Conservation of energy and power, Reducing unwanted energy transfers, Efficiency
Conservation of energy and power
You need to know the conservation of energy principle
Energy is transferred between stores, not all of the energy is transferred usefully in the store they want it to go to, some is dissipated into the surroundings when the energy transfer takes place
Dissipated energy is sometimes called waste energy as it is stored in a way that isn't useful, usually converted to thermal energy
Energy can be transferred usefully, stored or dissipated but can never be created or destroyed
You need to be able to describe energy transfers for closed systems
A cold spoon is dropped into an insulated flask of soup which is then sealed, this creates a closed system because the flask is a perfect thermal insulator. Energy is transferred from thermal energy of the soup to useless thermal energy of the spoon, causing the soup to cool down slightly. The nets change is zero.
Power is the 'rate of doing work' - I.e. How much per second
Measured in watts. One watt = 1 joule of energy transferred per second
Calculate power with these equations
Power is the rate of energy transfer
A powerful machine is one that transfers a lot of energy in a short space of time
Power = Energy transferred/time
Power = Work done/time
Reducing Unwanted energy transfers
Lubrication reduces frictional forces
Whenever something moves, theres usually at least one frictional force acting against it, this causes some energy in the system to be dissipated
For objects that are being rubbed together, lubricants can be used to reduce the friction between the objects surfaces when the move, they are usually liquids, so they can flow easily between objects and coat them
Heating can occur by conduction and convection
When the particles are free to move the particles move faster so the space between them increases, causing the density to decrease
Liquids and gasses can flow, the warmer and less dense region will rise above denser, cooler regions so particles move away from hot to cool regions, this is CONVECTIONS
Thermal conductivity is a measure of how quickly energy is transferred through the material. The higher the thermal conductivity the higher the rate of reaction
This causes the particles to vibrate more and collide with each other, during this the energy us transferred to kinetic this is CONDUCTION
When an object is heated, the energy is transferred kinetic energy stores
Insulation reduces the rate of of energy transfer by heating
Have thick walls that are made from a material with low thermal conductivity
Use thermal insulation
The thicker the walls and the lower the thermal conductivity is the slower the rate of energy will be
Efficiency
Most energy transfers involve some waste energy
Less energy wasted in this energy store, the more efficient it is
You can improve the efficiency by insulating object, lubricating them or making them more streamlined
input energy is usually wasted by being transferred to a useless energy store, usually thermal energy
Efficiency of any energy transfer can be worked out by this
Useful devises are only useful because they can transfer energy from one store to another
Also by using this but using power input and output
Efficiency = Useful energy output/ total input energy
Efficiency = useful power output/ total power input
Useful energy input isn't equal to total energy output
No device is 100% efficient and the wasted energy is used as thermal energy
Electric heaters are the exception to this, they're usually 100% efficient because all the electric energy is used for useful thermal energy
All energy transferred to thermal energy stores, e.g an electric drill its energy transfers to lots of different energy stores, but end up all in thermal energy