Undergraduate
  1. Classical mechanics  1.1. dynamics  1.1.1. Motion in one dimension  1.1.2. Motion in two dimensions  1.1.3. projectile motion  1.1.4. Relative speed  1.1.5. Uniform circular motion  1.1.6. Non-uniform circular motion  1.1.7. Reference frames and transformations  1.2. Newton's Laws of Motion  1.2.1. First law of motion  1.2.2. Second law of motion  1.2.3. Third Law of Motion  1.2.4. Applications of Newton's Laws  1.2.5. Friction and its types  1.2.6. Free Body Diagram  1.2.7. Constraints and pseudo-forces  1.3. Work and Energy  1.3.1. work done by the force  1.3.2. Work–energy theorem  1.3.3. Kinetic energy  1.3.4. Potential energy in classical mechanics  1.3.5. Conservative and non-conservative forces  1.3.6. Energy Conservation  1.3.7. Power and efficiency  1.4. Speed and collisions  1.4.1. Linear momentum  1.4.2. Conservation of momentum  1.4.3. Impulse and impact force  1.4.4. Elastic and inelastic collision  1.4.5. Center of mass and speed  1.4.6. Rocket Propulsion  1.5. Rotational motion  1.5.1. Torque and Angular Momentum  1.5.2. Moment of inertia  1.5.3. Rotational Kinematics  1.5.4. Rotational Energy  1.5.5. Rolling Motion  1.5.6. Precession and gyroscopic motion  1.6. Gravitational force  1.6.1. Newton's law of universal gravitation  1.6.2. Gravitational potential energy  1.6.3. Orbital mechanics  1.6.4. Kepler's laws of planetary motion  1.6.5. Gravitational field and potential  1.7. Fluid mechanics  1.7.1. Pressure and Pascal's Principle  1.7.2. Buoyancy and Archimedes' principle  1.7.3. Fluid Dynamics and Bernoulli's Principle  1.7.4. Viscosity and Poiseuille's law  1.7.5. Surface tension and capillarity  1.8. Oscillations and waves  1.8.1. Simple Harmonic Motion  1.8.2. Damped and driven oscillations  1.8.3. Coupled oscillations and common modes  1.8.4. Wave properties and types  1.8.5. Sound Waves and the Doppler Effect  1.8.6. Wave interference and superposition
  2. Electromagnetism  2.1. Electrostatics  2.1.1. Electric charge and properties  2.1.2. Coulomb's law  2.1.3. Electric field and electric potential  2.1.4. Gauss's Law  2.1.5. Capacitance and Dielectric  2.1.6. Electric dipole and potential energy  2.2. Electric circuits  2.2.1. Current and Resistance  2.2.2. Ohm's law  2.2.3. Kirchhoff's Laws  2.2.4. RC Circuit  2.2.5. AC Circuits and Reactance  2.2.6. Electric power and energy  2.3. Magnetism  2.3.1. Magnetic Fields and Forces  2.3.2. Ampere's Law  2.3.3. Magnetic dipole moment  2.3.4. Biot-Savart law  2.3.5. Magnetic Materials and Hysteresis  2.4. Electromagnetic induction  2.4.1. Faraday's Law  2.4.2. Lenz's law  2.4.3. Self and mutual induction  2.4.4. Transformers and Inductive Circuits  2.5. Maxwell's equations  2.5.1. Gauss's law for electricity  2.5.2. Gauss's law for magnetism  2.5.3. Faraday's law of induction  2.5.4. Ampere-Maxwell law  2.5.5. Electromagnetic waves
  3. Thermodynamics  3.1. Laws of Thermodynamics  3.1.1. Zeroth law of thermodynamics  3.1.2. First law of thermodynamics  3.1.3. Second Law  3.1.4. Third Law  3.2. Heat and work  3.2.1. Heat transfer (conduction, convection, radiation)  3.2.2. Thermal expansion  3.2.3. Heat engines and refrigerators  3.2.4. Carnot cycle and efficiency  3.3. Statistical mechanics  3.3.1. Maxwell–Boltzmann distribution  3.3.2. Entropy and Probability  3.3.3. Partition function  3.3.4. Fermi–Dirac and Bose–Einstein statistics
  4. Optics  4.1. Geometrical Optics  4.1.1. Reflection and Refraction  4.1.2. Mirrors and lenses  4.1.3. Optical Instruments  4.1.4. Fermat's principle  4.2. Wave optics  4.2.1. Interference in wave optics  4.2.2. Diffraction  4.2.3. Polarization  4.2.4. Coherence and holography
  5. Quantum mechanics  5.1. Wave–particle duality  5.1.1. Blackbody radiation in wave–particle duality in quantum mechanics  5.1.2. Photoelectric effect  5.1.3. Compton scattering  5.1.4. De Broglie wavelength  5.2. Schrödinger Equation  5.2.1. Time-independent Schrödinger equation  5.2.2. Particle in a box  5.2.3. Quantum Tunneling  5.2.4. Potential wells and obstacles  5.3. Quantum States  5.3.1. Wave function  5.3.2. Quantum operators  5.3.3. Heisenberg uncertainty principle  5.3.4. Angular momentum and spin
  6. Relativity  6.1. Special relativity  6.1.1. Lorentz transformations  6.1.2. Time expansion and length contraction  6.1.3. Relativistic energy and momentum  6.2. General relativity  6.2.1. Principle of Equivalence  6.2.2. Schwarzschild metric  6.2.3. Gravitational waves  6.2.4. Black holes and event horizons
  7. Solid state physics  7.1. Crystal structure  7.1.1. Bravais lattices  7.1.2. X-ray diffraction  7.1.3. Band theory  7.1.4. Phonons and lattice vibrations  7.2. Electrical and Magnetic Properties  7.2.1. Conductors, Semiconductors and Insulators  7.2.2. Superconductivity  7.2.3. Hall effect
  8. Nuclear and particle physics  8.1. Atomic Structure  8.1.1. Atomic Model  8.1.2. Nuclear binding energy  8.2. Radioactivity  8.2.1. Alpha, beta, gamma decay  8.2.2. Half life  8.2.3. Nuclear Fission and Fusion  8.3. Particle physics  8.3.1. Standard Model  8.3.2. Quarks and Leptons  8.3.3. antimatter  8.3.4. Fundamental interactions
  9. Astrophysics and cosmology  9.1. Stellar evolution  9.1.1. Star formation  9.1.2. Black holes and neutron stars  9.1.3. White dwarfs and supernovae  9.2. Cosmology  9.2.1. The Big Bang Theory  9.2.2. Dark matter and dark energy  9.2.3. Cosmic microwave background

Undergraduate → Thermodynamics → Laws of Thermodynamics ↓


First law of thermodynamics


The first law of thermodynamics is a fundamental principle in physics that establishes the basic concept of energy conservation. It states that energy cannot be created or destroyed in an isolated system. Rather, it can only be converted from one form to another or transferred from one part of the system to another. This means that the total energy of an isolated system remains constant.

Conceptual understanding

Imagine you have a sealed box that does not allow any energy to pass in or out. Inside this box, if you have a hot object, it may cool down over time, but somewhere else in the box, something else will gain some energy. This could be the movement of gas molecules that leads to an increase in temperature inside the box. The total energy in this closed box remains the same.

To put this into a simple equation form, we write:

Energy in = Energy out + Change in energy

In a more detailed thermodynamic sense, this can be expressed as:

ΔU = Q – W

Where:

  • ΔU is the change in the internal energy of the system.
  • Q is the heat added to the system.
  • W is the work done by the system.

Visual example

Let us consider a simple piston system to understand the first law of thermodynamics.

 ,
| | 
,
|  | |
, | ,
| │ cylinder ├───────┘
,
,

In this piston system, if heat Q is added (for example, by combusting some fuel), this will accelerate the piston, which will do work W on the area around it as it expands. The first law tells us that the amount of heat converted into work plus any change in the internal energy ΔU of the gas inside the piston will still be equal to the initial heat Q

Textual examples and applications

Consider holding a metal rod and heating one end of it. You will soon feel the heat coming toward your hand. What is happening here is that thermal energy is being transferred along the length of the rod from the hot end to the cold end. No energy is lost; it only moves from one place to another. That is the first law.

The first law is important in understanding engines such as cars. Internal combustion engines, which push a piston outward as gases expand, were designed based on the first law. During each engine cycle, some of the chemical potential energy stored in the fuel is converted into heat energy by combustion. This heat energy then does work on the piston, moving your vehicle forward.

Mathematical aspects

Let us look at some mathematical aspects of this law:

We have the formula:

ΔU = Q – W

Here, both Q and W can be positive or negative, depending on the direction of heat flow and work done.

  • If heat is added to the system then Q is positive.
  • If heat is removed then Q is negative.
  • If work is done by the system, then W is positive.
  • If work is done on the system, then W is negative.

Let us consider an example:

Suppose a system absorbs 500 joules of heat and does 225 joules of work, what will be the change in internal energy?

ΔU = Q – W
ΔU = 500 J – 225 J
ΔU = 275 J

Therefore, the change in internal energy is 275 J.

Implications in the real world

An interesting implication of the first law is determining energy efficiency. Consider a power plant that produces electricity. Not all the energy produced by burning fuel is converted into electricity; some of it is lost to the environment as waste heat. By applying the first law, engineers can figure out how much useful work is produced compared to the total energy input and try to improve efficiency, leading to more sustainable energy use and a reduced environmental impact.

In a refrigerator, the first law helps us understand how heat is drawn in and expelled from within the appliance, lowering the internal temperature and keeping food safe.

Conclusion

The first law of thermodynamics highlights the irreversible nature of energy conservation. Its principles are foundational not only in theoretical physics but also in practical applications such as engines, refrigerators, and even biological processes within our bodies. Whether through equations or everyday examples, understanding how energy is transitioned within and between systems is vital to advancing technology and improving the quality of life.


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First law of thermodynamics

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