Keith Mack
The first law of thermodynamics, also known as the law of conservation of energy, states that energy can neither be created nor destroyed, but it can be transformed from one form to another. This law applies the conservation of energy principle to systems where heat and work are the methods of transferring energy into and out of the system. The equation for this law is ΔU = Q + W, where ΔU is the change in internal energy of the gas, Q is the amount of heat energy added to the gas, and W is the amount of work done on the gas. This law can be used to describe how energy transferred by heat is converted and transferred again by work.
| Characteristics | Values |
|---|---|
| First Law of Thermodynamics | The internal energy of a system will change when heat is added or taken away from the gas or when work is done on the gas or by the gas. |
| Equation | ΔU = Q + W |
| Where | ΔU = Change in internal energy of the gas |
| Q = Amount of heat energy added to the gas | |
| W = Amount of work done on the gas | |
| Conservation of Energy | Energy can neither be created nor destroyed, but it can be changed from one form to another. |
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What You'll Learn
Conservation of energy
The first law of thermodynamics is a formulation of the law of conservation of energy in the context of thermodynamic processes. This law of conservation of energy is a fundamental concept in physics, alongside the conservation of mass and momentum. The principle states that within a given system, the amount of energy remains constant—energy is neither created nor destroyed.
The first law of thermodynamics applies this principle to systems where heat transfer and work are the methods of energy transfer. The law distinguishes two principal forms of energy transfer: heat and thermodynamic work. The internal energy of a system can change when heat is added or removed, or when work is done on the system or by the system. This is expressed in the equation: ΔU = Q + W, where ΔU is the change in internal energy, Q is the amount of heat energy added, and W is the amount of work done on the gas. When heat energy is taken from the gas or the gas does work on its environment, those values become negative because energy is leaving the system.
The first law of thermodynamics also accounts for the transfer of energy between thermodynamic systems. Energy can be transferred adiabatically as work, or by a non-adiabatic path unaccompanied by matter transfer. In the case of a transfer of matter between two systems, there is an accompanying transfer of internal energy that cannot be solely attributed to heat and work components. However, there may be pathways for heat and work transfer independent of and simultaneous with the matter transfer, and energy is conserved in such transfers.
The first law of thermodynamics is useful for describing complex processes, such as metabolism and photosynthesis, without needing to account for the intricacies of the processes themselves. For example, while body fat can be converted to do work and produce heat transfer, work done on the body and heat transfer into it cannot be converted to body fat.
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Heat transfer
The First Law of Thermodynamics, also known as the Law of Conservation of Energy, states that energy can neither be created nor destroyed, but it can be converted from one form to another. This law applies to systems where heat and work are the methods of energy transfer.
Heat is the transfer of thermal energy between two bodies at different temperatures, while work is the force used to transfer energy between a system and its surroundings. Both heat and work can change the internal energy of a system, which is the sum of the kinetic and potential energy of a system's atoms and molecules.
The equation for the First Law of Thermodynamics is:
ΔU = Q + W
Where:
- ΔU is the change in internal energy of the system
- Q is the net heat transferred into the system (the sum of all heat transfers into and out of the system)
- W is the net work done by the system (the sum of all work done on or by the system)
When solving problems related to the First Law of Thermodynamics and heat transfer, it is important to identify the values of Q and W. If Q is positive, there is a net heat transfer into the system, and if W is positive, there is net work done by the system.
For example, consider a gas that has 1000 Joules of heat energy added to it and does 700 Joules of work on its environment. In this case, Q is positive (1000 Joules) and W is negative (the gas loses energy by doing work on its surroundings). By substituting these values into the equation, we can find the change in internal energy of the gas.
Another example involves a gas that is cooled, resulting in a loss of 650 Joules of heat, while 700 Joules of work is done on the gas. In this case, Q is negative (heat is removed from the gas) and W is positive (work is done on the gas). By applying these values to the equation, we can determine the change in internal energy of the gas.
By understanding the principles of heat transfer and the First Law of Thermodynamics, we can analyze and solve various problems related to energy conversion and transfer in thermodynamic systems.
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Work done
The first law of thermodynamics is a statement of conservation: any energy gained by a system must be lost by another system, such that the total amount of heat and work equals the total change in the system's energy. This law applies to systems where heat transfer and work done are the methods of energy transfer.
Work is the force used to transfer energy between a system and its surroundings. Work done on a system can be calculated using the equation ΔU = Q + W, where ΔU is the change in internal energy of the system, Q is the amount of heat energy added to the system, and W is the amount of work done on the system. When heat energy is removed from the system or the system does work on its environment, these values become negative because energy is leaving the system.
For example, consider a gas in a system with constant pressure. The surroundings around the system lose 62 J of heat and do 474 J of work on the system. To find the internal energy of the system, we must consider the relationship between the system and its surroundings. The internal energy of the system is 536 J.
The first law of thermodynamics can be applied to understand complex processes, such as metabolism and photosynthesis. For example, body fat can be converted to do work and produce heat transfer, but work done on the body and heat transfer into it cannot be converted to body fat.
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Internal energy
The first law of thermodynamics is a fundamental principle that describes the relationship between heat, work, and internal energy in a system. It establishes the principle of energy conservation in thermodynamic processes, allowing us to analyse how energy is transferred and transformed within a closed system.
Mathematically, the first law of thermodynamics can be expressed as:
ΔU = Q + W
Where:
- ΔU represents the change in internal energy of the system.
- Q is the net heat transferred into the system, accounting for all heat transfers into and out of the system.
- W is the net work done by or on the system, encompassing all work done on or by the system.
The signs of Q and W are crucial in determining the direction of energy transfer. When heat is added to the system (Q is positive), the internal energy increases. Conversely, when heat is removed from the system (Q is negative), the internal energy decreases. Similarly, work done by the system (W is positive) decreases internal energy, while work done on the system (W is negative) increases internal energy.
For example, consider a scenario where 1000 joules of heat energy are added to a gas, and it simultaneously performs 700 joules of work on its environment. To calculate the change in internal energy, we use the equation:
ΔU = Q + W
ΔU = 1000 J - 700 J
ΔU = 300 J
So, the gas has gained 300 joules of internal energy.
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Kinetic and potential energy
The first law of thermodynamics is a statement of conservation, meaning that the total amount of energy in the universe is fixed. In other words, energy cannot be created or destroyed, only transferred or transformed. This law can be expressed mathematically as:
$$Δ U = Q + W$$
Where:
- Δ U is the change in internal energy of the gas
- Q is the amount of heat energy added to the gas
- W is the amount of work done on the gas
When heat energy is removed from the gas or the gas does work on its environment, Q and W become negative because energy is leaving the gas.
$$Δ E_{tot} = Δ U + Δ KE + Δ PE = Q - W$$
Where:
- Δ E_{tot} is the total energy in the system
- Δ U is the change in internal energy
- Δ KE is the change in kinetic energy
- Δ PE is the change in potential energy
This equation shows that the change in total energy of a closed system is equal to the change in internal energy, kinetic energy, and potential energy, accounting for heat ($Q$) and work ($W$).
It is important to note that the first law of thermodynamics applies to various situations, including biological metabolism. For example, humans can convert the chemical energy in food into kinetic energy by riding a bicycle. This illustrates the transfer and transformation of energy described by the first law.
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Frequently asked questions
The first law of thermodynamics states that energy can neither be created nor destroyed, but it can be transformed from one form to another.
The equation for the first law of thermodynamics is ΔU = Q + W, where ΔU is the change in internal energy of a gas, Q is the amount of heat energy added to the gas, and W is the amount of work done on the gas.
The change in internal energy of a system is calculated by subtracting the net work done by the system from the net heat transfer into the system.
Some examples of the first law of thermodynamics in everyday situations include boiling a kettle, metabolism, and photosynthesis.
