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Physics Chapter 6 : Topic 6
Continuity and Conservation of Mass
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The continuity equation is based on the principle of conservation of:
Reasoning: Continuity equation states that mass entering a system equals mass leaving it.
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Conservation of mass in fluid flow means:
Reasoning: The total mass of fluid remains constant during flow.
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In a steady fluid flow through a pipe, the mass entering per second is:
Reasoning: Conservation of mass requires equal mass flow rates.
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The continuity equation represents:
Reasoning: It mathematically expresses the conservation of mass in fluid motion.
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If fluid accumulates inside a pipe, the flow is:
Reasoning: In steady flow, mass cannot accumulate at any point.
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The mass flow rate of a fluid is given by:
Reasoning: $\mathbf{Mass\ flow\ rate = \rho Av}$
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The SI unit of mass flow rate is:
Reasoning: Mass flow rate represents mass passing per second.
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In the equation $\mathbf{\rho}_{\mathbf{1\ }}\mathbf{A}_{\mathbf{1\ }}\mathbf{v}_{\mathbf{1}}\mathbf{=}\mathbf{\rho}_{\mathbf{2\ }}\mathbf{A}_{\mathbf{2\ }}\mathbf{v}_{\mathbf{2\ }}$, the quantity conserved is:
Reasoning: Both sides represent mass flow rate.
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If the density of fluid increases while area and velocity remain constant, mass flow rate:
Reasoning: $\mathbf{m = \rho Av}$ ; Mass flow rate depends directly on density.
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A fluid with greater density flowing at the same speed carries:
Reasoning: Higher density means more mass in the same volume.
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The general continuity equation is:
Reasoning: It includes density and represents mass conservation.
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The equation $\mathbf{A}_{\mathbf{1\ }}\mathbf{v}_{\mathbf{1}}\mathbf{=}\mathbf{A}_{\mathbf{2\ }}\mathbf{v}_{\mathbf{2}}$ is obtained from continuity equation when:
Reasoning: For incompressible fluids, density remains constant and cancels out.
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The continuity equation can be derived by considering:
Reasoning: It compares mass flow at two sections of a pipe.
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In a pipe, if no fluid is added or removed, then:
Reasoning: This is the basis of continuity equation.
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The continuity equation fails if:
Reasoning: Leakage violates the condition of constant mass flow.
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For an incompressible fluid, density:
Reasoning: Incompressible means constant density.
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For incompressible flow, continuity equation becomes:
Reasoning: Constant density allows simplification.
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Water flowing through a pipe is usually treated as:
Reasoning: Water density changes very little under normal conditions.
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If the cross-sectional area decreases in an incompressible fluid, velocity increases because:
Reasoning: The same mass must pass through a smaller area.
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The continuity equation connects:
Reasoning: Mass flow depends on $\mathbf{\rho Av}$.
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Water of density $\mathbf{1000\ kg\ /\ }\mathbf{m}^{\mathbf{3}}$ flows through an area of $\mathbf{2\ }\mathbf{m}^{\mathbf{2}}$ with velocity $\mathbf{3\ m/s}$. Mass flow rate is:
Reasoning: Mass flow rate $\mathbf{=}\overset{˙}{\mathbf{m}}\mathbf{= \rho Av}$ $\mathbf{= 1000}\left( \mathbf{2} \right)\left( \mathbf{3} \right)\mathbf{= 6000\ kg/s}$
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A pipe has area $\mathbf{4}\mathbf{\ m}^{\mathbf{2}}$ and velocity $\mathbf{2\ m/s}$. It narrows to $\mathbf{2}\mathbf{\ m}^{\mathbf{2}}$. The new velocity is:
Reasoning: $\mathbf{4(2) = 2}\mathbf{v}_{\mathbf{2}}$ $\mathbf{\Longrightarrow}$ $\mathbf{v}_{\mathbf{2}}\mathbf{= 4\ m/s}$
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A fluid enters a pipe at mass flow rate $\mathbf{500\ kg/s}$. In steady flow, it leaves at:
Reasoning: Mass conservation requires equal flow rates.
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If density doubles while area and velocity remain the same, mass flow rate:
Reasoning: Mass flow rate $\mathbf{=}\overset{˙}{\mathbf{m}}\mathbf{= \rho Av}$ depends directly on density.
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A liquid flows from a wider pipe to a narrower pipe. The continuity equation predicts:
Reasoning: Smaller area requires greater velocity.
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Why does fluid not pile up inside a pipe during steady flow?
Reasoning: Conservation of mass prevents accumulation.
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The continuity equation is important because it allows us to determine:
Reasoning: It relates area and velocity.
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If a pipe diameter decreases, the mass flow rate:
Reasoning: Conservation of mass keeps mass flow rate constant.
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A leaking pipe violates continuity equation because:
Reasoning: Leakage means mass entering does not equal mass leaving.
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The continuity equation assumes:
Reasoning: It is based on conservation of mass.
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In a steady flow system, if $\mathbf{\rho}_{\mathbf{1\ }}\mathbf{A}_{\mathbf{1\ }}\mathbf{v}_{\mathbf{1}}\mathbf{\ }\mathbf{> \ }\mathbf{\rho}_{\mathbf{2\ }}\mathbf{A}_{\mathbf{2\ }}\mathbf{v}_{\mathbf{2}}$ then:
Reasoning: More mass enters than leaves, causing accumulation.
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The continuity equation is a direct consequence of:
Reasoning: It mathematically represents mass conservation.
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In a pipe with changing diameter, which quantity remains constant?
Reasoning: Mass conservation keeps flow rate constant.
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The expression $\mathbf{\rho Av\ }$represents:
Reasoning: It is the mass flow rate.
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Continuity equation cannot be applied directly if:
Reasoning: Leakage violates mass conservation.
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The fundamental idea behind continuity equation is:
Reasoning: The equation is based on conservation of mass.
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