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1.7.4Motion, Forces and Energy

Conservation of Energy — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 9 parts Lesson 75 of 91 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
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Lesson summary

1.7.4 Conservation of Energy — IGCSE PHYSICS notes: State and apply the principle of conservation of energy to systems with and without friction.Conservation of energy = energy cannot be created or destroyed, only transferred from one store to another or from one object to anothe…

  • Falling object
  • Roller coaster
  • Mass on a spring
  • The energy lost to friction can be found from W
  • Bat hitting a ball
  • Energy flow diagram

Section 1 of 9The principle

Conservation of energy
Conservation of energy energy cannot be created or destroyed, only transferred from one store to another or from one object to another.
The total energy of a closed system stays constant.
Total energy before equals total energy after when all stores are counted.
Energy that seems to disappear has gone to a store not noticed, normally the thermal store of the surroundings.

Section 2 of 9Changes without friction

Without friction or air resistance, mechanical energy (gravitational potential + kinetic + elastic) stays constant.
Falling object
  • Falling object gravitational potential energy decreases and kinetic energy increases by the same amount, so mgh + ½mv2 is constant.
Pendulum
  • Pendulum energy swaps between gravitational potential energy at the ends and kinetic energy at the bottom, so the bob returns to the same height.
Roller coaster
  • Roller coaster a car can reach any later point of equal or lower height.
Mass on a spring
  • Mass on a spring elastic energy and kinetic energy swap.

Section 3 of 9Changes with friction

1
Friction and air resistance do work against the motion and transfer energy to the thermal store of the object, surface and air.
2
Total energy is still conserved but the mechanical energy decreases.
3
A real pendulum swings lower each time and a dropped ball bounces to a lower height each time.
4Method
  • Method initial energy = final energy + energy transferred to thermal (wasted).
5The energy lost to friction can be found from W
Ffriction × d.
6
The energy is not lost; it is in a less useful form because it is spread out in the surroundings.

Section 4 of 9Sankey diagrams

1
A Sankey diagram shows transfers as arrows whose widths are proportional to the energy.
2
The input arrow splits into a useful arrow and wasted arrows.
3
The sum of the outputs equals the input.
4
A filament lamp has a thin arrow for light and a thick arrow for thermal energy.

Section 5 of 9Energy flow diagrams and dissipation

1
Energy is dissipated (spread out) to the surroundings by heating and radiation.
2
Dissipated energy is usually not useful and is described as wasted energy; always say which store it ends up in.
3Bat hitting a ball
  • Bat hitting a ball some kinetic energy of the bat goes usefully to the ball and the rest goes to the thermal stores of the bat, ball and surroundings.
4Kettle
  • Kettle electrical energy goes to the heating element, then to the water, with waste to the kettle and surroundings.
5Energy flow diagram
  • Energy flow diagram total energy in equals total energy out.
6
Stating the principle requires saying energy cannot be created or destroyed, only transferred.

Section 6 of 9Calculations

Calculations
Write one energy equation, such as mgh1
Write one energy equation, such as mgh1 mgh2 + ½mv2 + dissipated energy for a roller coaster car.
Without friction the mass cancels, so v2
2g(h1 − h2).
With friction, include the dissipated energy, found from F × d or given.
The speed can never be higher than the frictionless value.
A perpetual motion machine cannot work, because any real machine dissipates some energy.

Section 7 of 9More from the specification

1
Thermodynamic energy balance
  • Thermodynamic energy balance equating decrease in chemical potential energy to generated thermal energy provides an empirical means to evaluate efficiency.
2
Conservation of energy
  • Conservation of energy energy is transferred between stores but the total remains constant
3
Efficiency
  • Efficiency useful energy output divided by total energy input, and wasted energy is usually dissipated as heat
4
Work done
  • Work done force times distance moved in the direction of the force, measured in joules
5
Power
  • Power energy transferred per second;
    • gravitational potential energy change equals mass times field strength times height change
6
Efficiency
  • Useful output divided by total input.
7
Joule
  • Unit of energy, 1 kg·m2/s2.
8
Distance
  • Total length of path travelled, a scalar.
9
Gravitational field
  • Region where mass experiences gravitational force.
10
Transfer pathway
  • The way energy moves between stores, such as heating or work done.

Section 8 of 9Key terms

TermMeaning
Conservation of energyTotal energy of a closed system remains constant.
Closed systemA system with no energy transferred in or out.
Sankey diagramDiagram where arrow width shows the amount of energy transferred.
Mechanical energySum of kinetic and potential energy.
DissipationSpreading of energy into the surroundings, usually as thermal energy.
FrictionForce opposing relative motion between surfaces.

Section 9 of 9Exam technique

1
Saying energy is lost when friction acts — it is transferred to the thermal store.
2
Using gravitational potential energy = kinetic energy when friction is significant.
3
Forgetting stores such as thermal or sound.
4
Believing mass changes the speed of a free-falling object in energy methods — mass cancels.

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