Work, Power and Efficiency Flashcards
What is meant by the term ‘‘work’’ ?
Work is the energy required for a force to move and object and transfer energy from one store to another
What is meant by mechanical work?
The energy shifted by a force (moving object)
What are the unit of work done?
Joules
What is the equation for calculating mechanical work?
Work done = Force x Distance
What force do you work against when you lift an object onto a shelf?
Gravity
What force do you do work against when you push an object across a rough surface?
Friction
What force do you work against when you push the opposite poles of two magnets together?
Magnetic force between the magnets
What is meant by the term power?
The rate of energy transfer
This is the energy transferred per second
What are the units of power?
Watts
What is the equation for calculating power>
Power = Energy / Time
If 50 J of energy is transferred in 10 seconds what is the power?
5 W
1 watt is the same as…
1 Joule per second
What is meant by the term efficiency?
Efficiency is the proportion of energy that is usefully transferred in a process
What is the equation for efficiency is terms of energy?
Efficiency = Useful energy/ Total energy
What is the equation for efficiency is terms of power?
Efficiency = Useful power/ Total power
What are the units of efficiency?
Efficiency has no units because it is a ratio.
They are written as decimals or percentages
What is the smallest value of efficiency?
0 or 0%
What is the largest possible value of efficiency?
1 or 100%
Explain why in a practice it is impossible to hace an efficiency of 1 for an energy transfer
Because some energy is dissipated to other non-useful stores (thermal)
State and explain how energy dissipation can be reduced for objects moving through air or water
Make them more streamllined, to reduce the amount of resistance on the object
This reduces thermal energy loss so more energy is used usefully
State how energy loss can be reduced for a house
Loft insulation
Cavity wall insulation
Double glazing
Use materials with low thermal condcutivity
What do materials with a high thermal conductivity have a high rate of cooling?
These materials have a higher rate of energy transfer across the material
Give an example of a material with a high thermal conductivity
Metal
Thermal insulators often have small pockets of trapped _______
Air
Gas is an excellent insulator true or false?
True
