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Theme A — Space, Time and Motion

A.3.8 Energy and Power — IB Physics HL Revision Notes & Topical Questions

5 min read 10 parts Lesson 39 of 63 1 quiz

📚 IB Physics HL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Lesson summary

A.3.8 Energy and Power — IB PHYSICS SL notes: Define power as the rate of energy transfer and use P = W/t and P = Fv in calculations.Power = the rate at which energy is transferred, which is the rate of doing work.Equation — P = ΔW/Δt = ΔE/Δt.P is the power in W, W the work in J …

  • SI base units
  • Two motors
  • Acceleration
  • Derivation
  • Fixed power
  • Average power

Part 1 of 8Definition of power

Power
Power the rate at which energy is transferred, which is the rate of doing work.
Equation
  • P = ΔW/Δt = ΔE/Δt.
  • P is the power in W, W the work in J and t the time in s.
The watt
  • 1 W = 1 J s−1, a transfer of 1 joule in 1 second.
SI base units
  • SI base units the watt is kg m2 s−3.
Scalar
  • Scalar power has no direction.
Multiples
  • 1 kW = 103 W and 1 MW = 106 W.
Rating
  • Rating a 60 W lamp transfers 60 J of electrical energy each second.

Part 2 of 8Why time matters

Two cars
  • Two cars if both transfer the same energy to reach the same speed, the more powerful one does it sooner.
Two motors
  • Two motors lifting the same weight through the same height, the one that lifts faster has more power.
Acceleration
  • Acceleration power is required to produce an acceleration.

Part 3 of 8Power and velocity

Equation
  • P = Fv, for a constant force moving a body at constant velocity.
Derivation
  • Derivation the work done in time t is Fvt, so the power is Fv.
Direction
  • Direction the force is applied in the same direction as the velocity.
Vehicles
  • Vehicles at constant speed the driving force equals the resistive force, so engine power = resistive force × speed.
Drag
  • Drag the force used is the one that balances the drag.
Fixed power
  • Fixed power a higher speed means a lower driving force.
  • This is why a vehicle accelerates more slowly at high speed and why gears exist.

Part 4 of 8Average and instantaneous power

1Average power
  • Average power total energy transferred divided by total time.
2Instantaneous power
  • Instantaneous power the rate at one moment, equal to F × the instantaneous speed.
3Energy–time graph
  • Energy–time graph the gradient is the power.
4Power–time graph
  • Power–time graph the area under the curve is the energy transferred.
5Constant acceleration from rest
  • Constant acceleration from rest the power needed rises linearly with time.
6Peak and average
  • Peak and average a sprinter may reach several kilowatts briefly but average far less.

Part 5 of 8Power in different contexts

ContextExpression
LiftingP = mgΔh/Δt = weight × vertical speed
ElectricalP = VI = I2R = V2/R
Wind turbineP = ½ρAv3
RadiationIntensity I = P/A

Part 6 of 8Input, output and cost

Efficiency
  • Efficiency the useful output power is less than the input power, and the difference is lost as thermal power.
Input needed
  • Input needed divide the output power by the efficiency.
Energy unit
  • 1 kW h = 3.6 × 106 J.
Cost
  • Cost energy in kW h is power in kW times time in hours; a 2.0 kW heater for 3.0 h uses 6.0 kW h.

Part 7 of 8Plausibility

Car
  • Car an engine delivering 15 kW at cruising speed is realistic.
Humans
  • Humans a person sustains 100 to 200 W, and over 1 kW for a second or two in a sprint or jump.
Power of ten errors
  • Power of ten errors estimation catches unit-conversion mistakes.

Part 8 of 8More from the specification

More from the specification
Gravitational and Elastic Potential Energy
  • Gravitational potential energy stored in an object elevated to height h is Ep = m·g·h,
  • while elastic potential energy stored in a stretched or compressed spring is given by Ee = ½·k·x².
Mechanical Energy Conservation
  • In an isolated mechanical system without resistive dissipation, total mechanical energy remains constant during motion.
    • (sum of kinetic energy Ek = ½·m·v² and potential energy)
Energy Transformations and Power
  • When an object moves along a horizontal or inclined surface, work done against friction results in wasted thermal energy dissipated to surroundings;
    • power represents the rate of mechanical energy transfer (P = W/t = F·v).
Kinetic energy
  • K = ½mv2, energy of motion.
Wasted energy
  • Energy transferred to stores that are not useful, usually thermal in the surroundings.
Mechanical energy
  • Sum of kinetic and potential energy.
Sankey diagram
  • Diagram where arrow width shows energy amount.
Energy conservation
  • Mgh = ½Mv2 + ½Iω2.
Resultant force
  • Another name for net force.
Final speed
  • Speed at the end, v
Elastic
  • Returns to original shape when force is removed.
Friction
  • Force parallel to the surface opposing relative motion or its tendency.
Density
  • Mass per unit volume.
Component
  • Part of a vector along one axis.
Position
  • Location relative to a reference point.
Distance
  • Total length of path travelled, a scalar.
Potential energy
  • Energy stored by the configuration of a system.
Gravitational potential
  • Work done per unit mass in bringing a test mass from infinity to the point.
W = Fs
What the symbols mean: W = work done = energy transferred (J) · F = force (N) · d, s or x = distance moved in the direction of the force (m)
W
Fs = 25.6 × 20 = 512 J
What the symbols mean: W = work done = energy transferred (J) · F = force (N) · d, s or x = distance moved in the direction of the force (m)

Key terms

TermMeaning
PowerThe rate at which work is done or energy is transferred
Watt (W)The unit of power; 1 J s−1
Kilowatt-hourUnit of energy equal to 3.6 × 106 J
Average powerTotal energy transferred divided by total time
Instantaneous powerRate of energy transfer at a single moment
Power ratingEnergy transferred per second by an appliance

Avoid these mistakes

1
Using P = Fv when the force or velocity is not constant — use instantaneous values or P = W/t.
2
Mixing kW h with J — convert with 3.6 × 106.
3
Using the average speed when the instantaneous speed is needed — use the speed at that moment.
4
Forgetting to divide by efficiency to find the input power — input = output / efficiency.
5
Confusing power with energy — power is energy per second.

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