Energy & Heat Transfer — Chapter review · Physical Science: Matter, Forces & Energy, Grade 8

Trace energy transfers and transformations and investigate how thermal energy moves.

Review the key ideas

Kinetic & Potential Energy

Kinetic energy (KE = ½mv²) is energy of motion; potential energy (PE = mgh) is stored energy due to position — height and speed constantly trade off.

Energy Transfer & Transformation

Energy is never created or destroyed — only transformed between forms (like chemical to electrical to light) or transferred between objects.

Thermal Energy & Heat Transfer

Heat flows from warm to cool by conduction (contact), convection (fluid movement), or radiation (waves, no medium needed) — until both reach the same temperature.

Conservation of Energy

Energy is never created or destroyed — only transformed or transferred; energy that seems 'lost' has really just changed into a less visible form, like heat or sound.

Electricity & Simple Circuits

Ohm's Law connects voltage, current, and resistance: V = I × R; series circuits share one path, parallel circuits offer independent paths.

Problem 1

Why does doubling speed give four times the kinetic energy?
Answer and explanation

The formula contains speed squared. Replacing v by 2v makes the squared term 4v² while mass is unchanged.

The formula contains speed squared. Replacing v by 2v makes the squared term 4v² while mass is unchanged.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 2

At 4 m/s, what does this model predict?
Answer and explanation

16 J for the same 2 kg mass. The prediction uses the squared-speed model rather than extending the last straight segment indefinitely.

16 J for the same 2 kg mass. The prediction uses the squared-speed model rather than extending the last straight segment indefinitely.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 3

Visual-model chapter review: A supplied 2 kg object moves at 3 m/s and then at 6 m/s in the same reference frame. Compare kinetic energies and say whether doubling speed doubles kinetic energy.
Answer and explanation

KE values are 9 J and 36 J, so doubling speed quadruples KE in this fixed-mass model.

Use ½mv² with the same mass and units.

Compare the ratio after squaring both speeds.

KE values are 9 J and 36 J, so doubling speed quadruples KE in this fixed-mass model.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

Problem 4

Does an electrical lamp convert all its input into visible light?
Answer and explanation

No. Some energy enters thermal stores and other radiation outputs. The relative amounts depend on the device and conditions.

No. Some energy enters thermal stores and other radiation outputs. The relative amounts depend on the device and conditions.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 5

Which quantity compares the useful light fraction?
Answer and explanation

60/300=0.20 or 20%, for the defined visible-light output.

60/300=0.20 or 20%, for the defined visible-light output.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 6

Use the illustrative service records to propose a conditional source/storage or demand plan. Explain one benefit, one limitation and two additional evidence needs, tracing rather than creating energy.
Hints

Use the illustrated study titled Energy sources and future choices. Keep the observations, model and conditions separate.

Answer and explanation

Supply X alone does not meet the stated 20-unit evening need, while storage with 15 units charged and 80% useful return delivers 12 evening units. The remaining 8 need another supply or a justified demand reduction. A proposal should compare resource/impact/reliability evidence for that equal service and acknowledge omitted costs and timing. No source is universally best from these few records, and lost useful fraction remains accounted energy elsewhere.

Supply X alone does not meet the stated 20-unit evening need, while storage with 15 units charged and 80% useful return delivers 12 evening units. The remaining 8 need another supply or a justified demand reduction. A proposal should compare resource/impact/reliability evidence for that equal service and acknowledge omitted costs and timing. No source is universally best from these few records, and lost useful fraction remains accounted energy elsewhere.

Response criteria

  • Use equal service and the supplied timing.
  • Calculate/trace storage output and demand gap.
  • Evaluate benefits, limits and missing evidence fairly.

Problem 7

Visual-model chapter review: A supplied motor system receives 150 J and gives 90 J to the useful job, 40 J to thermal increase and 20 J to sound/other measured changes. Build a complete account and distinguish a transfer from a store.
Answer and explanation

150 = 90+40+20 J. Transfers name energy moving between systems, while stores/contributions name energy accounted for within chosen objects/arrangements. No supplied energy is missing.

Sum all stated outputs and changes.

Identify actual receiving systems rather than treating arrows as material flowing through every object.

150 = 90+40+20 J. Transfers name energy moving between systems, while stores/contributions name energy accounted for within chosen objects/arrangements. No supplied energy is missing.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

Problem 8

Why is a warming solid handle an example of conduction rather than bulk fluid flow?
Answer and explanation

Energy spreads through interacting particles in the solid without the entire handle material flowing from one end to the other.

Energy spreads through interacting particles in the solid without the entire handle material flowing from one end to the other.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 9

Which pictured mechanism requires a moving bulk fluid?
Answer and explanation

Convection. Conduction involves local interactions without that required bulk motion, and radiation can cross empty space.

Convection. Conduction involves local interactions without that required bulk motion, and radiation can cross empty space.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 10

Visual-model chapter review: An object receives energy across a vacuum gap from a warmer source. Which of conduction, convection or radiation can explain the gap transfer? What would a full net-exchange account include?
Answer and explanation

Radiation can cross the vacuum. Convection needs moving material and conduction needs an interacting material pathway. A full net account includes both objects’ emission/absorption and their temperatures.

Use the presence or absence of matter to distinguish mechanisms.

Do not mistake a selected one-way arrow for every transfer in the full exchange.

Radiation can cross the vacuum. Convection needs moving material and conduction needs an interacting material pathway. A full net account includes both objects’ emission/absorption and their temperatures.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

Problem 11

How does a system gain stored energy while also sending energy outward?
Answer and explanation

If incoming energy exceeds outgoing energy, the net input increases the stored amount. Both transfers can occur in the same interval.

If incoming energy exceeds outgoing energy, the net input increases the stored amount. Both transfers can occur in the same interval.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 12

How much mechanical energy is redistributed?
Answer and explanation

25 J, accounted for by 20 J additional thermal energy in the chosen system and 5 J transferred to surroundings.

25 J, accounted for by 20 J additional thermal energy in the chosen system and 5 J transferred to surroundings.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 13

Visual-model chapter review: A full supplied energy account starts with 80 J GPE. Later it has 15 J GPE, 45 J KE and 20 J thermal increase. Compare mechanical and total energy before and after.
Answer and explanation

Mechanical energy falls from 80 to 15+45 = 60 J. The stated total stays 80 J once the 20 J thermal increase is included.

Distinguish the mechanical subset from the full account.

Use a common boundary and reference for both states.

Mechanical energy falls from 80 to 15+45 = 60 J. The stated total stays 80 J once the 20 J thermal increase is included.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

Problem 14

Why should a lamp not automatically be treated as a constant resistor?
Answer and explanation

Its filament temperature can change during operation, changing resistance. The fixed-resistance model needs a stated condition or supplied approximation.

Its filament temperature can change during operation, changing resistance. The fixed-resistance model needs a stated condition or supplied approximation.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 15

What changes with a 12 Ω resistor at the same ideal voltage?
Answer and explanation

The modeled current is 6/12=0.5 A; the complete return path remains required.

The modeled current is 6/12=0.5 A; the complete return path remains required.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 16

Complete source/return diagrams, compare the three supplied records and explain evidence for energy transfer by current. Identify which observations would be needed to measure energy quantitatively.
Hints

Use the illustrated study titled Observe energy transfer in circuit paths. Keep the observations, model and conditions separate.

Answer and explanation

With the supplied series loop closed, both lamps respond; opening the single path makes both go out. In the parallel model, opening one branch leaves the other complete branch operating. Light/thermal responses provide qualitative energy-transfer evidence, not charge consumption. Quantitative energy needs voltage, current and duration or another justified method, not brightness alone.

With the supplied series loop closed, both lamps respond; opening the single path makes both go out. In the parallel model, opening one branch leaves the other complete branch operating. Light/thermal responses provide qualitative energy-transfer evidence, not charge consumption. Quantitative energy needs voltage, current and duration or another justified method, not brightness alone.

Response criteria

  • Complete conducting paths and distinguish arrangements.
  • Use observed device behavior as qualitative evidence.
  • Separate charge, energy and measurement limits.

Problem 17

Visual-model chapter review: A supplied receiver transfers 18 J for 6 C of charge while 6 C passes in 3 s. Find potential difference and current, then explain why the receiver does not use up the charge.
Answer and explanation

Potential difference 18/6 = 3 V; current 6/3 = 2 A. Charge circulates through the complete circuit while energy is transferred to the receiver.

Use joules per coulomb for voltage and coulombs per second for current.

Keep conserved charge flow distinct from energy transfer.

Potential difference 18/6 = 3 V; current 6/3 = 2 A. Charge circulates through the complete circuit while energy is transferred to the receiver.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.
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