physics - energy - forces doing work (8.1 - 8.15)

8.1 changes involved in way energy is stored when systems change

system changes - energy transferred from one form to another

e.g. electricity —transfers energy→ electric bulb, electric bulb —transfers energy (by light & heating)→ surroundings

8.2 Sankey diagrams

8.3 energy changes in closed system

no net change to total energy in system - no energy transferred to/from surroundings

8.4 energy of system changed - work done by forces

human doing work to push ball up hill

8.4 energy of system changed - in electrical equipment

electricity —transfers energy→ electric bulb

8.4 energy of system changed - in heating

bulb —transfers energy (by light & heating)→ surroundings

8.5 how to measure work done by force

energy transferred (J) = work done (J)

8.6 work done equation

work done (J) = force (N) x distance moved in direction of force (m)

E = F x d

8.7 changes in energy involved when system changed by work done by forces

force makes something move → energy transferred

energy transferred by force = work done

8.8 change in GPE equation

change in GPE (J) = mass (kg) x grav. field strength (N/kg) x change in height (m)

∆GPE = mg∆h

8.9 kinetic energy equation

KE (J) = ½ x mass (kg) x (speed)² ((m/s)²)

KE = ½mv²

8.10 energy is dissipated in all system changes

energy is dissipated - stored in less useful ways

e.g. light bulb bulb transfers energy to surroundings by light & heat

8.11 mechanical processes becoming wasteful

become wasteful when they cause rise in temp. - dissipate energy in heating to surroundings

8.12 power definition

rate at which energy is transferred

e.g. 2 birds lift same sized fish 2 metres in air - both done same amount of work; 1 lifts its fish in shorter time - bird produced greater power

8.13 power equation

power (W) = work done (J)/time taken (s)

P = E/t

8.14 watts to joules

1 watt = 1 joule per second

1W = 1J/s

8.15 efficiency equation

efficiency = useful energy transferred/total energy supplied