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Q2.3:
A calorie is a unit of heat (energy in transit) and it equals about 4.2 J where (1J = 1 kg m^{2} s^{-2}). Suppose we employ a system of units in which the unit of mass equals (\alpha) kg, the unit of length equals (\beta) m, the unit of time is (\gamma) s. Show that a calorie has a magnitude (4.2 \alpha^{-1} \beta^{-2} \gamma^{2}) in terms of the new units.

Solution :

Step 1: Dimensional Analysis of Energy

The unit of energy (Joule) is defined by the physical quantity of Work/Energy. The dimensional formula for energy is derived from the formula $W = F \cdot d = (ma) \cdot d$.

  • Mass ($M$): $[M^1]$
  • Acceleration ($a$): $[L^1 T^{-2}]$
  • Distance ($d$): $[L^1]$
  • Dimensional Formula for Energy $[E] = [M^1 L^2 T^{-2}]$

[By the Principle of Dimensional Homogeneity, any physical quantity can be expressed as $M^a L^b T^c$. For energy, $a=1, b=2, c=-2$.]

Step 2: Defining the Systems of Units

Let the two systems of units be represented by subscripts 1 (SI system) and 2 (New system). The magnitude of a physical quantity remains invariant regardless of the system used: $n_1 u_1 = n_2 u_2$.

Quantity System 1 (SI) System 2 (New)
Mass $M_1 = 1 \text{ kg}$ $M_2 = \alpha \text{ kg}$
Length $L_1 = 1 \text{ m}$ $L_2 = \beta \text{ m}$
Time $T_1 = 1 \text{ s}$ $T_2 = \gamma \text{ s}$

Step 3: Calculating Magnitude in the New System

Given the magnitude of energy in the SI system is $n_1 = 4.2$, we find $n_2$ using the conversion formula:

$n_2 = n_1 \left[ \frac{M_1}{M_2} \right]^a \left[ \frac{L_1}{L_2} \right]^b \left[ \frac{T_1}{T_2} \right]^c$

Substituting the dimensional exponents ($a=1, b=2, c=-2$):

$n_2 = 4.2 \left[ \frac{1 \text{ kg}}{\alpha \text{ kg}} \right]^1 \left[ \frac{1 \text{ m}}{\beta \text{ m}} \right]^2 \left[ \frac{1 \text{ s}}{\gamma \text{ s}} \right]^{-2}$

Step 4: Algebraic Simplification

Applying the exponents to the ratios:

$n_2 = 4.2 \left( \frac{1}{\alpha} \right) \left( \frac{1}{\beta^2} \right) \left( \frac{1}{\gamma} \right)^{-2}$

[By the Law of Exponents, specifically $(x^n)^{-1} = x^{-n}$ and $(x/y)^{-n} = (y/x)^n$]:

$n_2 = 4.2 \cdot \alpha^{-1} \cdot \beta^{-2} \cdot (\gamma^{-1})^{-2}$

$n_2 = 4.2 \cdot \alpha^{-1} \cdot \beta^{-2} \cdot \gamma^{2}$

Visual Representation of Dimensional Scaling

SI Units New Units $n_2 = 4.2 \alpha^{-1} \beta^{-2} \gamma^2$

Final Solution: The magnitude of a calorie in the new system of units is $4.2 \alpha^{-1} \beta^{-2} \gamma^{2}$.


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