Physics in society

A metal-cased appliance develops a fault that connects its live wire to the case. Which protection is designed to provide a low-resistance path that helps a large fault current operate the protective device?

1. A

   The earth wire

2. B

   The plastic switch label

3. C

   A thicker live wire only

4. D

   A higher-power appliance

1. a

   Select and explain Select the protection and explain how it reduces electric-shock risk.

   3
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   Correct choiceA

   1. 1

      Selects A, the earth wire.

   2. 2

      Explains that the earth connection keeps the case near Earth potential and provides a low-resistance fault path.

   3. 3

      Explains that the resulting large current operates a fuse or circuit breaker, disconnecting the supply.

An ultrasound pulse travels through soft tissue at 1540 m s⁻¹. The strongest echo returns 80 μs after the pulse is sent.

1. a

   Explain Explain why the distance to the boundary is half the total pulse travel distance.

   2
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   1. 1

      States that the pulse travels from the probe to the boundary.

   2. 2

      States that the echo then returns from the boundary to the probe, so the recorded time is for a round trip.

2. b

   Calculate Calculate the depth of the reflecting boundary.

   3
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   1. 1

      Converts 80 μs to 80 × 10⁻⁶ s.

   2. 2

      Uses depth = vt ÷ 2.

   3. 3

      Obtains 1540 × 80 × 10⁻⁶ ÷ 2 = 0.0616 m, or 6.16 cm.

3. c

   State State one reason the real depth may differ from this result.

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   1. 1

      States a valid limitation such as wave speed varying between tissues, uncertain echo origin, probe angle, timing resolution, or layered propagation.

A rural clinic can fund one first-line imaging service. Ultrasound is relatively portable, produces real-time soft-tissue images, uses non-ionising waves, and depends strongly on operator skill. X-ray equipment is better for many bone and chest images, uses ionising radiation, and needs controlled shielding and power.

1. a

   Compare Compare how the two technologies form useful information and state one suitable use for each.

   3
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   1. 1

      Explains that ultrasound uses returning echoes from tissue boundaries to build an image.

   2. 2

      Explains that X-ray imaging uses differences in attenuation through the body to create contrast.

   3. 3

      Gives suitable examples, such as pregnancy or soft-tissue motion for ultrasound and fracture or chest imaging for X-ray.

2. b

   Analyse Identify two pieces of local evidence the clinic should collect before deciding.

   2
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   1. 1

      Identifies one relevant evidence need such as patient case mix, referral distance, staff skills, power reliability, maintenance, cost, or building suitability.

   2. 2

      Identifies a second distinct evidence need and links it to the choice.

3. c

   Evaluate Make a conditional recommendation that considers access, diagnostic benefit, safety, and training.

   3
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   1. 1

      Uses at least one evidence-based benefit and one limitation of the recommended technology.

   2. 2

      Considers safety and the training or quality-assurance needed for reliable use.

   3. 3

      Makes a conditional recommendation tied to the clinic's likely cases and a referral plan for needs the chosen system cannot meet.