What is the first law of thermodynamics?

The first law of thermodynamics states that within an isolated system, the total energy remains constant, even when converted from one form to another. This principle is essentially the law of conservation of energy . For systems that are not isolated, the change in internal energy (\u0394 U ) is equal to the heat ( Q ) added to the system minus the work ( W ) done by the system: \u0394 U = Q \u2212 W . This law is fundamental to thermodynamics , which studies heat, work, temperature, and energy.

How does the first law of thermodynamics relate to energy?

The first law of thermodynamics is a fundamental principle that directly relates to energy by stating that within an isolated system, the total energy remains constant, even when converted from one form to another. This is essentially another way of expressing the law of conservation of energy . The law quantifies energy changes by considering the flow of energy across a system's boundary. If a system absorbs heat ( Q ) and does work ( W ), the change in its internal energy (\u0394 U ) is given by the equation \u0394 U = Q \u2212 W . This means that energy is meticulously accounted for; any increase in internal energy comes from heat added or work done on the system, and any decrease is due to heat lost or work done by the system.

What is meant by conservation of energy?

The conservation of energy, also known as the first law of thermodynamics , is a fundamental principle in physics stating that the total energy within an isolated system remains constant. Energy cannot be created or destroyed; it can only be transformed from one form to another. For example, potential energy can convert into kinetic energy as an object moves, and friction can transform kinetic energy into thermal energy. This principle applies across various forms of energy, including kinetic, potential, thermal, electrical, chemical, and nuclear energy. The concept is so fundamental that even mass is now understood as equivalent to energy, as expressed by Einstein's famous equation, (E=mc^2).

How does the first law explain what happens when heat is added to a system?

The first law of thermodynamics, also known as the law of conservation of energy, explains that when heat is added to a system, the total energy of that system remains constant. If a system absorbs heat ((Q)) and does work ((W)), the change in its internal energy ((\Delta U)) is represented by the equation (\Delta U = Q - W). This means the added heat increases the system's internal energy, or it can be used by the system to perform work, or a combination of both. The internal energy of a substance is the sum of its kinetic and potential energies at a molecular level.

What is the mathematical formula for the first law of thermodynamics?

The first law of thermodynamics, also known as the law of conservation of energy, states that energy cannot be created or destroyed, only converted from one form to another. For a system not isolated from its surroundings, the change in its internal energy ((\Delta U)) is equal to the heat ( Q ) added to the system minus the work ( W ) done by the system on its surroundings. The mathematical formula is: [ \Delta U = Q - W ] Here, (\Delta U) represents the change in internal energy, Q is the heat added to the system, and W is the work done by the system. Internal energy is a state function, meaning its value depends only on the current state of the system, not on how it reached that state. However, heat and work are path-dependent quantities.

How does the first law of thermodynamics apply to everyday examples, like boiling water or running a car?

The first law of thermodynamics, also known as the law of conservation of energy , states that energy cannot be created or destroyed, only converted from one form to another. When boiling water, the heat energy ( Q ) added to the water increases its internal energy (\u0394 U ) and can also do work ( W ) as the water expands into steam. The change in internal energy is represented by the equation \u0394 U = Q \u2212 W . In a car engine, fuel is burned, releasing heat energy. This heat is used to do work by expanding gases that move pistons, propelling the vehicle. Some energy is lost as waste heat, but the total energy remains constant throughout the process.

first law of thermodynamics

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The law of conservation of energy explained

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first law of thermodynamics

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Also known as: law of conservation of energy

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Top Questions

What is the first law of thermodynamics?
The first law of thermodynamics states that within an isolated system, the total energy remains constant, even when converted from one form to another. This principle is essentially the law of conservation of energy.

For systems that are not isolated, the change in internal energy (\u0394U) is equal to the heat (Q) added to the system minus the work (W) done by the system: \u0394U = Q \u2212 W. This law is fundamental to thermodynamics, which studies heat, work, temperature, and energy.
6 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How does the first law of thermodynamics relate to energy?
The first law of thermodynamics is a fundamental principle that directly relates to energy by stating that within an isolated system, the total energy remains constant, even when converted from one form to another. This is essentially another way of expressing the law of conservation of energy.

The law quantifies energy changes by considering the flow of energy across a system's boundary. If a system absorbs heat (Q) and does work (W), the change in its internal energy (\u0394U) is given by the equation \u0394U = Q \u2212 W. This means that energy is meticulously accounted for; any increase in internal energy comes from heat added or work done on the system, and any decrease is due to heat lost or work done by the system.
5 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What is meant by conservation of energy?
The conservation of energy, also known as the first law of thermodynamics, is a fundamental principle in physics stating that the total energy within an isolated system remains constant. Energy cannot be created or destroyed; it can only be transformed from one form to another.

For example, potential energy can convert into kinetic energy as an object moves, and friction can transform kinetic energy into thermal energy. This principle applies across various forms of energy, including kinetic, potential, thermal, electrical, chemical, and nuclear energy. The concept is so fundamental that even mass is now understood as equivalent to energy, as expressed by Einstein's famous equation, (E=mc^2).
9 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How does the first law explain what happens when heat is added to a system?
The first law of thermodynamics, also known as the law of conservation of energy, explains that when heat is added to a system, the total energy of that system remains constant. If a system absorbs heat ((Q)) and does work ((W)), the change in its internal energy ((\Delta U)) is represented by the equation (\Delta U = Q - W). This means the added heat increases the system's internal energy, or it can be used by the system to perform work, or a combination of both. The internal energy of a substance is the sum of its kinetic and potential energies at a molecular level.
7 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What is the mathematical formula for the first law of thermodynamics?
The first law of thermodynamics, also known as the law of conservation of energy, states that energy cannot be created or destroyed, only converted from one form to another. For a system not isolated from its surroundings, the change in its internal energy ((\Delta U)) is equal to the heat (Q) added to the system minus the work (W) done by the system on its surroundings.

The mathematical formula is:
[ \Delta U = Q - W ]

Here, (\Delta U) represents the change in internal energy, Q is the heat added to the system, and W is the work done by the system. Internal energy is a state function, meaning its value depends only on the current state of the system, not on how it reached that state. However, heat and work are path-dependent quantities.
5 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How does the first law of thermodynamics apply to everyday examples, like boiling water or running a car?
The first law of thermodynamics, also known as the law of conservation of energy, states that energy cannot be created or destroyed, only converted from one form to another.

When boiling water, the heat energy (Q) added to the water increases its internal energy (\u0394U) and can also do work (W) as the water expands into steam. The change in internal energy is represented by the equation \u0394U = Q \u2212 W.

In a car engine, fuel is burned, releasing heat energy. This heat is used to do work by expanding gases that move pistons, propelling the vehicle. Some energy is lost as waste heat, but the total energy remains constant throughout the process.
5 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

first law of thermodynamics, thermodynamic relation stating that, within an isolated system, the total energy of the system is constant, even if energy has been converted from one form to another. This law is another way of stating the law of conservation of energy. It is one of four relations underlying thermodynamics, the branch of physics concerning heat, work, temperature, and energy.

The first law of thermodynamics is put into action by considering the flow of energy across the boundary separating a system from its surroundings. Consider the classic example of a gas enclosed in a cylinder with a movable piston. The walls of the cylinder act as the boundary separating the gas inside from the world outside, and the movable piston provides a mechanism for the gas to do work by expanding against the force holding the piston (assumed frictionless) in place. If the gas does work W as it expands, and/or absorbs heat Q from its surroundings through the walls of the cylinder, then this corresponds to a net flow of energy W − Q across the boundary to the surroundings. In order to conserve the total energy U, there must be a counterbalancing change ΔU = Q − W in the internal energy of the gas. The first law provides a kind of strict energy accounting system in which the change in the energy account (ΔU) equals the difference between deposits (Q) and withdrawals (W).

There is an important distinction between the quantity ΔU and the related energy quantities Q and W. Since the internal energy U is characterized entirely by the quantities (or parameters) that uniquely determine the state of the system at equilibrium, it is said to be a state function such that any change in energy is determined entirely by the initial (i) and final (f) states of the system: ΔU = U_f − U_i. However, Q and W are not state functions. Just as in the example of a bursting balloon, the gas inside may do no work at all in reaching its final expanded state, or it could do maximum work by expanding inside a cylinder with a movable piston to reach the same final state. All that is required is that the change in energy (ΔU) remain the same. By analogy, the same change in one’s bank account could be achieved by many different combinations of deposits and withdrawals. Thus, Q and W are not state functions, because their values depend on the particular process (or path) connecting the same initial and final states. Just as it is more meaningful to speak of the balance in one’s bank account than its deposit or withdrawal content, it is only meaningful to speak of the internal energy of a system and not its heat or work content.

first law of thermodynamics

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