A-Level AQA Chapter 10: Work, energy, and power

Name four kinds of energy stores

1) Gravitational potential stores

2) Kinetic stores

3) Thermal stores

4) Elastic stores

Name three way energy can be transferred

1) By radiation (e.g. light)

2) Electrically

3) Mechanically (e.g. sound)

Principle of conservation of energy

Energy cannot be created or destroyed, only transferred or stored

Describe one joule (J)

Energy needed to raise a 1N weight through a vertical height of 1m

Work done

1) Work is done on an object when a force acting on it makes it move

2) Work Done (J) = Force (N) * Distance (m)

3) Remember its distance in direction of motion

Area under force-distance graph

Equal to the total work done

Are under force-extension graph

1) Work done to stretch string by extension L

Kinetic energy equation

Ek = 1/2mv^2

Gravitational potential energy equation

GPE = mgh

Kinetic energy of object falling through heigh h

1) Ek = GPE

2) 1/2mv^2 = mgh

Kinetic energy of a pendulum bob

1) Ek = GPE

2) 1/2mv^2 = mg(h1 - h2)

3) Where h1 is the initial height and h2 the final height

Work done by fairground vehicle to overcome friction and air resistance

E = mgh - 1/2mv^2

What is power

Rate of transfer of energy, measured in Watts (W)

Power equation

1) P = ΔE/Δt , when energy 'E' is transferred in time 't'

2) P = ΔW/Δt , when work 'W' is done in time 't'

How to test your muscle power

1) Time how long it takes you to walk up a flight of stairs

2) Measure your weight and height gain of stairs

3) GPE = Your Weight * Total Height Gain

4) Muscle Power = GPE / Time Taken

Calculate electrical power of component

P = Energy Supplied / Time Taken to Supply it

How to calculate engine power of vehicle moving at constant velocity at constant height

P = Fv , where F=Force and v=velocity

Describe the motive power and how to calculate it

1) When a powered object gains speed, the output force exceeds the resistive forces on it

2) Motive Power = Energy wasted per second due to resistive forces + gain of kinetic energy per second

Equation for power of a machine

P = Fv

Useful energy

Energy transferred to where it is wanted in the desired form

Efficiency equation

1) Efficiency=Useful Energy Transferred/Energy supplied

2) Efficiency=Work Done/Energy Supplied

How to calculate percentage efficiency

Efficiency * 100

Ways to increase efficiency

1) Insulate to stop heat loss

2) Replace burning fuels with electricity

3) But burning fuels is required to make electricity generators work

4) And insulating them would prevent them from working