Conservation and Dissipation of energy

Law of energy conservation

Energy cannot be created or destroyed

• can only be transferred into different forms

changes in total energy on a ball that is kicked (closed system)

the total energy of the system remains constant due to the conservation of energy

what is waste energy?

energy that is not used by the device for its desired purpose

describe the energy changes that occur in a filament light bulb

• electrical energy is transferred into light and heat energy

• light is a useful energy form, heat is waste energy

two equations to calculate efficiency

• useful output energy transfer / total input energy transfer

• useful power output / total power output

how can the efficiency of a system be increased?

1. reducing waste output (by lubrication, thermal insulation etc)

2. recycling waste output (eg. recycling thermal waste energy as input energy)

the consequence for energy transfer of a material with a high thermal conductivity

the rate of energy transfer through the material is higher than for a material with a lower thermal conductivity

do double glazed windows have a higher or lower thermal conductivity than single-glazed?

• lower - meaning less energy transfers through them

what key factors affect the rate of cooling of a building?

• the thickness of the walls

• the thermal conductivity of the walls

methods of reducing heat loss in a building

1. double glazing

2. loft and wall insulation

3. thicker walls

how can the efficiency of a mechanical machine with moving parts be improved?

lubricate any moving parts to reduce the friction and therefore energy loss due to heating

how can the efficiency of a radiator be improved?

insulating metal foil sheets behind the radiator to reflect the heat back into the room rather than it being absorbed into the walls

how can the efficiency of boiling water in a pan be improved?

by placing a lid on the pan to reduce the heat loss from the top.

what does the thermal conductivity of a material tell us

how quickly energy is transferred through the material via thermal conduction

• the higher the rate of thermal conductivity the higher the rate of energy transfer

how can you experiment thermal conductivity?

• using wax on different kinds of identical metal rods

  • the rod that wax melts first has the highest thermal conductivity

what does the rate of energy transfer depend on?

1. the thermal conductivity

2. the thickness of the material

3. the difference in temperature (in and out)