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Physics Chapter 8 : Topic 11
First Law and Conservation of Energy
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The first law of thermodynamics expresses the principle of:
Reasoning: The first law states that energy cannot be created or destroyed; it can only be transferred between heat, work, and internal energy.
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The first law of thermodynamics is mathematically represented as:
Reasoning: Heat supplied to a system is used to increase internal energy and perform work on the system.
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According to the first law, energy supplied to a system as heat can be converted into:
Reasoning: The supplied heat may increase internal energy or be used to do work.
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Why is the first law of thermodynamics considered a statement of energy conservation?
Reasoning: Heat, work, and internal energy are different forms of energy transfer, but total energy is conserved.
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A gas receives $\mathbf{500\ J}$ of heat and its internal energy increases by $\mathbf{300\ J}$. The remaining energy:
Reasoning: According to energy conservation, the remaining $\mathbf{(200\ J)}$ must be transferred as work.
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If no heat is supplied to a system, an increase in internal energy can occur by:
Reasoning: Energy can enter the system through work even when heat transfer is zero.
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A machine cannot produce unlimited energy without an energy input because it violates:
Reasoning: The first law prevents creation of energy from nothing.
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A gas expands and performs work on the surroundings. According to energy conservation:
Reasoning: Work done by the gas transfers energy from the system to surroundings.
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When a gas is compressed and work is done on it:
Reasoning: Work done on the gas is an energy transfer into the system.
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Which equation best demonstrates conservation of energy in thermodynamics?
Reasoning: The first law divides supplied energy into stored energy and transferred energy.
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If a system absorbs $\mathbf{1000\ J}$ heat and does $\mathbf{400\ J}$ work on surroundings, the increase in internal energy is:
Reasoning: $\mathbf{\mathrm{\Delta}U = Q - W = 1000 - 400 = 600\ J}$
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A system loses $\mathbf{300\ J}$ heat but gains $\mathbf{500\ J}$ energy by work done on it. The change in internal energy is:
Reasoning: Energy gained by work exceeds energy lost by heat, giving a net increase of $\mathbf{(200\ J)}$.
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In an isolated system:
Reasoning: An isolated system does not exchange energy with surroundings, so total energy remains conserved.
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Which statement correctly describes heat in the first law?
Reasoning: Heat is energy transferred due to temperature difference.
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Which statement correctly describes work in thermodynamics?
Reasoning: Work transfers energy between a system and surroundings.
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During adiabatic compression, the temperature of a gas may increase because:
Reasoning: Although no heat enters, work input increases the internal energy of the gas.
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If a system returns to its original state after a cycle:
Reasoning: Internal energy is a state function, so it returns to its original value after a complete cycle.
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In a cyclic process, if the system absorbs $\mathbf{800\ J}$ heat, the net work done by the system is:
Reasoning: For a cycle, $\mathbf{\mathrm{\Delta}U = 0}$, so $\mathbf{Q = W}$.
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The first law rules out the possibility of a machine that:
Reasoning: Creating energy violates conservation of energy.
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A refrigerator does not violate the first law because:
Reasoning: A refrigerator uses external work to transfer heat from a colder region to a warmer region.
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If $\mathbf{(Q = 0)}$ and work is done on a system, then:
Reasoning: Work input becomes an increase in internal energy.
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The total energy of a system and surroundings together:
Reasoning: Energy may transfer between system and surroundings, but total energy remains constant.
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A gas loses internal energy while doing work. This is possible because:
Reasoning: The gas uses its internal energy to perform work on surroundings.
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Which process shows conversion of mechanical energy into internal energy?
Reasoning: Compression transfers mechanical energy into the gas, increasing internal energy.
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The first law of thermodynamics connects:
Reasoning: The law describes the relationship between different forms of energy transfer.
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A system receives $\mathbf{600\ J}$ heat and its internal energy decreases by $\mathbf{100\ J}$. The work done by the system is:
Reasoning: Using, $\mathbf{Q = \mathrm{\Delta}U + W}$ $\mathbf{\Longrightarrow}$ $\mathbf{600 = - \ 100 + W}$ $\mathbf{\Longrightarrow}$ $\mathbf{W = 700\ J}$
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Why does the first law not allow a perpetual motion machine of the first kind?
Reasoning: Such a machine would create energy without input, contradicting the first law.
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When energy enters a system as heat and leaves as work, the system:
Reasoning: The balance between heat input and work output determines internal energy change.
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The conservation principle in thermodynamics means:
Reasoning: The first law states that energy is conserved while being transformed between different forms.
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