Waves, Information & a Speaker System — Practice · Grade 8 Science

Chapter: Waves, Sound & Light

What this practice covers

By the end of Waves, Information & a Speaker System you should be able to:

12 questions, written answer, each with a worked explanation.

Vocabulary used: signal, membrane, analog, digital, threshold, model, evidence, system boundary, assumption.

Problem 1

Why does a fixed magnet alone not explain a complete changing sound?
Answer and explanation

The time-varying input and resulting changing force are also needed to produce the membrane’s varying motion and air disturbance.

The time-varying input and resulting changing force are also needed to produce the membrane’s varying motion and air disturbance.

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 must two people agree on before flashes form a reliable message?
Answer and explanation

The encoding rules, timing and how to distinguish message boundaries and symbols; the physical carrier alone does not define the meaning.

The encoding rules, timing and how to distinguish message boundaries and symbols; the physical carrier alone does not define the meaning.

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

Does digital transmission prevent all error?
Answer and explanation

No. Noise can cause incorrect symbol decisions, especially near a threshold. Reliability depends on the system and code, not merely the digital label.

No. Noise can cause incorrect symbol decisions, especially near a threshold. Reliability depends on the system and code, not merely the digital label.

Response criteria

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

Problem 4

What distinguishes the sound pathway from the magnetic pathway?
Answer and explanation

Sound requires material interactions in a medium. Magnetic interactions can occur across a vacuum; the two effects do not share the same air requirement.

Sound requires material interactions in a medium. Magnetic interactions can occur across a vacuum; the two effects do not share the same air requirement.

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

Illustrative received values are 0.15, 0.85, 0.48 and 0.88 with threshold 0.5. A drawing shows electrical input jumping directly to an ear with no speaker or air. Decode the values, identify the less robust reading and revise the drawing with intermediate interactions.
Answer and explanation

The decoded sequence is 0101. The 0.48 value is closest to the threshold, so small noise could change its symbol. Add the coil–magnet interaction, attached membrane vibration, successive air interactions and receiver. The drawing should not imply that charge or air particles travel the entire information path unchanged.

The decoded sequence is 0101. The 0.48 value is closest to the threshold, so small noise could change its symbol. Add the coil–magnet interaction, attached membrane vibration, successive air interactions and receiver. The drawing should not imply that charge or air particles travel the entire information path 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 6

A magnet still attracts another magnet in a vacuum, but a distant ordinary sound cannot be heard through the gap. Is the evidence contradictory?
Answer and explanation

No. Magnetic fields can mediate an interaction through a vacuum, while ordinary sound requires a material medium. Different mechanisms predict the different observations.

No. Magnetic fields can mediate an interaction through a vacuum, while ordinary sound requires a material medium. Different mechanisms predict the different observations.

Response criteria

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

Problem 7

Evaluate and correct: A digital message cannot be corrupted.
Answer and explanation

Noise can cause a reading to cross the decision threshold; a code and receiver design influence error detection and reliability.

Noise can cause a reading to cross the decision threshold; a code and receiver design influence error detection and reliability.

Response criteria

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

Problem 8

Choose one prediction from this unit. Explain a comparison that could test it, specifying observations, changed and matched conditions, and a result that would challenge it.
Answer and explanation

In a supplied vacuum comparison, magnets can still interact while no ordinary airborne sound crosses an empty gap. The magnetic force and sound transmission need different models. Use evidence about the actual gap rather than claim that all effects require air. Sound requires material interactions in a medium. Magnetic interactions can occur across a vacuum; the two effects do not share the same air requirement.

In a supplied vacuum comparison, magnets can still interact while no ordinary airborne sound crosses an empty gap. The magnetic force and sound transmission need different models. Use evidence about the actual gap rather than claim that all effects require air. Sound requires material interactions in a medium. Magnetic interactions can occur across a vacuum; the two effects do not share the same air requirement.

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

Received values 0.3 and 0.7 use threshold 0.5. Decode the two symbols and explain why replacing an air gap with a vacuum distinguishes sound from magnetic interaction.
Hints

Distinguish the observed quantities, the proposed mechanism and its conditions.

Answer and explanation

Symbols are 0 and 1. Ordinary sound requires material interactions and cannot use a vacuum gap as an air medium. Magnetic interaction can act across a vacuum. The threshold rule can still make errors if noise alters the values.

Symbols are 0 and 1. Ordinary sound requires material interactions and cannot use a vacuum gap as an air medium. Magnetic interaction can act across a vacuum. The threshold rule can still make errors if noise alters the values.

Problem 10

A magnet still attracts another magnet in a vacuum, but a distant ordinary sound cannot be heard through the gap. Is the evidence contradictory? Explain a condition that would change this conclusion.
Hints

Distinguish the observed quantities, the proposed mechanism and its conditions.

Answer and explanation

No. Magnetic fields can mediate an interaction through a vacuum, while ordinary sound requires a material medium. Different mechanisms predict the different observations.

No. Magnetic fields can mediate an interaction through a vacuum, while ordinary sound requires a material medium. Different mechanisms predict the different observations.

Problem 11

A physical signal and its interpreted message differA physical signal and its interpreted message differOne chosen code: three short pulses means “ready”.pulsepulsepulseCarrier: changing physical pattern. Message: meaning assigned by a code.A larger pulse does not automatically change this three-pulse message.
Three pulses form a changing physical pattern. Under the explicitly chosen classroom code they mean ready. The material disturbance is the carrier, while the meaning depends on a shared code and receiver. A different code could assign different meaning to the same pulses. Changing pulse size within a readable range need not change the encoded pulse count.
For this unit’s illustrated case, would a receiver that has never learned the three-pulse code automatically know the message? What information is missing?
Answer and explanation

No. It can detect a pattern but needs the agreed code/context to interpret ready. Physical detection and message interpretation are different steps.

A signal is an observable changing pattern used to convey information. A code links a pattern to meaning. Explain both the physical mechanism carrying it and the rule used to interpret it.

No. It can detect a pattern but needs the agreed code/context to interpret ready. Physical detection and message interpretation are different steps.

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 12

A continuous signal is recorded every 0.1 s and rounded to only three allowed levels. Name two kinds of lost detail and explain why merely taking samples faster does not solve both.
Answer and explanation

Changes between sample times and finer level differences are lost. Faster sampling improves temporal coverage but does not add more allowed amplitude levels; the encoding and reconstruction also matter.

Separate sampling times from quantized levels.

State which omitted detail each proposed change addresses.

Changes between sample times and finer level differences are lost. Faster sampling improves temporal coverage but does not add more allowed amplitude levels; the encoding and reconstruction also matter.

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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