Energy Stores & Transfers (Edexcel IGCSE Physics)

The four energy transfer pathways used in physics are:

• mechanical

• electrical

• heating

• radiation

True.

An example of energy transfer by heating is a hot chocolate warming up cold hands.

False.

The total amount of energy in a closed system remains constant; it cannot change over time according to the principle of conservation of energy.

An energy flow diagram represents energy stores and transfers within a system, showing both the stores and the transfers taking place.

The principle of conservation of energy states that energy cannot be created or destroyed; it can only be transferred from one energy store to another.

When energy is dissipated to the surroundings, it means that energy has been transferred to the surroundings and cannot easily be used for a specific purpose. Dissipated energy is a form of wasted energy.

The equation representing the conservation of energy is:

total energy input = useful energy output + wasted energy

Sankey diagrams are graphical representations of energy transfers, characterised by splitting arrows that show the proportions of energy transfers taking place.

The left-hand side of a Sankey diagram represents the energy transferred into the system, while the straight arrow pointing to the right represents the useful energy output. The curved arrows represent wasted energy.

The equation for calculating wasted energy is:

True.

According to the principle of conservation of energy, the total energy input in a closed system is equal to the useful energy output plus the wasted energy.

If an energy transfer has an efficiency of 0.4, 40% of the energy is transferred usefully and 60% of the energy is wasted.

The energy transfer is 80% efficient, so 20% of the energy is wasted. The total energy input is 200 J; therefore, the wasted energy is 40 J.

• wasted energy is equal to 20% of the total energy input

• 

True.

Thermal conduction in solids occurs when hotter, more energetic atoms or ions vibrate faster and collide with neighbouring atoms or ions, transferring energy throughout the structure of the solid.

Thermal conduction occurs faster in metals because metals have free (delocalised) electrons, which speed up the energy transfer throughout the substance.

False.

Thermal conduction does happen in liquids and gases, but they tend to be poor conductors because the particles are further apart than in solids.

True.

Convection only happens in liquids and gases because the particles are free to move around. It cannot happen in solids because the particles are held in a rigid structure.

When a fluid is heated, the space between particles increases, so the density of the fluid decreases which causes it to rise, and cooler, denser fluid takes its place.

True.

All objects emit thermal radiation. The hotter an object is, the more thermal radiation it emits.

True.

The colour of an object does affect its ability to emit and absorb thermal radiation.

False.

White surfaces are poor emitters and poor absorbers of thermal radiation.

Black surfaces are the best emitters of thermal radiation.

False.

Shiny surfaces are very poor emitters of thermal radiation. They are also very poor absorbers because they reflect the infrared radiation.

Energy transfer by conduction can be reduced by using an insulator.

The three factors that the effectiveness of an insulator are dependent on are:

• the thermal conductivity of the material

• the density of the material

• the thickness of the material

The air trapped between the fibres of fibreglass insulation makes it a good insulator.

The purpose of cavity wall insulation is to reduce energy transfer by conduction through the walls of buildings.