20.6
First Law of Thermodynamics
A change in the internal energy of a system depends on the the net heat transfer into the system and the net work done by the system. The first law of…
Consider a sky lantern with a candle. The candle's heat increases the internal energy of the air molecules inside the lantern, which causes the expansion of air and inflates the lantern. Due to the expansion, work is done by the air molecules on the lantern. The air density inside the inflated lantern is reduced, which creates lift.
In this example, the change in internal energy of the system is equal to the net heat transfer into the system minus the net work done by the system. This equation is a generalized form of energy conservation known as the first law of thermodynamics.
Conventionally, Q is positive or negative, depending on whether heat is added to the system or removed from the system. Also, W is positive when work is done by the system and negative when work is done on the system.
For any thermodynamic process, the change in internal energy is path independent and depends only on the system's initial and final equilibrium states. Thus, the internal energy is a state function.
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Q1: What does the first law of thermodynamics state?
The first law of thermodynamics is a generalized form of energy conservation stating that the change in internal energy equals the heat added to the system minus the work done by the system. Mathematically, ΔU = Q - W. This principle applies to all thermodynamic processes and relates three fundamental quantities: internal energy change, heat transfer, and work performed.
Q2: Why is internal energy considered a state function?
Internal energy is a state function because its change depends only on the system's initial and final equilibrium states, not on the path taken between them. Although heat and work individually depend on the thermodynamic path, their difference—the change in internal energy—is path independent. This makes internal energy similar to potential energy in mechanics.
Q3: How do heat and work affect a system's internal energy?
Heat added to a system increases its internal energy, while heat removed decreases it. When a gas expands and does work on its surroundings, its internal energy decreases. Conversely, when work is done on the system, internal energy increases. The net effect on internal energy depends on the balance between heat transfer and work done.
Q4: What is the sign convention for heat and work in thermodynamics?
Heat is positive when added to the system and negative when removed from it. Work is positive when done by the system and negative when done on the system. This convention ensures consistent application of the first law equation ΔU = Q - W across all thermodynamic processes and calculations.
Q5: How does a sky lantern demonstrate the first law of thermodynamics?
In a sky lantern, the candle's heat increases the internal energy of air molecules inside, causing air expansion and inflation. As air expands, it does work on the lantern walls. The reduced air density creates lift. This example illustrates how heat transfer increases internal energy, which then performs work, demonstrating energy conservation in action.
Q6: What thermodynamic variables determine a system's internal energy?
Internal energy is a function of thermodynamic variables including pressure, temperature, and volume. These state variables completely describe a system's condition. Since internal energy depends only on these variables and not on how the system reached its current state, it qualifies as a state function, similar to other path-independent quantities.
Q7: How does the first law apply to cyclic processes?
In cyclic processes, a system returns to its initial state, so the total change in internal energy is zero. According to the first law, the net heat transfer into the system must equal the net work done by the system. This relationship holds for cyclic processes and helps analyze engines and refrigeration cycles.