Q11 · Waves

Problem

High-pressure pulse sent down a pipe. What reflects when it hits the other end?

Options

Option	Value
(A)	High pulse up if open
(B)	Low pulse up if open
(C)	Low pulse up if closed
(D)	High pulse up if closed

Key insight: Open end: pressure wave reflects with π phase shift (compression → rarefaction). Closed end: pressure reflects with no phase change.

Setup

flowchart TB
  O["Open end"] --> Op["Pressure reflects with π phase shift"]
  Cl["Closed end"] --> Cp["Pressure reflects in phase"]

1. Open end

Phase reversal of pressure wave.

$$\text{high P} \to \text{low P reflected} \Rightarrow (B)$$

2. Closed end

No phase reversal.

$$\text{high P} \to \text{high P reflected} \Rightarrow (D)$$

Deep dive

At an open boundary, the pressure must match atmospheric (fixed), so a compression pulse reflects as rarefaction. At a closed boundary, velocity is zero but pressure can double — compression stays compression.

Hints

• Open end: phase reversal of pressure. Closed end: no reversal.

Answer

(B), (D)