Fluids – Study Notes for ANM Admission (Physics)
Why you need this?
Fluids are everywhere – water, blood, air, and the many liquids we use in medicine. Understanding how they behave helps you interpret clinical situations (e.g., blood pressure, fluid balance, ventilation).
1. Simple Explanation
- Fluid: A substance that can flow (liquids and gases).
- Pressure: Force exerted perpendicular to a surface per unit area.
- Density (ρ): Mass per unit volume.
- Buoyancy: Upward force on an object immersed in a fluid, equal to the weight of the fluid displaced.
- Viscosity: Internal resistance to flow – “thickness” of a fluid.
- Flow rate (Q): Volume of fluid that passes through a cross‑section per unit time.
2. Key Points
| Concept | What It Means | Formula | Practical Example |
|---|---|---|---|
| Pressure | Force per area | (P = \frac{F}{A}) | Blood pressure (= \frac{Force;on;arterial;wall}{Cross;sectional;area}) |
| Hydrostatic Pressure | Pressure due to fluid column | (P = \rho g h) | Pressure at 1 m depth in water |
| Pascal’s Law | Pressure applied to an enclosed fluid is transmitted undiminished | (P_1 = P_2) | Hydraulic lift |
| Archimedes’ Principle | Buoyant force equals weight of displaced fluid | (F_b = \rho_{\text{fluid}};V_{\text{displaced}};g) | Balloon floats in air |
| Density | (\rho = \frac{m}{V}) | – | Blood density ≈ 1.06 g cm⁻³ |
| Viscosity | Resistance to flow | (\eta) (dynamic) | Blood viscosity higher in disease |
| Continuity Equation | (A_1v_1 = A_2v_2) | – | Narrowing of a tube increases velocity |
| Bernoulli’s Equation | Energy conservation in fluid flow | (P + \frac{1}{2}\rho v^2 + \rho gh = \text{constant}) | Venturi tube measurement |
3. Worked Example (Hydrostatic Pressure)
Problem:
What is the hydrostatic pressure at a depth of 2 m in freshwater?
(Assume (ρ_{\text{water}} = 1000\ \text{kg m}^{-3}), (g = 9.81\ \text{m s}^{-2}))
Solution:
- Identify the formula:
[ P = ρ g h ] - Plug in the numbers:
[ P = (1000\ \text{kg m}^{-3})(9.81\ \text{m s}^{-2})(2\ \text{m})
] - Calculate:
[ P = 19,620\ \text{Pa} \quad(\text{or } 19.6\ \text{kPa}) ]
Interpretation:
The pressure 2 m below the surface of still water is about 19.6 kPa.
4. Quick Revision Summary
- Fluid basics – Liquids & gases flow, pressure acts perpendicular to a surface.
- Hydrostatics – (P = ρgh); deeper → higher pressure.
- Pascal’s law – Pressure change in a closed fluid transmits uniformly.
- Archimedes – Buoyant force = weight of displaced fluid.
- Density – Heavier fluid (higher ρ) exerts more pressure at same depth.
- Viscosity – Thicker fluids flow slower (important for blood).
- Continuity – Narrower tubes → faster flow.
- Bernoulli – Higher speed → lower pressure (used in medical devices).
Tip for exams:
When a question mentions “depth” or “height,” think hydrostatic pressure.
When it mentions “flow through a tube,” use continuity or Bernoulli.
For “floating” or “sinking,” apply Archimedes’ principle.
Good luck! 🚑