PHYSICS FINAL
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187 Terms
Energy store: chemical
Energy stored in fuels, food, batteries due to bonds
Energy store: kinetic
Energy an object has due to its motion
Energy store: gravitational potential
Energy stored when an object is raised in a gravitational field
Energy store: elastic potential
Energy stored when an object is stretched or compressed
Energy store: thermal (internal)
Energy stored in particles due to their random motion (temperature-related)
Energy store: magnetic
Energy stored in magnetic fields due to attraction/repulsion
Energy store: electrostatic
Energy stored due to separated electric charges
Energy store: nuclear
Energy stored in the nucleus of atoms (released in fission/fusion)
Energy transfer: mechanically
Energy transferred by forces doing work (pushing, friction)
Energy transfer: electrically
Energy transferred by moving charges in a circuit
Energy transfer: heating
Energy transferred due to temperature difference
Energy transfer: radiation
Energy transferred by waves (light or sound)
Conservation of energy
Energy cannot be created or destroyed, only transferred or stored
Closed system
Total energy remains constant in a system
Useful energy output
Energy transferred to the desired form
Wasted energy
Energy transferred to unwanted forms (usually heat/sound)
Efficiency definition
How much input energy becomes useful output energy
Efficiency formula
Efficiency = (useful energy output ÷ total energy input) × 100%
High efficiency
A large proportion of input energy is useful
Low efficiency
Most energy is wasted to surroundings
Sankey diagram purpose
Shows energy transfers using arrows
Wide arrow
Large energy transfer
Thin arrow
Small energy transfer
Input arrow
Total energy entering a system
Useful output arrow
Energy used for the intended purpose
Waste output arrow
Energy lost to surroundings
Lamp energy transfer
Electrical → light (useful) + thermal (wasted)
Kettle energy transfer
Electrical → thermal energy in water + wasted heat
Car energy transfer
Chemical → kinetic + thermal + sound
Bicycle rider
Chemical → kinetic + thermal
Wind turbine
Wind kinetic → electrical + thermal/sound losses
Friction energy transfer
Kinetic energy → thermal energy
Air resistance
Kinetic energy → thermal energy in air
Battery lamp energy path
Chemical → electrical → light + thermal
Food energy transfer
Chemical → kinetic + thermal
Improving efficiency methods
Reduce friction, lubrication, insulation, streamline design
Energy units
Joules (J)
Difference between energy store and transfer (A*)
Stores show where energy is held; transfers show how energy moves between stores
Common mistake with energy wording
Saying “energy is transferred as kinetic energy” instead of “to the kinetic energy store”
A* correct wording for energy loss
Energy is transferred to the thermal energy store of the surroundings
Braking car energy pathway (A*)
Kinetic energy store decreases → thermal energy stores of brakes and surroundings increase
Energy transfers in braking
Mechanical work (friction) → thermal energy in brakes + surroundings
Falling object with air resistance (A*)
GPE decreases → KE increases → thermal energy in air increases
Effect of air resistance
Transfers kinetic energy into thermal energy of the surroundings
Conservation of energy (A* definition)
Energy cannot be created or destroyed, only transferred between stores
Closed system definition
A system where no energy enters or leaves; total energy remains constant
Why energy is never fully useful
Some energy is always transferred to thermal energy stores of surroundings due to friction/resistance
Correct “wasted energy” definition
Energy transferred to less useful thermal or sound energy stores of surroundings
Efficiency meaning (A*)
How much input energy is transferred into useful output energy
High efficiency meaning
A large proportion of input energy is transferred to useful stores
Low efficiency meaning
Most energy is transferred to unwanted thermal/sound stores
Efficiency formula
Efficiency = (useful energy output ÷ total energy input) × 100%
Rearranged efficiency (useful)
Useful = efficiency × total ÷ 100
Rearranged efficiency (total)
Total = useful × 100 ÷ efficiency
How to improve efficiency (A*)
Reduce friction, reduce air resistance, use lubrication, insulation, streamline design
Sankey diagram meaning (A*)
Shows energy transfers with arrow width proportional to energy size
What wide arrows mean
Large energy transfer
What thin arrows mean
Small energy transfer
Sankey diagram key rule
Total input energy = sum of all output energies
Useful Sankey calculation method
Use proportional widths or given values to find missing energy
Why Sankey diagrams are useful
They visually show efficiency and energy losses
Electric motor energy pathway
Electrical → kinetic + thermal + sound energy
Kettle energy pathway
Electrical → thermal (water) + thermal (surroundings) + sound
Power station energy pathway
Chemical/nuclear → thermal → kinetic → electrical + thermal losses
How friction affects energy transfers
Kinetic energy is transferred to thermal energy stores of surroundings
How air resistance affects energy
Kinetic energy is transferred to thermal energy of air
Correct explanation of energy “loss”
Energy is not lost; it spreads into surroundings as thermal energy
6-mark answer structure for energy questions
State initial store → describe transfers → final stores → surroundings → waste energy → link to efficiency
What examiners look for in A* answers
Correct terminology, full energy pathways, mention of surroundings, and conservation of energy
Key A* phrase for top marks
“Energy is transferred to the thermal energy stores of the surroundings and is dissipated”
Insulation purpose (A*)
Reduces unwanted energy transfer to the surroundings, usually thermal energy loss
Methods of reducing heat loss
Insulation, double glazing, cavity walls, loft insulation, draught excluders
How insulation works
Traps air (poor conductor) and reduces energy transfer by conduction, convection, and radiation
Work done definition (A*)
Energy transferred when a force moves an object through a distance
Work done formula
W = F × d
Work done units
Joules (J)
Meaning of work done
Work done equals energy transferred
Why work done = energy transferred
Because energy is transferred when a force causes movement
Force in work equation (A*)
Force must act in the direction of motion
Gravitational potential energy meaning
Energy stored when an object is raised in a gravitational field
GPE formula
GPE = m × g × h
Mass unit in GPE
kilograms (kg)
Gravitational field strength unit
N/kg (or N kg⁻¹)
Height unit
metres (m)
Meaning of GPE equation
GPE increases with mass, height, and gravitational field strength
Kinetic energy meaning
Energy an object has due to its motion
Kinetic energy formula
KE = 1/2 × m × v²
Speed unit
metres per second (m/s)
Mass unit in KE
kilograms (kg)
Key idea in kinetic energy (A*)
Speed has a squared effect on kinetic energy (doubling speed quadruples KE)
Link between GPE and KE (A*)
GPE is converted into KE when an object falls
Energy conservation link
GPE lost = KE gained (in ideal conditions)
Energy transfer in falling object
GPE → KE → thermal energy (if air resistance present)
Work-energy principle (A*)
Work done on an object equals the change in its energy store
Work done and energy transfer link
Work done by a force transfers energy to or from an object
Power definition (A*)
Power is the rate of energy transfer or the rate of doing work
Power formula
P = W / t
Power unit
Watts (W)
Meaning of power equation
More power means more energy transferred per second
High power meaning
Energy is transferred quickly