Work, Power and Efficiency

Question 1

What is meant by the term ‘‘work’’ ?

Answer 1

Work is the energy required for a force to move and object and transfer energy from one store to another

Question 2

What is meant by mechanical work?

Answer 2

The energy shifted by a force (moving object)

Question 3

What are the unit of work done?

Answer 3

Joules

Question 4

What is the equation for calculating mechanical work?

Answer 4

Work done = Force x Distance

Question 5

What force do you work against when you lift an object onto a shelf?

Answer 5

Gravity

Question 6

What force do you do work against when you push an object across a rough surface?

Answer 6

Friction

Question 7

What force do you work against when you push the opposite poles of two magnets together?

Answer 7

Magnetic force between the magnets

Question 8

What is meant by the term power?

Answer 8

The rate of energy transfer

This is the energy transferred per second

Question 9

What are the units of power?

Answer 9

Watts

Question 10

What is the equation for calculating power>

Answer 10

Power = Energy / Time

Question 11

If 50 J of energy is transferred in 10 seconds what is the power?

Answer 11

5 W

Question 12

1 watt is the same as…

Answer 12

1 Joule per second

Question 13

What is meant by the term efficiency?

Answer 13

Efficiency is the proportion of energy that is usefully transferred in a process

Question 14

What is the equation for efficiency is terms of energy?

Answer 14

Efficiency = Useful energy/ Total energy

Question 15

What is the equation for efficiency is terms of power?

Answer 15

Efficiency = Useful power/ Total power

Question 16

What are the units of efficiency?

Answer 16

Efficiency has no units because it is a ratio.
They are written as decimals or percentages

Question 17

What is the smallest value of efficiency?

Answer 17

0 or 0%

Question 18

What is the largest possible value of efficiency?

Answer 18

1 or 100%

Question 19

Explain why in a practice it is impossible to hace an efficiency of 1 for an energy transfer

Answer 19

Because some energy is dissipated to other non-useful stores (thermal)

Question 20

State and explain how energy dissipation can be reduced for objects moving through air or water

Answer 20

Make them more streamllined, to reduce the amount of resistance on the object
This reduces thermal energy loss so more energy is used usefully

Question 21

State how energy loss can be reduced for a house

Answer 21

Loft insulation
Cavity wall insulation
Double glazing
Use materials with low thermal condcutivity

Question 22

What do materials with a high thermal conductivity have a high rate of cooling?

Answer 22

These materials have a higher rate of energy transfer across the material

Question 23

Give an example of a material with a high thermal conductivity

Answer 23

Metal

Question 24

Thermal insulators often have small pockets of trapped _______

Answer 24

Air

Question 25

Gas is an excellent insulator true or false?

Answer 25

True