Energy can be transferred, stored, or dissipated (wasted). However, energy cannot be created or destroyed.

This means that in a closed system (where energy cannot enter or leave), the net change in energy will always be zero.

When energy is transferred between stores, not all of it will be directed to the intended energy store. The energy that is successfully transferred to the desired store is called useful energy. The energy that ends up in other stores is termed wasted energy. We say that wasted energy has been dissipated.

For example, a light bulb emits both heat and light. The energy used to produce light is useful energy, while the energy that goes into heating the bulb is wasted, as the goal is not to heat the bulb.

Some materials make better thermal insulators than others. This is because of a property called thermal conductivity. Thermal conductivity tells you how well a material transfers heat energy through it.

Materials with a low thermal conductivity transfer less heat energy and therefore make good thermal insulators. Materials with a high thermal conductivity transfer more heat energy and therefore make poor thermal insulators.

For example, if you heat one end of a metal rod, the other end will also get hot. This is because metal has a high thermal conductivity and so the heat is transferred quickly to the metal. However, if you heat one end of a plastic rod, the other end will stay cool because plastic has low thermal conductivity.

Buildings are constructed with materials with a low thermal conductivity in order to insulate the rooms inside. This allows the building to stay warm in cold weather but cool in the summer.