Medical Physics: Specific Gravity & Pressure

Vocabulary flashcards covering the concepts, definitions, formulas, and unit conversions of Specific Gravity and Pressure in Medical Physics.

How is Specific Gravity ($S.G$) defined?

It is the ratio of the density of a substance to the density of water at $4^\circ\text{C}$.

What is the density of water ($\rho_w$) at $4^\circ\text{C}$ in both $kg/m^3$ and $g/cm^3$?

$$\rho_w = 1000\,kg/m^3 = 1\,g/cm^3$$

Why is Specific Gravity ($S.G$) a dimensionless quantity without units?

Because it is a ratio of two densities.

What is the general formula for pressure ($P$) in terms of perpendicular force ($F$) and area ($A$)?

$$P = \frac{F}{A}$$

What is the formula for pressure ($P$) when the acting force is the weight ($W = mg$) of an object?

$$P = \frac{mg}{A}$$

Is pressure classified as a vector quantity or a scalar quantity?

Scalar quantity.

What is the formula for pressure when an inclined force $F$ acts at an angle $\theta$ with the surface?

$$P = \frac{F\sin(\theta)}{A}$$

What is the formula for the pressure exerted by an object of mass $m$ on a surface inclined at an angle $\theta$?

$$P = \frac{mg\cos(\theta)}{A}$$

What happens to the pressure exerted on an inclined plane as the angle of inclination increases?

The pressure decreases.

What is the equivalence between $1\,Pa$ and $dyn/cm^2$, and how does $Pa$ relate to Joules and cubic meters?

$1\,Pa = 10\,dyn/cm^2$, and $Pa = \frac{J}{m^3}$

Specific Gravity ($S.G$)

The dimensionless ratio of the density of a substance to the density of water at $4^\bot\circ\text{C}$, expressed as $S.G = \frac{\rho_{\text{substance}}}{\rho_{\text{water}}}$.

Density of Water at $4^\circ\text{C}$ ($\rho_w$)

The standard reference density value for water, equal to $1000\,kg/m^3$ or $1\,g/cm^3$.

Pressure ($P$)

A scalar physical quantity defined as the perpendicular force acting per unit area of a surface ($P = \frac{F}{A}$).

Pascal ($Pa$)

The SI unit of pressure, defined as one newton per square meter ($1\,Pa = 1\,N/m^2$).

Pressure Under Gravitational Force ($P$)

The pressure calculated when the acting force is weight ($F = W = mg$), expressed as $P = \frac{mg}{A}$, where $g = 9.8\,m/s^2$.

Normal-Angle Inclined Force Pressure

The pressure produced when an applied force forms an angle $\theta$ with the normal (perpendicular) to the surface: $P = \frac{F\cos(\theta)}{A}$, where $F_\perp = F\cos(\theta)$.

Surface-Angle Inclined Force Pressure

The pressure produced when an applied force forms an angle $\theta$ with the surface itself: $P = \frac{F\sin(\theta)}{A}$, where $F_\perp = F\sin(\theta)$.

Pressure on an Inclined Surface

The pressure exerted by an object on a ramp inclined at angle $\theta$, given by $P = \frac{mg\cos(\theta)}{A}$; as the angle of inclination increases, the pressure decreases.

Dyne per Square Centimeter Conversion ($dyn/cm^2$)

A pressure unit conversion derived from $1\,N = 10^5\,dyn$ and $1\,m^2 = 10^4\,cm^2$, resulting in $1\,Pa = 10\,dyn/cm^2$.

Energy Density Pressure Equivalent

An alternative expression showing that pressure is equivalent to energy per unit volume: $1\,J/m^3 = 1\,(N\cdot m)/m^3 = 1\,N/m^2 = 1\,Pa$.