A3 Work, Energy and Power

Work energy power

A3.1 Energy

Energy is measure of work done. Amount of energy transferred = work done. Both measured in Joules kg m² s^-2

A3.2 Energy conservation

In closed system; no change in total E. Never created or destroyed, changes form. Universe is closed system, no change total E.

A3.3 Work done formula

Work done = Force x distance moved in direction of force (Nm = J)
Work done by resultant force = change in energy in system
transfer of energy when external force causes an object to move over a certain distance
F-s graph; area = W, when F is not paralel: W=Fs cosθ (θ is between F and s)

A3.4 Kinetic energy

Energy of a moving object (translational): E_k = ½ mv² = p²/2m

A3.5 Gravitational Potential energy

When something is lifted off the ground, gains E_p, can lose when dropped/falling again
E_p = mgh. Uniform grav. field assumed.

A3.6 Elastic potential energy

Energy stored in material ( spring/string/elastic) when stretched/compressed
E_H = ½ kx² Hookes law. k=spring constant , x=extension of spring

E_H = ½ Fx (F=kx)=restoring force). E_H turns into E_k and other way around etcetc…

A3.7 Chemical energy

In a substance; in bonds

A3.8 Nuclear energy

Within nucleus of atom

A3.9 Internal energy

E something has due to its T (or state) - thermal/heat energy

A3.10 Energy dissipation

No transfer is 100% efficient, dissipated as thermal energy to the surroundings that is not used for another purpose; wasted energy. Useful/wasted energy.

A3.11 Examples of energy transfers

falling: grav potential → kinetic + Q

spring: elastic potential → kinetic + Q

battery: chemical → electrical

car: chemical (fuel) → kinetic

trampoline: elastic potential → kinetic → grav potential

A3.12 Sankey Diagram

Width indicates energy, used to display efficiency of a process. Total in = useful out + wasted energy.

A3.13 Mechanical Energy

sum = E_k + E_p + E_H, change in mech → work done. Conserved when drag forces are neglible

A3.14 Energy and Power

Power = rate at which energy is transferred / work is done

P = Fv = ^W/_t (Watts = Js^-1)

A3.15 Efficiency formula

^{useful P or E out}/_{total P or E in} = η, unitless, between 0 and 1.

A3.16 Energy density

Energy / unit volume, Jm^-3 used for fuels etc…coal, diesel, oil….
Energy / unit mass, Jkg^-1

A3.17 Explosions

Kinetic energy and momentum conserved