The First Law of Thermodynamics, also known as the Law of the Conservation of Energy, says that energy cannot be created or destroyed, it can only change form. Energy comes in a ton of different forms, including:
Energy is not created or destroyed; it simply changes from one form to another. Turning on a light switch does not create energy, it simply converts electrical energy into radiant energy (light) and thermal energy (heat).
Within the First Law are three related concepts – work, heat, and internal energy. Heat is the transfer of thermal energy between two systems. Work is the force that transfers energy between a system and its surroundings. By producing work either within a system or outside of it, you create heat. Then there’s internal energy, which is all the energy within a system. When heat, work, and internal energy interact together, energy is transformed. You can express this relationship mathematically as:
Here, ΔU is the total change in internal energy within a system, Q is the heat exchanged between the system and its surroundings, and W is the work done to or by the system.
When a system releases heat or does some kind of work, the internal energy of the system decreases. Likewise, if heat is added into a system, or work is done to a system, the internal energy of the system will increase. Any kind of energy that is released by a system is absorbed by its surroundings, and any kind of energy lost by a surrounding is absorbed into a system. In all of these examples, you aren’t creating or destroying energy; it’s just moving from one place to another. Expressed mathematically, this looks like:
Here, ΔUsystem is the total internal energy in a system and is always equal to ΔUsurroundings the total energy in the surroundings.
One important thing to keep in mind about the First Law is that the transformation of energy is not 100% efficient. In our light bulb example you can transform electrical energy into a usable form of light energy, but in the process, you create unusable energy in the form of heat.
When related to electronics, First Law of Thermodynamics has a similar resemble to Kirchhoff’s Current Law. This well-known law states that the amount of current that enters a node is equal to the amount of current leaving a node. It doesn’t matter how many nodes you have, what goes in, must come out.
In the image below we have two currents entering a node, and three currents leaving the node. According to Kirchhoff’s Current Law, the relationship between the current entering and exiting the nodes can be represented as:
Doesn’t that look strangely familiar to our equation for the balance between systems and surroundings?
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ReplyDeleteThis is a nicely written explanation of the First Law of Thermodynamics, clear, concise, and accessible. You do a great job relating the concepts of heat, work, and internal energy with real-world examples. The comparison to Kirchhoff’s Current Law is a smart way to bridge thermodynamics with electrical systems. I particularly liked how you emphasise that energy transformations are never 100% efficient; that’s a key insight that many overlook.
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