroyed.

Dissipated energy

Also called 'wasted energy', this is energy that is not transferred usefully and usually ends up in thermal energy stores.

Power

The rate of energy transfer or the rate of doing work, measured in watts ($W$).

Watt ($W$)

The unit of power, where $1\,W = 1\,J/s$ ($1$ joule of energy transferred per second).

Power formulas

The equations used to calculate power: $P = \frac{E}{t}$ (Power = Energy transferred $\div$ time) or $P = \frac{W}{t}$ (Power = Work done $\div$ time).

Energy stores (list)

The eight stores are: Thermal (internal), Kinetic, Gravitational potential, Elastic potential, Chemical, Magnetic, Electrostatic, and Nuclear.

Limit of proportionality

The point beyond which a stretched object will no longer return to its original shape and the elastic potential energy formula no longer applies.