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7.2Physics 01. Forces And Energy

Know that energy is conserved, meaning it cannot be created or… — Cambridge Checkpoint Science 0893 Revision Notes & Topical Questions

5 min read 8 parts Lesson 2 of 12 1 quiz

📚 Cambridge Checkpoint Science 0893 Hub / Topical Revision
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Lesson summary

7.2 Conservation of energy — CHECKPOINT - Lower Secondary Science 0893 notes: Know that energy is conserved, meaning it cannot be created or destroyed, and use energy input and output to find missing values.Law — energy cannot be created or destroyed, only changed from one form t…

  • Meaning of conserved
  • Everyday meaning
  • Energy output
  • Dissipated energy
  • Not destroyed
  • Conservation in a swing

Part 1 of 6Law of conservation of energy

Law
  • Law energy cannot be created or destroyed, only changed from one form to another.
Meaning of conserved
  • Meaning of conserved the total quantity of energy stays the same when energy is stored, changed, transferred or dissipated.
Everyday meaning
  • Everyday meaning using less energy is a different idea from the energy being conserved in physics.

Part 2 of 6System, input and output

System, input and output
System
  • System something that has been chosen to study, especially in terms of energy changes.
Energy input
  • Energy input the energy supplied to the system.
Energy output
  • Energy output the energy leaving the system as useful and wasted energy together.
Rule
  • Rule total energy input = total energy output.
  • The output can never be greater than the input because energy cannot be created.

Part 3 of 6Energy diagrams

SystemInputUseful outputWasted output
Electric lamp100 J electrical10 J light90 J thermal
Car engine100% chemical25% kinetic35% thermal and sound, 40% exhaust gas
Electric motor1000 J electrical500 J kinetic500 J thermal

Part 4 of 6Dissipation

Dissipated energy
  • Dissipated energy spreads out into the surroundings and becomes less useful.
Not destroyed
  • Not destroyed dissipated energy still exists, so it must be included in the energy balance.

Part 5 of 6Calculating a missing energy value

Calculating a missing energy value
Write the total energy input.
Add all the energy outputs, both useful and wasted.
The two totals are equal, so the missing value is the total minus the known values.
  • Formula
    • missing output = input - sum of the other outputs.

Part 6 of 6Energy stores

Conservation in a swing
  • Conservation in a swing a pendulum moves between gravitational potential energy and kinetic energy.
  • It slows down and stops because energy is dissipated as thermal and sound energy.
Elastic band racers
  • Elastic band racers twisting an elastic band stores elastic potential energy that is transferred to kinetic energy of the bottle.

Key terms

TermMeaning
ConservedStaying the same in total.
SystemThe object or group of objects chosen for study.
Energy inputThe energy supplied to a system.
Energy outputThe energy transferred out of a system.
DissipatedSpread out and becoming less useful.

Avoid these mistakes

1
Energy is lost in a lamp — energy output equals energy input when thermal energy is included.
2
Conserving energy means switching things off — in physics it means the total stays constant.
3
Wasted energy is destroyed — wasted energy is dissipated.
4
A machine can give out more energy than it takes in — output can never exceed input.

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