Unwanted energy transfers

1. An unwanted energy transfer is any transfer that takes energy away from the job a system is meant to do.
2. The two most common causes are work done against friction between moving parts and heating of the surroundings by anything that is hotter than them.
3. Both end in the same way, with energy dissipated into the thermal energy stores of the surroundings.
4. Reducing an unwanted transfer has one of two benefits, depending on the system.
   1. In a machine, more of the input energy reaches the useful store, such as the kinetic energy store of the moving parts.
   2. In a house, a hot drink or a fridge, energy stays in the store you want to keep it in for longer, so less energy has to be supplied to replace what escapes.
5. The two main ways of reducing unwanted transfers are lubrication, which tackles friction, and thermal insulation, which tackles heating.
6. Unwanted transfers can be made much smaller, but they can never be removed completely.

Lubrication

1. Even surfaces that look and feel smooth are covered in tiny peaks and hollows when seen under a microscope.
2. When two dry surfaces slide over each other, these peaks press together, catch on one another and have to be dragged past, which is the cause of friction.
3. The work done against friction transfers energy into the thermal energy stores of both surfaces, which is why dry moving parts get hot and also wear away.
4. A lubricant such as oil or grease spreads into a thin layer between the surfaces and keeps the peaks apart.
5. The surfaces now slide over the layer of lubricant instead of grinding against each other, and the particles of a liquid move past one another easily.
6. The frictional force is therefore much smaller, so less work is done against friction as the parts move.
7. Less work done against friction means less energy is transferred to thermal energy stores, so more of the input energy goes into the useful kinetic energy store of the moving parts.
8. Lubrication also cuts the sound made by rubbing parts, which is why oiling a squeaky door hinge or bicycle chain stops the noise.
9. Common examples include engine oil between the pistons and cylinders of a car engine, grease in the hubs of bicycle wheels and oil on the moving parts of a sewing machine.
10. A lubricant slowly becomes dirty and breaks down, so it has to be topped up or replaced to keep friction low.

How energy escapes by heating

1. Energy is always transferred by heating from a hotter object to cooler surroundings, and it can travel by three routes.
2. Conduction: in a solid, particles that vibrate more strongly pass energy to their neighbours, so energy travels through the material from the hot side to the cold side.
   1. Metals conduct very well, most non-metals conduct poorly, and gases conduct very poorly because their particles are far apart.
3. Convection: in a liquid or gas, the part next to a hot surface warms up, expands and becomes less dense, so it rises and is replaced by cooler fluid.
   1. This sets up a circulating convection current that carries energy away from the hot surface.
4. Infrared radiation: every object gives out infrared radiation, and hotter objects give out more of it.
   1. Infrared needs no material to travel through, dull dark surfaces give out and absorb it well, and shiny light surfaces reflect it.
5. Thermal insulation works by making one or more of these three routes slower.

Thermal insulation

1. Trapping air: air is a very poor conductor, and when it is trapped in tiny pockets it cannot move around to form convection currents, so trapped air reduces energy transfer by both conduction and convection.
   1. Most insulating materials, including mineral wool, polystyrene foam, fleece and feathers, are mostly trapped air held in place by a small amount of solid.
2. Reflecting infrared: a shiny foil layer reflects infrared radiation back towards the warm side, so it reduces energy transfer by radiation.
3. Sealing gaps: blocking gaps around doors and windows stops warm air flowing out and cold air flowing in, which reduces energy transfer by convection.
4. Using a vacuum: a vacuum contains no particles, so energy cannot cross it by conduction or convection at all.
5. These ideas are put to work around the home in several ways.
   1. Loft insulation: a layer of mineral wool laid between the joists of the loft traps air above the ceiling, so less energy is transferred out through the roof by conduction and convection.
   2. Cavity wall insulation: foam or mineral wool is blown into the gap between the inner and outer brick walls, which stops convection currents circulating in the gap and fills it with trapped pockets of air.
   3. Double glazing: two panes of glass are separated by a sealed, narrow gap of dry air or argon, and the gas conducts poorly while the gap is too narrow for strong convection.
   4. Draught excluders: strips of brush or foam around doors and windows seal the gaps and reduce convection.
   5. Radiator foil: a shiny panel fixed to the wall behind a radiator reflects infrared back into the room instead of letting it warm the outside wall.
   6. Hot water tank jackets and pipe lagging: a foam layer around the tank and pipes keeps the water hot for longer.
6. The same methods keep cold things cold, so the foam walls of a cool box slow the transfer of energy from the warm air outside into the cold food inside.
7. Insulation reduces the rate of energy transfer but never stops it, so a well-insulated house still cools when the heating is off, but more slowly, and less fuel is needed to keep it warm.