🔥 Heat, Internal Energy & Work
1 – Temperature tells heat where to go 🌡️
When two bodies touch, energy (heat) always flows from the hotter one to the colder one until they reach the same temperature – that’s thermal equilibrium. Temperature is simply our “hotness scale.” :contentReference[oaicite:0]{index=0}
2 – Meet internal energy (U) 🏃♂️🎢
Inside any material a huge crowd of molecules is rushing, spinning and vibrating. The sum of all their kinetic plus potential energies is its internal energy, \(U\). It’s measured in the frame where the object’s center of mass is at rest, so bulk motion doesn’t count. :contentReference[oaicite:1]{index=1}
- Depends only on the current state (P, V, T) of the system, not on how it got there – that makes \(U\) a state variable. :contentReference[oaicite:2]{index=2}
3 – Two ways to change \(U\) ⬆️⬇️
Imagine a gas in a cylinder with a movable piston:
- Heat, \( \Delta Q \) – put the cylinder in contact with something hotter. Energy sneaks in because of the temperature difference. 🔥 :contentReference[oaicite:3]{index=3}
- Work, \( \Delta W \) – push the piston down (you do work on the gas) or let the gas push it up (gas does work on surroundings). 🛠️ :contentReference[oaicite:4]{index=4}
4 – Heat vs Internal Energy vs Work 🤔
Heat and work are not “contents” you store inside a system; they’re processes – energy in transit while the state changes. Only \(U\) describes the system itself. :contentReference[oaicite:5]{index=5}
5 – The First Law of Thermodynamics 💡
The grand energy bookkeeping rule:
\( \Delta Q = \Delta U + \Delta W \) or \( \Delta Q – \Delta W = \Delta U \)
Energy you supply as heat either bumps up internal energy or shows up as work done by the system. :contentReference[oaicite:6]{index=6}
Path matters for \( \Delta Q \) and \( \Delta W \), but not for \( \Delta U \) 🛤️
You can reach the same final state through many routes (e.g., change volume first, then pressure, or the other way around). \( \Delta U \) stays the same, yet individual \( \Delta Q \) and \( \Delta W \) numbers differ. However, their combination \( \Delta Q-\Delta W \) is always the same – a handy check! :contentReference[oaicite:7]{index=7}
Quick example – isothermal expansion 🧊➡️🚀
For an ideal gas kept at constant temperature, \( \Delta U = 0 \). The First Law collapses to:
\( \Delta Q = \Delta W \)
All the heat you feed in goes straight into pushing the piston (doing work). :contentReference[oaicite:8]{index=8}
🎯 High-Yield Ideas for NEET
- The First Law equation \( \Delta Q = \Delta U + \Delta W \) and sign convention (heat added to the system positive, work done by the system positive).
- Internal energy as a state variable – depends only on P, V, T, not the path.
- Difference between heat & work vs. internal energy – heat/work are processes, internal energy is property.
- Isothermal vs. adiabatic ideas (here: isothermal ⇒ \( \Delta U = 0 \)).
✨ Keep practicing, and may your thermodynamics journey be smooth and heat-efficient! ✨
