Efficiency Flashcards
(24 cards)
What principle states that energy is only useful when transferred from one store to a useful store?
The principle of conservation of energy
Energy must be transferred to a useful store for it to be considered useful.
What happens to some of the input energy in useful devices?
It is dissipated or wasted
This dissipation often occurs as heat.
What forces must be overcome whenever work is done mechanically?
Frictional forces
This includes forces from moving parts rubbing together and air resistance.
Where is the energy needed to overcome frictional forces usually transferred?
To the thermal energy stores of the surroundings
This energy is typically not useful.
What does the conservation of energy principle state?
Total energy input = useful energy output + wasted energy
This reflects the balance of energy in any process.
How is the efficiency of a device defined?
Efficiency = useful energy transferred by device (J) / total energy supplied to device (J)
Efficiency can be expressed as a decimal or a percentage.
What is the maximum efficiency a device can achieve?
Less than 1 (or 100%)
Due to the unavoidable waste of energy.
Calculate the efficiency of a toaster that transfers 84,000 J to the bread out of 216,000 J supplied.
0.39 (to 2 significant figures)
Efficiency is calculated as 84,000 J / 216,000 J.
Fill in the blank: The less energy that’s wasted, the more _______ the device is said to be.
efficient
Efficiency indicates how much useful energy is obtained from total energy supplied.
True or False: All devices have an efficiency of 100%.
False
Some energy is always wasted, making it impossible for efficiency to be 100%.
What does a motor do in terms of energy transfer?
Transfers energy usefully from kinetic energy store to gravitational potential energy store
It also transfers energy to thermal energy stores of its moving parts.
What are two main methods to reduce energy waste?
Lubrication and thermal insulation
These methods help increase efficiency by minimizing wasted energy.
What is the maximum efficiency of any device?
No device is 100% efficient
This means some energy is always wasted in any process.
What type of diagram is used to show energy transfers in a device?
Sankey diagrams
These diagrams visually represent energy input, useful energy output, and wasted energy.
In a Sankey diagram for an electric motor with 80% efficiency, what is the total energy supplied?
100 J
This is the energy input into the motor.
What is the useful energy transferred to kinetic energy stores in the motor with 80% efficiency?
80 J
This represents the energy that is effectively used by the motor.
How much energy is wasted in the electric motor with 80% efficiency?
20 J
This is the energy that does not contribute to the useful work done by the motor.
What role does lubrication play in energy transfer?
Reduces energy transferred by friction
This helps in minimizing energy loss to thermal energy stores.
What happens when an object is heated?
Particles vibrate more and collide with each other
This leads to energy transfer from kinetic energy stores to other particles.
What is conduction in the context of energy transfer?
The process of energy transfer through an object
It occurs when particles collide and transfer energy.
What does thermal conductivity describe?
How well a material transfers energy by conduction
Metals have high thermal conductivity; gases have low thermal conductivity.
How does the thermal conductivity of walls affect energy transfer in buildings?
Lower thermal conductivity means slower energy transfer
This results in buildings cooling more slowly.
What design feature in some houses helps reduce energy transfer by conduction?
Cavity walls with an air gap
The air gap has very low thermal conductivity, slowing down energy transfer.
What is the effect of wall thickness on energy transfer?
Thicker walls reduce the rate of energy transfer
This means the building retains heat better.
