🔍 Key Concepts

• Absolute pressure vs. gauge pressure vs. atmospheric pressure – know how each is defined and what they’re referenced to • Perfect vacuum (zero pressure reference) – understand what this means in theory for pressure measurement • How pressure at a point can be expressed as the sum of atmospheric pressure and gauge pressure

💭 Think About

• Ask yourself: When we say absolute in physics, what is usually the reference point – something variable like weather, or something fixed and theoretical? • Think about gauge pressure you see on a typical pressure gauge. What is that gauge using as its zero reference? How would you modify that to get absolute pressure? • Which option describes a pressure measured from the lowest possible pressure, not from the surrounding air?

✅ Before You Answer

• Identify which choice clearly uses a fixed zero reference (not changing with weather or location). • Confirm which option distinguishes absolute pressure from gauge pressure, rather than confusing the two. • Eliminate any option that compares two pressures which are both already measured relative to atmosphere (that would not be absolute).