Energy Transfers
Energy cannot be created or destroyed, but it can undergo transformations from one form to another. Any energy that is not converted into a useful form dissipates into the surroundings as heat. Sankey diagrams are a visual representation of how energy is converted from one form into another.
Energy Stores
Energy is ‘stored’ in a particular form. There are eight different types of energy stores:
Nuclear: when atomic nuclei react, they release energy. For example, in an atomic bomb uranium nuclei will split apart to release an explosive amount of energy.
Chemical: the bonds between atoms store energy. When bonds break during a reaction, this chemical energy is released.
Magnetic: any object which is magnetised will be able to attract and repel other magnets and can attract magnetic materials.
Gravitational potential: when an object is raised in height and can fall, it gains gravitational potential energy. GPE depends on its mass, the height it is raised to and the strength of the gravitational field.
Kinetic: anything that moves has kinetic energy. The amount of kinetic energy an object has depends on its size and speed.
Thermal: thermal energy is caused by the vibrations of molecules. The hotter the object is, the more its molecules vibrate and the more thermal energy it has. Cold object still have thermal energy, but much less than hot ones.
Elastic potential: materials that can be stretched or squashed, such as rubber bands and springs, will have elastic potential energy.
Electrostatic: two oppositely charged objects will attract one another while two similarly charged objects will repel one another.
Use the mnemonic ‘Naomi Campbell met Grace Kelly to eat enchiladas’ to help you remember the different energy stores!
Energy transfers
There are four main ways in which energy can be transferred from one form to another:
Mechanically: moving parts can transfer energy from one store to another by exerting a force on it. For example, a cue hitting a ball on a pool table
Electrically: energy is transferred through the movement of charge through a potential difference. For example, in an electric circuit, chemical energy in a battery can be transferred into to kinetic energy of the electrons moving through the wire.
By heating: energy is transferred from a hotter object to a cooler object. For example, a toaster transfers thermal energy to bread.
By radiation: energy is transferred through the movement of light or sound waves. For example: lamps transfer visible light and infra-red radiation to their surroundings.
Conservation of Energy
We have seen how energy can be transferred from one store to another, but it is important to remember that it cannot be created or destroyed. This means that the total energy of a system always remains the same and is referred to as the conservation of energy.
Efficiency
When energy is transferred between different stores, not all of it is transferred into a useful form of energy. For example, when I use a kettle to boil water, electrical energy is transferred into thermal energy to heat the water. However, some energy is also wasted by heating the surrounding air. This energy spreads out to the surroundings and is lost - we say that the energy is dissipated.
We can calculate the efficiency of a particular energy transfer using the equation:
The higher the proportional of total energy that has been converted into a useful form, the more efficient the device. We want appliances to have a high efficiency because it means less energy is wasted, which is cheaper and better for the environment. However, even with really efficient devices some energy will always be lost which means efficiency will never be 100%.
Worked example:
A washing machine has an input energy of 30 000 J and converts 26 500 J of energy into kinetic energy of the drum and thermal energy of the water. The remaining energy dissipates to the surroundings as sound and heat energy. Calculate the efficiency of the washing machine.
Percentage efficiency = useful energy / total energy x 100
Percentage efficiency = (26,500 / 30,000) x 100
Efficiency = 88.3%
Sankey diagrams
Sankey diagrams allow us to visualise the amount of energy being transferred into useful and non-useful forms of energy. The thicker the arrow, the larger the amount of energy that has been converted into that particular form. In the picture below, you can see that for every 100 J of energy that is used by a television, 40 J is usefully converted into sound, 15 J is usefully converted into light energy and the remaining 45 J is wasted as thermal energy.
Conduction, convection and radiation
We’ve already seen how heating is a way of transferring energy from one object to another. This can occur in three different ways - by conduction, convection or radiation. Conduction and convection both rely on particles to transfer energy so this type of energy transfer cannot take place in a vacuum. In contrast, radiation uses electromagnetic waves instead of particles to transfer heat so radiation can take place within a vacuum.
Heat moves from the hob, through the saucepan and into the egg by conduction.
Conduction
Conduction is the transfer of thermal energy through solid materials by direct contact. For example, frying an egg relies on the conduction of heat from the hob, through the saucepan and to the egg. Solids can conduct thermal energy because they contain tightly packed atoms which bump into each other when they vibrate. This ‘mexican wave’ of vibration transfers heat energy from hotter parts of the solid to cooler parts.
Convection
Lava lamps use convection currents.
Convection occurs in fluids (liquids and gases) because their particles are free to move. Let’s say we have a radiator heating the air in a cold room. The particles next to the radiator will gain heat energy causing these particles to move faster and move apart from one another. This makes the hot air expand and become less dense, causing it to rise and displace colder air. The colder air will fall to spot next to the radiator and itself heat up and rise. This creates cycles of fluid movement which we call convection currents. You can also see convection in action if you look at a lava lamp - the heating element at the bottom heats the waxy blobs, causing them to rise and displace cooler blobs which fall back towards the bottom of the lamp.
Radiation
All objects emit heat as infra-red radiation which can be detected by thermal imaging cameras.
Heat can also be transferred by infra-red radiation, which involves the transmission of thermal energy by electromagnetic waves. All objects are continually emitting and absorbing infrared radiation. The hotter the object, the more infrared radiation it emits. Dull surfaces are good absorbers and emitters of infrared radiation whereas shiny surfaces are poor absorbers and emitters (although they easily reflect heat). This is why in hot countries such as Greece, the houses are painted white to keep the inside of the house cool. Similarly, the inside of an oven is painted black to absorb infra-red radiation generated by the heating element and emit it back towards the centre of the oven.
Reducing heat loss from a house
Heat energy is lost from a house in different ways:
By conduction through any solid material such as walls, floors, windows and the roof. Conduction can be reduced using material which is bad at conducting heat. Cavity wall insulation involves inserting an insulating material between the outer brick wall and the inside layer, preventing the conduction of heat through the walls.
By convection through gaps in the doors and windows. To reduce convection, you need to stop air from moving in order to prevent convection currents. Draught excluders prevent convection, along with double glazed windows. These windows have a vacuum between two panes of glass. Since convection relies on particles to transfer energy, double glazing prevents convection through the windows.
By radiation through the walls, roofs and windows. To reduce energy transfers by radiation, parts of the house should be painted light colours which are poor emitters of infra-red.