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

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

Problem 1

Illustrative kinetic-energy records for a 2 kg cart at speeds 1, 2 and 3 m/s are 1, 4 and 9 J. A same-speed 4 kg cart has twice the corresponding energy. Describe and graph the separate speed and mass relationships. Explain why equal changes in speed do not give equal changes in kinetic energy.
Answer and explanation

For fixed mass, kinetic energy grows with speed squared: the record 1,4,9 is nonlinear. For fixed speed, doubling mass doubles kinetic energy. Plot separately and label units; do not change mass and speed together and call that an isolated test. Equal speed increments add different energy amounts because the square changes differently.

For fixed mass, kinetic energy grows with speed squared: the record 1,4,9 is nonlinear. For fixed speed, doubling mass doubles kinetic energy. Plot separately and label units; do not change mass and speed together and call that an isolated test. Equal speed increments add different energy amounts because the square changes differently.

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

What does a smaller kinetic-energy gain than gravitational loss suggest?
Answer and explanation

Other energy transfers or stores matter. Account for them, and check measurement uncertainty, before claiming energy has disappeared.

Other energy transfers or stores matter. Account for them, and check measurement uncertainty, before claiming energy has disappeared.

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

Independent chapter check: 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

An illustrative lighting device receives 300 J electrical input, produces 60 J visible light and transfers 240 J by other mainly thermal pathways. Draw a labeled energy-transfer model and calculate the visible-light fraction. Distinguish energy form from crossing a boundary.
Answer and explanation

The visible-light fraction is 60/300=20%, and outputs sum to 300 J. Arrows crossing the device boundary show transfer; different labels can describe electrical, light and thermal pathways. Converting and transferring are related but distinct ideas. Calling thermal output less useful for lighting does not mean it ceased to exist.

The visible-light fraction is 60/300=20%, and outputs sum to 300 J. Arrows crossing the device boundary show transfer; different labels can describe electrical, light and thermal pathways. Converting and transferring are related but distinct ideas. Calling thermal output less useful for lighting does not mean it ceased to exist.

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

Why should an unexplained remainder not automatically be labeled sound?
Answer and explanation

The data have not identified that destination. It could be another store, transfer, or measurement discrepancy; the account should state what is unknown.

The data have not identified that destination. It could be another store, transfer, or measurement discrepancy; the account should state what is unknown.

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

Independent chapter check: 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

Illustrative equal warm-water samples cool from 40 °C to 31 °C in an uncovered cup and to 36 °C in an otherwise matched insulated lidded cup over 10 min. Compare changes and propose follow-up tests separating insulation from a lid. Explain conduction, convection and evaporation in plain words.
Answer and explanation

The drops are 9 and 4 °C. The treatment changes both walls and lid, so separate wall-only and lid-only comparisons can distinguish effects. Contact interactions transfer energy through walls; moving fluid transfers energy between regions; escaping water removes matter and energy. A lid and insulation reduce some rates but do not guarantee zero exchange.

The drops are 9 and 4 °C. The treatment changes both walls and lid, so separate wall-only and lid-only comparisons can distinguish effects. Contact interactions transfer energy through walls; moving fluid transfers energy between regions; escaping water removes matter and energy. A lid and insulation reduce some rates but do not guarantee zero exchange.

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

What would make an insulation comparison hard to interpret?
Answer and explanation

Changing water amount, starting temperature, lid use, and insulation simultaneously makes it difficult to attribute a difference to one intended feature.

Changing water amount, starting temperature, lid use, and insulation simultaneously makes it difficult to attribute a difference to one intended feature.

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

Independent chapter check: 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

An illustrative rolling system starts with 80 J mechanical energy. Later it has 55 J mechanical energy, 20 J additional thermal energy in the system and 5 J transferred to surroundings. Write a complete energy account and explain the system boundary. Distinguish reduced mechanical energy from reduced total accounted energy.
Answer and explanation

55+20+5=80 J accounts for the initial amount. Mechanical energy fell by 25 J, but energy is redistributed within and across the boundary. A closed boundary for matter can still exchange energy; an isolated ideal system exchanges neither. State the boundary and include transfers before claiming energy disappeared.

55+20+5=80 J accounts for the initial amount. Mechanical energy fell by 25 J, but energy is redistributed within and across the boundary. A closed boundary for matter can still exchange energy; an isolated ideal system exchanges neither. State the boundary and include transfers before claiming energy disappeared.

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

What is a useful response to an unexplained energy remainder?
Answer and explanation

State the remainder, identify plausible missing transfers or stores, and propose measurements that could distinguish them. Keep uncertainty explicit.

State the remainder, identify plausible missing transfers or stores, and propose measurements that could distinguish them. Keep uncertainty explicit.

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

Independent chapter check: 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

Illustrative 6 V circuits have resistances 3 Ω and 12 Ω in separate trials. A drawing omits the return connection to the battery. Find currents under the simple Ohm’s-law model and repair the circuit drawing. Explain why current is not used up in a bulb.
Answer and explanation

Currents are I=V/R=2 A and 0.5 A for the respective simple resistors. Add the continuous return path; an open circuit does not sustain the modeled current. Energy is transferred to the bulb and surroundings, while charge is not consumed. This is a model using ideal sources and appropriate resistive components, not every real device.

Currents are I=V/R=2 A and 0.5 A for the respective simple resistors. Add the continuous return path; an open circuit does not sustain the modeled current. Energy is transferred to the bulb and surroundings, while charge is not consumed. This is a model using ideal sources and appropriate resistive components, not every real device.

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

Why is a short circuit different from a proper lamp connection?
Answer and explanation

It provides a very low-resistance path that can draw excessive current and heat components. It bypasses the intended controlled energy transfer.

It provides a very low-resistance path that can draw excessive current and heat components. It bypasses the intended controlled energy transfer.

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

Independent chapter check: 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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